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1.
A cycloconverter can be A.
step down
B.
step up
C.
step down or step up
D.
none of the above
The output frequency can be more or less than input frequency. 2.
In the below figure the average load current is 15 A. The rms value of transformer secondary current is
A.
15 A
B.
10.61 A
C.
7.5 A
D.
14.14 A
RMS secondary current =
3.
= 15/1.414 = 10.608 A.
In a 3 phase fully controlled converter the firing frequency is A.
3 times the line frequency
B.
6 times the line frequency
C.
9 times the line frequency
D.
12 times the line frequency
It is a 6 pulse converter. 4.
Two thyristor of same rating and same specifications A.
will have equal turn on and turn off periods
B.
will have equal turn on but unequal turn off periods
C.
may have equal or unequal turn on and turn off periods
D.
will have unequal turn on and turn off periods
Turn on and turn off times of similar SCRs may not be same. 5.
6.
The amount of dielectric heating is inversely proportional to frequency. A.
True
B.
False
A semiconverter feeding an RLE load operates in A.
first quadrant only
B.
first and second quadrants
C.
first and fourth quadrants
D.
first and third quadrants
Semiconvertor operates only in first quadrant. 7.
The phenomenon of overlap in converter operation due to A.
source resistance
B.
source inductance
C.
both source resistance and source inductance
D.
high value of firing angle
Source inductance delays turn off and hence causes overlap. 8.
A thyristor has a maximum allowable junction temperature of 120°C and the ambient temperature is 40°C. If thermal resistance is 1.6° C/W, the maximum allowable internal power dissipation is A.
20 W
B.
50 W
C.
92 W
D.
128 W
. 9.
For a 12 pulse operation, the two 6 pulse units are fed by A.
Y - Y and Y - Y transformers
B.
Δ - Δ and Δ - Δ transformers
C.
Y - Y and Y - Δ transformers
D.
Y - Δ and Y - Δ transformers
30° phase difference is necessary. 10. In a single phase full wave converter (M - 2 connection) feeding a highly inductive load, the firing angle for each thyristor is a in the respective half cycle. The period of conduction of each thyristor is A.
-
B.
C.
+
D.
- 2
Since conduction is from
to
+ ,
period of conduction is
in each half cycle.
11. An SCR is triggered at 40° in the positive half cycle only. The average anode current is 50 A. If the firing angle is changed to 80°, the average anode current is likely to be A.
50 A
B.
25 A
C.
less than 50 A but more than 25 A
D.
less than 25 A
. 12. The characteristics of a non-linear resistance is i = kv4. If i becomes 100 times, vbecomes A.
about 100 times
B.
about 10 times
C.
about 3 times
D.
about twice
v ∝ (i)1/4 ∝ (100)1/4 ≈ 3. 13. In a step down chopper using pulse width modulation, Ton = 3 x 10 -3 and Toff = 1 x 10 -3s. The chopping frequency is A.
333.33
B.
250
C.
500
D.
1000
. 14. Thyristors are suitable for dc circuit breakers but not for ac circuit breakers. A.
True
B.
False
They are suitable for both dc and ac circuit breakers.
15. Assertion (A): Inverters and choppers use fast switching thyristors Reason (R): Fast switching SCR has low turn off time. A.
Both A and R are correct and R is correct explanation of A
B.
Both A and R correct but R is not correct explanation of A
C.
A is correct but R is wrong
D.
A is wrong but R is correct
Low turn off time enables fast switching. 16. The value of capacitor C for dynamic equalising circuit of series connected thyristors is determined by A. B.
forward characteristics of thyristors reverse recovery characteristics of thyristors
C.
both forward and reverse recovery characteristics of thyristors
D.
none of the above
Value of C is proportional to maximum difference between reverse charge of SCRs in series.
17. A chopper A. B.
converts constant voltage dc into ac and then into variable vo ltage dc converts constant voltage dc into variable voltage dc directly
C.
converts ac of one frequency into ac of another frequency
D.
converts ac to dc
A chopper is a dc-ac converter. It converts a fixed dc voltage into a variable dc voltage.
18. Assertion (A): If the duty cycle of chopper is low the current would not become discontinuous Reason (R): If the time constant L/R of the load in chopper is low the current may become discontinuous. A.
Both A and R are correct and R is correct explanation of A
B.
Both A and R correct but R is not correct explanation of A
C. D.
A is correct but R is wrong
A is wrong but R is correct
If duty cycle is low, current may become discontinuous.
19. In a single phase full wave regulator, the firing angles in the positive and negative half cycles are generally A.
equal
B.
different
C.
equal or different
D.
different but sometimes equal
Firing angles are kept equal to get symmetrical output voltage.
20. An electric heater is controlled by thyristors through on-off control. If = 0.4, the heating is A. B.
16% maximum 40% of maximum
C.
60% of maximum
D.
84% of m ximum
Vl = V
. Therefore Vl2 - V2
(a). Heating is proportional to
.
21. A two winding transformer is feeding a single phase half wave rectifier circuit. The load is purely resistive. The rms value of transformer secondary current is I s and rms value of load current in Irms. Then A.
Is = Irms irrespective of the value of firing angle
B.
Is = 0.5 Irms irrespective of the value of firing angle
C.
Is = Irms if firing angle is less than 30°
D.
Is = Irms if firing angle is more than 30°
.
22. The terminals of a power BJT are called A.
emitter, base, collector
B.
emitter, base, drain
C.
source, base, drain
D.
source, base, collector
In a BJT terminals are emitter, base and collector irrespective of rating.
23. In single phase half wave regulator, the average current over one full cycle A.
is always positive
B.
may be positive or negative
C.
may be negative
D.
is always negative
The input current during positive half cycle is less than that during negative half cycle.
24. The dynamic equalising circuit consists of a series combination of capacitor C and resistor RC across each thyristor. This resistance RC (along with parallel connected diode) A. B.
limits charging current limits discharging current
C.
limits both charging and discharging currents
D.
none of the above
Discharge current flows through Re .
25. A single phase semiconverter is feeding a highly inductive load and has freewheeling diode across the load. The waveshapes of output voltage and output current A. B.
are similar are not similar
C.
may be similar or dissimilar
D.
are similar only if firing angle is zero
When freewheeling diode conducts output current exists but output voltage is zero.
26. Current derating may be necessary when thyristors are operating in A. B.
series parallel
C.
series or parallel
D.
none of the above
Since current does not divide equally, current derating is necessary.
27. If duty cycle is
and the load is a resistance R, the rms value of output voltage is
A.
B.
V
2V
C.
D.
RMS output voltage VL =
V.
28. For a BJT
= 0.98, then β =
A.
0.02
B.
C.
4.9
D.
0.5 48
29. Assertion (A): Power transistor has lower switching time than SCR Reason (R): Power transistor has greater switching time than SCR A.
Both A and R are correct and R is correct explanation of A
B.
Both A and R correct but R is not correct explanation of A
C.
A is correct but R is wrong
D.
A is wrong but R is correct
Power transistor does not have high
rating.
30. A 3 kV circuit uses SCR of 800 V rating. If derating is 25%, the number of SCRs in series is A.
4
B.
C.
6
D.
5
8
.
31. In a circuit using a full wave converter (M - 2 connection) the PIV of each thyristor is 400 V. For the same output voltage and fully controlled bridge converter, PIV will be A.
400 V
B.
800 V
C. D.
200 V
2002 V
In bridge connection PIV is half of that in M-2 connection.
32. A dc separately excited motor has constant field current. The armature is fed from a s ingle phase supply through a full converter. When = 0, speed is 500 rpm. If = 45°, the speed is likely to be A.
about 350 rpm
B.
about 250 rpm
C.
about 175 rpm
D.
about 125 rpm
Average output voltage ∝ cos
and therefore reduced to 0.707 of the value for = 0. Speed is
nearly proportional to voltage.
33. In a multiphase chopper, all choppers operate together. A.
True
B.
False
They operate in a sequence.
34. The terminals of a power MOSFET are called A. B.
emitter, base, collector source, gate, drain
C.
source, base, drain
D.
emitter, gate, drain
In a MOSFET terminals are source, gate, drain irrespective of rating.
35. Assertion (A): The surge current which an SCR can withstand is much higher than rms on state current. Reason (R): The duration of surge current is very small. A.
Both A and R are correct and R is correct explanation of A
B.
Both A and R correct but R is not correct explanation of A
C.
A is correct but R is wrong
D.
A is wrong but R is correct
Since duration of surge current is small, the associated energy is small.
36. A single phase half wave controlled rectifier circuit has a free wheeling diode. The load is a combination of R and L. The firing angle is . The period of conduction of SCR and free wheeling diode respectively are
< ωt <
and <
ω t < (2 +
B.
< ωt <
and <
ω t < 2
C.
< ω t < ( +
D.
< ωt <
A.
) and (
and <
+
)
) < ω t < 2
ω t < (2 -
)
SCR conducts from to
and free wheeling diode from to (2
+
).
37. Assertion (A): A transistor requires a continuous base signal for conduction but a thyristor requires a gate pulse.Reason (R): Transistor find widespread application in power electronic circuits. A.
Both A and R are correct and R is correct explanation of A
B.
Both A and R correct but R is not correct explanation of A
C. D.
A is correct but R is wrong
A is wrong but R is correct
Transistors are used but not very widely.
38. In a thyristor the gate current is increased, then A.
anode current will increase
B.
anode current will decrease
C. D.
anode current will remain constant
anode current may increase or decrease
Gate current does not affect the magnitude of anode current.
39. Thyristors are not suitable for logic circuits. A.
True
B.
False
Thyristors can be used for logic circuits.
40. A single phase half wave rectifier is feeding a resistive load. Input voltage v = Vm sin ω t. The output dc voltage is Vdc and output rms voltage is Vrms . If firing angle is 180°, Vdcand Vrms respectively are A.
0 and 0
B.
C.
0 and - Vm
D.
Put = 180° in Eqs Otherwise also = 180° means no conduction. Hence zero output voltage.
41. The number of leads in an SCR are : A.
2
B.
C.
4
D.
3
5
Anode, cathode and gate.
42. In a single phase full wave converter (M-2 connection) feeding on R - L load, the input voltage is v = Vm sin ω t. The expression for dc output voltage is
A.
B.
C.
D.
Due to inductive load the period of conduction is
to +
in each half cycle.
43. A thyristor has a turn on time of 6 μs. If the anode circuit is inductive, the turn on time will be A.
6 μs
B.
less than 6μs
C. D.
more than 6 μs
either 6 μs or less
Inductive load increases the turn on time.
44. The total number of leads in reverse conducting thyris tor is A.
2
B.
C.
1
D.
3
4
45. Assertion (A): A TRIAC is a bidirectional SCR. Reason (R): A TRIAC is a four layers 3 terminal device and can conduct in both directions. A.
Both A and R are correct and R is correct explanation of A
B.
Both A and R correct but R is not correct explanation of A
C.
A is correct but R is wrong
D.
A is wrong but R is correct
Since a TRIAC can conduct in both directions it is bi directional SC R.
46. If Vdc1 is the dc output voltage in half wave controlled rectifier circuit feeding resistive load and Vdc2 is the dc output voltage in full wave controlled rectifier circuit (M - 2 connection) feeding resistive load, then A.
Vdc1 = 0.5 Vdc2 for all values of firing angle
B.
Vdc1 = Vdc2 for all values of firing angle
C.
Vdc1 ≤ 0.5 Vdc2 for all values of firing angle
D.
Vdc1 ≥ 0.5 Vdc2 for all values of firing angle
and Vdc =
(1 + cos
).
47. A semi converter operates in two quadrants. A.
True
B.
False
It operates only in quadrant.
48. Figure shows a chopper feeding RLE load, The free wheeling diode conducts when
A. B.
thyristor is on thyristor is off
C.
both when thyristor is on and thyristor is off
D.
partly when thyristor is off and partly when thyristor is on
FD is forward biased only when thyristor is off.
49. Assertion (A): An ideal rectifier should have unity harmonic factor Reason (R): An ideal rectifier should have unity displacement factor. A.
Both A and R are correct and R is correct explanation of A
B.
Both A and R correct but R is not correct explanation of A
C.
A is correct but R is wrong
D.
A is wrong but R is correct
Ideal rectifier should have zero harmonic factor and unity displacement factor.
50. A single phase half wave rectifier is feeding a resistive load. When V. When = 30°, the dc output voltage will be about A. B.
127 V 120 V
C.
110 V
D.
100 V
Using Eq
= 0.933.
= 0, dc output voltage is 127
Hence Vdc for 30° firing angle = 0.933 x 127 = ≈ 120 V.
SECTION 2 1.
A dc circuit breaker must use A.
natural commutation
B.
forced commutation
C.
both natural and forced commutation
D.
either natural or forced commutation
Answer & Explanation
Answer: Option B Explanation: Natural commutation is possible only in ac circuits.
2.
In a single phase full wave regulator feeding R-L load, the minimum firing angle can be zero. A.
True
B.
False
Answer & Explanation
Answer: Option B Explanation: Range of control of is 0 ≤
3.
≤
.
An RC snubber circuit is used to protect a thyristor against : A.
false triggering
B.
failure to turn on
C.
switching transients
D.
failure to commutate
Answer & Explanation
Answer: Option C Explanation:
Especially high
4.
during switching.
A DIAC has two terminals, anode and cathode. A.
True
B.
False
Answer & Explanation
Answer: Option B Explanation: They are designated as T1 and T2 because diac can conduct in both directions.
5.
A single phase semiconverter has a freewheeling diode. If the firing angle is
and the load is
purely resistive, the periods of conduction and freewheeling respectively are A.
-
and 0
B.
-
and
C.
+
and 0
D.
+
and
Answer & Explanation
Answer: Option A Explanation: Freewheeling diode conducts only when load is inductive. If load is purely resistive free wheeling diode does not come into operation.
6.
A dc separately excited motor has a constant field current. The armature is fed from a single phase supply through a semi-converter. When = 0, speed is 600 rpm. If = 90°, the speed is likely to be A.
about 300 rpm
B.
about 150 rpm
C.
about 75 rpm
D.
about zero
Answer & Explanation
Answer: Option A Explanation: Average output voltage is reduced to 50% if = 90°. Hence speed is also reduced to 50%.
7.
A static induction thyristor A.
is a majority carrier device
B.
is a minority carrier device
C.
either a majority carrier or minority carrier device
D.
has a total number of 2 leads
Answer & Explanation
Answer: Option B Explanation: It can be turned on by positive gate pulse and turned off by negative gate pulse. It is minority carrier device. It can be manufactured in high current and high voltage ratings and its on state voltage drops is low.
8.
A single phase step up cycloconverter changes 50 Hz to 100 Hz. Then one half wave of input will give rise to
A.
one half wave of output
B.
one full wave of output
C.
two full waves of output
D.
either (b) or (c)
Answer & Explanation
Answer: Option B Explanation: Since output frequency is 2f, one half wave of input must equal one full wave of output.
9.
Which of these triggering methods for series connected SCRs use LASCR in the gate circuit? A.
Resistance triggering
B.
Simultaneous triggering
C.
Sequential triggering
D.
Optical triggering
Answer & Explanation
Answer: Option D Explanation: Optical triggering uses LASCR.
10. A 3 phase ac regulator uses A.
3 thyristors
B.
15 thyristors
C.
9 thyristors
D.
12 thyristors
Answer & Explanation
Answer: Option B Explanation: Two thyristors for each phase.
11. The average load current supplied by a thyristor depends on A.
firing angle
B.
firing frequency
C.
magnitude of gate current
D.
all of the above
Answer & Explanation
Answer: Option A Explanation: The firing angle controls the period of conduction and hence average load current.
12. A dc chopper is feeding an RLE load. The maximum steady state ripple is A.
B.
C.
D.
Answer & Explanation
Answer: Option A Explanation:
(Imax - Imin)max =
.
13. In a 3 phase full wave regulator feeding a star connected resistance load the input voltage is 400 V line to line. The firing angle is 160°. The line to line output voltage would be A.
400 V
B.
about 100 V
C.
about 20 V
D.
zero
Answer & Explanation
Answer: Option D Explanation: In a three-phase full wave regulator, output voltage is zero if ≥ 150°.
14. A thyristor is reverse biased. A positive gate pulse is applied. The thyristor A.
will be turned on
B.
may or may not turn on
C.
will not turn on
D.
will turn on after sometime
Answer & Explanation
Answer: Option C
Explanation: Thyristor can turn on only when it is forward biased.
15. A single phase series converter is used for A.
high voltage output
B.
high current output
C.
high voltage and high current output
D.
improving the circuit efficiency
Answer & Explanation
Answer: Option A Explanation: Series connection increases the voltage.
16. A single phase half wave controlled rectifier circuit has an R-L load. A freewheeling diode is also in the circuit. When freewheeling diode is conducting the SCR A.
is forward biased
B.
is reverse biased
C.
may be forward biased or reverse biased
D.
forward biased initially but reverse biased afterwards
Answer & Explanation
Answer: Option B Explanation: Free wheeling diode is forward biased only when SCR is reverse biased.
17. An SBS can conduct is one direction only. A.
True
B.
False
Answer & Explanation
Answer: Option B Explanation: It is a bilateral switch.
18. A 750 A circuit uses thyristor of current rating 175 A. If derating is 25%, the number of thyristors in parallel is A.
3
B.
4
C.
6
D.
10
Answer & Explanation
Answer: Option C Explanation:
.
19. Assertion (A): An inverter does not require forced commutation Reason (R): A series inverter is a forced commutation inverter. A.
Both A and R are correct and R is correct explanation of A
B.
Both A and R correct but R is not correct explanation of A
C.
A is correct but R is wrong
D.
A is wrong but R is correct
Answer & Explanation
Answer: Option D Explanation: Some inverters require forced commutation.
20. In a single phase full wave converter (M - 2 connection), the rms transformer voltage is 300 V across each half of secondary. The PIV for each thyris tor is A.
300 V
B.
C.
6002 V
D. Answer & Explanation
Answer: Option C Explanation: PIV is twice the peak voltage across each half of secondary.
21. The number of doped regions in a SCR is A.
2
B.
3
C.
4
D.
5
Answer & Explanation
Answer: Option C Explanation: p = n = p = n.
22. All inverters use forced commutation. A.
True
B.
False
Answer & Explanation
Answer: Option B Explanation: Line commutated inverters are also used especially in HVDC.
23. In 180° mode of operation of a 3 phase bridge inverter, two thyristors conduct at one time. A.
True
B.
False
Answer & Explanation
Answer: Option B Explanation: In 180° mode three thyristors conduct simultaneously.
24. In a single phase full wave ac regulator the rms value of load current is 20 A load is purely resistive. Then A.
average thyristor current will be less than 20 A but rms thyristor current will be more than 20 A
B.
both the average and rms thyristor currents will be less than 20 A
C.
both the average and rms thyristor curre nts will be more than 20 A
D.
rms thyristor current will be 20 A and average thyristor current will be 10 A
Answer & Explanation
Answer: Option B Explanation:
Each thyristor conducts for 50% of time.
25. A reactor is used in a dual converter operating in non-circulating current mode. A.
True
B.
False
Answer & Explanation
Answer: Option B Explanation: Reactor is used only for circulating current mode.
26. Which of these commutation methods uses an auxiliary SCR? A.
Class A
B.
Class B
C.
Class C
D.
Class D
Answer & Explanation
Answer: Option C Explanation: Class C commutation uses an auxiliary SCR and it is also called auxiliary commutation.
27. A thyristor needs protection against A.
B.
high
high
C.
D.
both high
and high
either high
or high
Answer & Explanation
Answer: Option C Explanation:
Both high
and high
can destroy SCR.
28. Two semiconverters are connected in series to form a single phase series converter. The input v = Vm sin ω t and 1 = 2 are the firing angles of the two converters. If 1 = 2= 0, the dc output voltage is A.
B.
C.
D. Answer & Explanation
Answer: Option C Explanation:
Output voltage is
for each.
29. In integral cycle control the period of on-off control should be more than mechanical time constant of load. A.
True
B.
False
Answer & Explanation
Answer: Option B Explanation: It should be less than mechanical time constant.
30. A single phase half wave converter is feeding a resistive load. Just before triggering, the voltage across thyristor is A.
zero
B.
about 1 V
C.
same as input voltage
D.
twice the input voltage
Answer & Explanation
Answer: Option C Explanation: Before triggering the whole of applied voltage is across SCR and load current is zero.
31. In a 3 phase full wave regulator feeding resistance load connected in star, the possible range of firing angle is A.
0≤
≤ 180°
B.
0≤
≤ 150°
C.
0≤
≤ 120°
D.
0≤
≤ 90°
Answer & Explanation
Answer: Option B Explanation: Since output voltage is zero if ≥ 150°, the possible range of firing angles is 0 ≤
≤ 150°.
32. Assertion (A): A dual converter may be circulating current type or non circulating current type. Reason (R): In circulating current dual converter only one converter can be operated at one time. A.
Both A and R are correct and R is correct explanation of A
B.
Both A and R correct but R is not correct explanation of A
C.
A is correct but R is wrong
D.
A is wrong but R is correct
Answer & Explanation
Answer: Option C Explanation: In circulating current dual converter both converters operate together.
33. Current chopping leads to A.
over voltage
B.
over current
C.
both over voltage and over current
D.
either over voltage or over current
Answer & Explanation
Answer: Option A
Explanation:
e=L
. In current chopping
is high. Hence e is high.
34. A 3 phase bridge inverter is fed by 400 V battery. The load is star connected and has a resistance of 10 Ω per phase. The peak value of load current is A.
40 A
B.
20 A
C.
10 A
D.
5A
Answer & Explanation
Answer: Option B Explanation: Two resistances are in series at any time.
35. In an SCR, the anode current is controlled by A.
gate current only
B.
external circuit only
C.
both gate current and external circuit
D.
none of the above
Answer & Explanation
Answer: Option B Explanation: Once SCR has been triggered the gate has no control. Only external resistance controls the anode current.
36. The rated current of a thyristor is 40 A. The holding current will be about : A.
20 A
B.
1A
C.
100 mA
D.
10 mA
Answer & Explanation
Answer: Option D Explanation: The holding current is much less then 1% of rated current.
37. A cycloconverter is A.
ac-dc converter
B.
dc-ac converter
C.
dc-dc converter
D.
ac-ac converter
Answer & Explanation
Answer: Option D Explanation: It changes ac at frequency f into ac at a frequency different from f.
38. The number of leads in a power semiconductor diode are A.
2
B.
3
C.
4
D.
2 or 3
Answer & Explanation
Answer: Option A Explanation: A diode has 2 leads irrespective of its rating.
39. If ton and toff are the turn on and turn off times of a thyristor, then A.
ton and toff are almost equal
B.
ton >> toff
C.
toff >> ton
D.
ton is about twice toff
Answer & Explanation
Answer: Option C Explanation: Turn on time is less than 3 μs while turn off time is between 10 -300 μs.
40. Assertion (A): The gate current of an SCR is always in the form of pulses. Reason (R): The gate losses, in pulse triggering, are low. A.
Both A and R are correct and R is correct explanation of A
B.
Both A and R correct but R is not correct explanation of A
C.
A is correct but R is wrong
D.
A is wrong but R is correct
Answer & Explanation
Answer: Option A Explanation: Gate current in the form of pulses is commonly used because gate losses are low.
41. UJT can be used for A.
firing an SCR
B.
turning off an SCR
C.
both firing and turning off an SCR
D.
none of the above
Answer & Explanation
Answer: Option A Explanation: UJT relaxation oscillator is used to provide gate pulses to SCR for triggering.
42. In a series inverter supplying a load resistance R, the commutating e lements L and C should be such that A.
B.
C.
D. Answer & Explanation
Answer: Option B Explanation: Load should be under damped.
43. In a full wave converter (M - 2 connection) feeding an RLE load, the peak thyristor current is 15 A. For fully controlled bridge circuit and same load current, the peak thyristor current will be A.
30 A
B.
152 A
C.
15 A
D. Answer & Explanation
Answer: Option C Explanation: In both the connections the output voltage and peak thyristor current are the same. Both are full wave rectifiers.
44. Induction heating systems require low frequency inverters. A.
True
Answer & Explanation
Answer: Option B Explanation: It uses high frequency inverter.
B.
False
45. A single phase half wave controlled rectifier circuit is feeding on RL load. The circuit has a freewheeling diode. The input voltage is v = Vm sin ω t. If I0 is the load current during the conduction of freewheeling diode, the KVL equation during the conduction of free wheeling diode is A.
B.
C.
D. Answer & Explanation
Answer: Option B Explanation: Load is short-circuited by free wheeling diode.
46. HVDC transmission systems generally use A.
12 pulse converters
B.
6 pulse converters
C.
3 pulse converters
D.
either 12 or 6 pulse converters
Answer & Explanation
Answer: Option A Explanation: To reduce ripple, 12 pulse converters are used in HVDC.
47. A single phase semiconverter is feeding a highly inductive load. The circuit has a freewheeling
diode across the load and firing angle is
. During
one cycle of input voltage, the total duration of
conduction of freewheeling diode is A.
B.
2
C.
4
D.
0.5
Answer & Explanation
Answer: Option B Explanation: Freewheeling diode conducts for ° in each half cycle.
48. During forward blocking state of SCR, the voltage and current respectively are A.
high and high
B.
low and low
C.
high and low
D.
low and high
Answer & Explanation
Answer: Option C Explanation: Voltage is high but current is nearly zero.
49. In a 3 phase full converter the average load current is 150 A. The average thyristor current is A.
150 A
B.
75A
C.
50 A
D.
25 A
Answer & Explanation
Answer: Option C Explanation: Each thyristor conducts for one -third of one cycle.
50. The two modes of operation of three phase bridge inverters are known as A.
60° mode and 120° mode
B.
90° mode and 120° mode
C.
120° mode and 180° mode
D.
120° mode and 240° mode
Answer & Explanation
Answer: Option C Explanation: In 180° mode each thyristor conducts for 180° and in 120° mode each thyristor conducts for 120°.
SECTION 3 1.
In a dual converter both converters work as rectifiers. A.
True
Answer & Explanation
Answer: Option B Explanation:
B.
False
One operators as rectifier and the other as inverter.
2.
A single phase full wave regulator feeds R-L load. The best gating signal is A.
short duration pulses
B.
long duration pulses
C.
pulse train
D.
either (a) or (b)
Answer & Explanation
Answer: Option C Explanation: To ensure proper triggering pulse train in necessary because load is R - L.
3.
A 3 phase induction motor is fed by a 3 phase ac regulator to change the stator voltage. The variation in speed will be more if A.
load torque is constant
B.
load torque increases with increase in speed
C.
load torque decreases with increase in speed
D.
either (a) or (b)
Answer & Explanation
Answer: Option B Explanation: If load torque is constant, the change in speed is very small.
4.
A cycloconverter cannot give a frequency higher than input frequency. A.
True
B.
False
Answer & Explanation
Answer: Option B Explanation: Step up cycloconverter can give a frequency higher than input frequency.
5.
A thyristor has internal power dissipation of 40W and is operated at an ambient temperature of 20°C. If thermal resistance is 1.6° C/W, the junction temperature is A.
114°C
B.
164°C
C.
94° C
D.
84° C
Answer & Explanation
Answer: Option D Explanation:
.
6.
In a single phase dual converter, 1, and A.
1 - 2 =
B.
1 + 2 =
C.
1 - 2 =
D.
1 + 2 =
2 are firing
angles of the two converters. Then
Answer & Explanation
Answer: Option B Explanation: Since one converter is rectifying and the other is inverting Vdc1 = -Vdc2 or
7.
1 + 2 = .
The number of gates in an SCR is A.
1
B.
3
C.
4
D.
6
Answer & Explanation
Answer: Option A Explanation: No answer description available for this question. Let us discuss.
8.
The waveshape of output voltage of half bridge inverter is A.
sinusoidal
B.
square
C.
triangular
D.
either (a) or (b)
Answer & Explanation
Answer: Option B Explanation:
No answer description available for this question. Let us discuss.
9.
A 3 phase induction motor is controlled by voltage and frequency control such that the ratio V/f is constant. At 50 Hz, the speed is 1440 rpm. At 25 Hz, the speed is likely to be A.
about 1350 rpm
B.
about 720 rpm
C.
about 360 rpm
D.
either (b) or (c)
Answer & Explanation
Answer: Option B Explanation: In this case synchronous speed at 50 Hz is 1500 rpm. Therefore synchronous speed at 25 Hz is 750 rpm. Since slip is about 3 - 5%, speed is about 720 rpm.
10. The gate current required to turn on an SCR is A.
few amperes
B.
a few mA
C.
almost equal to anode current
D.
about 50% of anode current
Answer & Explanation
Answer: Option B Explanation: Only a very small gate current is required to trigger SCR into conduction.
11. A single phase half wave converter is used to charge a battery of 50 V. The thyristor is continuously fired by dc signal. Input voltage is v = Vm sin ω t. The thyristor will conduct A.
from 0 to 180°
B.
when v > 50 V
C.
from 0 to 90°
D.
When v < 50 V
Answer & Explanation
Answer: Option B Explanation: Thyristor can conduct only when it is forward biased. This occurs only when v > 50 V.
12. The symbol in figure is for
A.
DIAC
B.
P-N junction diode
C.
Schottky diode
D.
None of the above
Answer & Explanation
Answer: Option C Explanation: No answer description available for this question. Let us discuss.
13. In a 3 phase semiconverter, the firing frequency is A.
equal to line frequency
B.
3 times the line frequency
C.
6 times the line frequency
D.
12 times the line frequency
Answer & Explanation
Answer: Option B Explanation: Since the circuit has 3 thyristors, firing frequency is three times the line frequency.
14. Present day HVDC converters are all A.
3 pulse converters
B.
6 pulse converters
C.
12 pulse converters
D.
either 6 pulse or 12 converters
Answer & Explanation
Answer: Option C Explanation: To reduce ripple 12 pulse converters are used.
15. A single phase full converter gives maximum and minimum output voltage a t firing angles of A.
0 and 180° respectively
B.
180° and 0° respectively
C.
0 and 90° respectively
D.
90° and 0° respectively
Answer & Explanation
Answer: Option A Explanation: Output voltage is maximum if
= 0 and is minimum if
= 180°.
16. A thyristor has its gate current zero. To start conduction the forward voltage must exceed A.
about 0.5 V
B.
forward breakover voltage
C.
on state voltage
D.
peak working off state forward voltage
Answer & Explanation
Answer: Option B Explanation: With zero gate current, conduction can be started only if forward voltage exceeds breakover voltage.
17. Assertion (A): Snubber circuit has an RC series circuit in parallel with SCR Reason (R): Voltage across a capacitor can not change instantaneously. A.
Both A and R are correct and R is correct explanation of A
B.
Both A and R correct but R is not correct explanation of A
C.
A is correct but R is wrong
D.
A is wrong but R is correct
Answer & Explanation
Answer: Option A Explanation:
Since voltage across capacitor cannot change instantaneously it protects SCR against high
18. In dielectric heating, the thyristor circuits consist of A.
rectifier-chopper combination
B.
controlled rectifier
C.
ac regulator
D.
rectifier-inverter combination
Answer & Explanation
Answer: Option D Explanation: Input ac is rectified and then converted to high frequency ac.
19. On load tap changer using thyristors can have a maximum of two taps. A.
True
Answer & Explanation
Answer: Option B Explanation: Any number of taps are possible.
B.
False
.
20. Assertion (A): A cycloconverter may be line commutated or forced commutated Reason (R): A step up cycloconverter requires forced commutation. A.
Both A and R are correct and R is correct explanation of A
B.
Both A and R correct but R is not correct explanation of A
C.
A is correct but R is wrong
D.
A is wrong but R is correct
Answer & Explanation
Answer: Option A
21. In the below figure the rms voltage across each half of secondary is 200 V. The peak inverse voltage across each of thyristor is
A.
200 V
B.
2(200) V
C.
2(400) V
D.
800 V
Answer & Explanation
Answer: Option C Explanation: PIV = 2(2 x 200).
22. Assertion (A): A GTO circuit is more compact than SCR circuit Reason (R): A GTO can be turned off by negative gate pulse. A.
Both A and R are correct and R is correct explanation of A
B.
Both A and R correct but R is not correct explanation of A
C.
A is correct but R is wrong
D.
A is wrong but R is correct
Answer & Explanation
Answer: Option A Explanation: Since commutation circuit is not required, GTO circuit is compact.
23. In a single phase full wave ac regulator, the firing angles in the two half cycles A.
are always equal
B.
are sometimes equal
C.
are never equal
D.
may be equal or unequal
Answer & Explanation
Answer: Option A Explanation: No answer description available for this question. Let us discuss.
24. In a single phase full bridge converter (B - 2 connection), the input voltage is v = Vm sin ω t. The output dc voltage for resistive load
A.
B.
C.
D.
Answer & Explanation
Answer: Option C Explanation: SCRs conduct from to
in each half cycle.
25. As regards single phase dual converter A.
the change in operation from one quadrant to the other is faster in non-circulating current mode
B.
the change in operation from one quadrant to the other is faster in circulating current mode
C.
the time response to change in operation from one quadrant to other is the same in both the modes
D.
the time response to change in operation from one quadrant to the other may be faster in any of the two modes
Answer & Explanation
Answer: Option B
26. To protect an SCR against high di/dt we connect A.
a capacitance in series with SCR
B.
a capacitance in parallel with SCR
C.
an inductance in parallel with SCR
D.
an inductance in series with SCR
Answer & Explanation
Answer: Option D Explanation: current through inductance cannot change instantaneously.
27. In a parallel inverter A.
the commutating capacitor C is across the primary of transformer and the load is across the secondary
B.
the commutating capacitor and load are connected across the secondary of transformer
C.
the commutating capacitor C and load are connected in series and the combination is across the secondary of transformer
D.
none of the above
Answer & Explanation
Answer: Option A
28. A silicon controlled switch has : A.
2 leads
B.
3 leads
C.
4 leads
D.
5 leads
Answer & Explanation
Answer: Option C Explanation: Anode, cathode, anode gate and cathode gate.
29. A dc chopper V = 500 V feeds an electric fraction dc series motor. The chopping frequency is 400 Hz. The series inductance required to limit the maximum current swing to 10 A is A.
20 mH
B.
about 31 mH
C.
about 51 mH
D.
about 92 mH
Answer & Explanation
Answer: Option B Explanation:
.
30. A single phase fully controlled bridge rectifier is fed by a source having inductance. The output voltage during overlap period is A.
Vm
B.
0.5 Vm
C.
zero
D.
0.5 Vm - voltage drop across inductance
Answer & Explanation
Answer: Option C Explanation: During the period of overlap all the four thyristors conduct and load is effectively short-circuited. Output voltage is zero.
31. In a semi converter feeding a resistive load, the firing angle is A.
B.
( -
C.
( +
D.
( - 0.5
. Each thyristor conducts for
radians
) radians ) radians ) radians
Answer & Explanation
Answer: Option B Explanation: The conduction is from
to
.
32. Between which points does UJT exhibit negative resistance characteristics A.
between peak and valley points
B.
before peak point
C.
after peak point
D.
after valley point
Answer & Explanation
Answer: Option A
33. A thyristor has maximum allowable junction temperature of 110° C and the ambient temperature is 40°C, If the maximum internal power dissipation is 50 W, the thermal resistance is A.
2° C/W
B.
1.4° C/W
C.
0.7°C/W
D.
2.2° C/W
Answer & Explanation
Answer: Option B Explanation:
.
34. A single phase electric heater uses phase co ntrolled thyristors. The firing angle is 90°. The degree of heating is A.
25% of maximum
B.
50% of maximum
C.
75% of maximum
D.
none of the above
Answer & Explanation
Answer: Option B Explanation: Conduction occurs only during 50% of wave.
35. Which of these commutation methods is used in AC -DC converters? A.
Class A
B.
Class C
C.
Class D
D.
Class F
Answer & Explanation
Answer: Option D Explanation: Class F is natural commutation AC = DC converter use natural (or line) commutation.
36. When thyristors are connected in series and parallel, it may be necessary to have : A.
current derating
B.
voltage derating
C.
both current and voltage derating
D.
none of the above
Answer & Explanation
Answer: Option C Explanation: Thyristors in series do not share the voltage equally and require voltage derating. Thyristors in parallel need current derating.
37. A single phase half wave regulator uses A.
2 diodes and 2 thyristors
B.
2 thyristors
C.
1 diode and 1 thyristor
D.
2 diodes
Answer & Explanation
Answer: Option C Explanation: Regulation is possible only in positive half cycle.
38. A Shockley diode can be used A.
in place of SCR
B.
as a trigger switch for SCR
C.
in place of UJT
D.
both in place of SCR and as a trigger switch for SCR
Answer & Explanation
Answer: Option B Explanation: It is a four layered PNPN device without a gate terminal. Its characteristics is similar to that of SCR with zero gate current. It is acting as a trigger switch for Thyristor.
39. An SCR has a turn on time of 4 μs. The gate pulse should have a duration of about A.
2 μs
B.
5 μs
C.
20 μs
D.
50 μs
Answer & Explanation
Answer: Option B Explanation: Duration of gate pulse should be slightly more than turn time of SCR.
40. In a multiphase chopper, the different choppers operate in A.
series and all simultaneously
B.
parallel and one at a time
C.
parallel and simultaneously
D.
either (b) or (c)
Answer & Explanation
Answer: Option B Explanation: Only one chopper operates at one time.
41. In a 3 phase semiconverter, the dc output voltage for a certain firing angle is 50% of the maximum possible value. The firing a ngle is A.
30°
B.
60°
C.
90°
D.
120°
Answer & Explanation
Answer: Option C Explanation: Vdc ∝ (1 + cos
). Maximum possible output voltage occurs when = 0.
42. A high frequency series inverter consists of
A.
a number of series inverters in series
B.
a number of series inverters in parallel
C.
a number of parallel inverters in parallel
D.
a number of parallel inverters in series
Answer & Explanation
Answer: Option B Explanation: A high frequency series inverter has many series inverters in parallel. Only one inverter operates at one time and they operate in fixed sequence.
43. In an SCR, the anode current consists of A.
only holes
B.
only electrons
C.
both electrons and holes
D.
either electrons or holes
Answer & Explanation
Answer: Option C Explanation: Both electrons and holes contribute to current.
44. In the below figure, the average value of load current is 15 A. The load is highly indu ctive. The peak value of thyristor current is
A.
15 A
B.
152 A
C.
D.
30 A
Answer & Explanation
Answer: Option A Explanation: Because of highly inductive load, output current is nearly constant.
45. A 3 phase semiconverter when supplying a certain load has a rectification efficiency of 0.6 and transformer utilisation factor of 0.35. When a 3 phase full converter is supplying the same load A.
rectification efficiency will be more than 0.6 and TUF will be more than 0.35
B.
rectification efficiency will be more than 0.6 but TUF will be less than 0.35
C.
rectification efficiency will be less than 0.6 and TUF will be less than 0.35
D.
rectification efficiency will be less than 0.6 but TUF will be more than 0.35
Answer & Explanation
Answer: Option A
46. Assertion (A): A high dv/dt may cause triggering of a thyristor. Reason (R): A high dv/dt results in charging current in the thyristor. A.
Both A and R are correct and R is correct explanation of A
B.
Both A and R correct but R is not correct explanation of A
C.
A is correct but R is wrong
D.
A is wrong but R is correct
Answer & Explanation
Answer: Option A Explanation: The charging current may cause triggering.
47. Assertion (A): When an SCR is conducting, the voltage drop across is about 1 V. Reason (R): When an SCR is conducting, the junction temperature is about 200°C. A.
Both A and R are correct and R is correct explanation of A
B.
Both A and R correct but R is not correct explanation of A
C.
A is correct but R is wrong
D.
A is wrong but R is correct
Answer & Explanation
Answer: Option C Explanation: Junction temperature is about 120°C. Hence R is wrong.
48. When the core of a saturable reactor is saturated, the inductance is high.
A.
True
B.
False
Answer & Explanation
Answer: Option B Explanation: Since core is saturated, change of flux is small and inductance is low .
49. In controlled heating circuits, thyristors act as A.
rectifier
B.
inverter
C.
ac regulator
D.
chopper
Answer & Explanation
Answer: Option C Explanation: The output voltage is controlled ac.
50. In a fully controlled bridge converter feeding a resistive load the waveshapes of output voltage and output current A.
are always similar
B.
can never be similar
C.
may be similar or dissimilar
D.
are similar only if
Answer & Explanation
=0
Answer: Option A Explanation: For resistive load output current is always proportional to output voltage.
SECTION 4 1.
A chopper cannot be line commutated. A.
2.
B.
False
DC circuits only AC circuits only
C.
both AC and DC circuits
D.
none of the above
In a thyristor the gate lead is connected to the n layer near the p layer to which anode is connected. A.
4.
B.
Natural commutation can be used in : A.
3.
True
True
B.
False
A single phase half wave rectifier circuit has a free wheeling diode. The free wheeling diode will conduct only if A.
load is purely resistive
B.
load is purely inductive
C.
load is combination of R and L
D.
load is purely inductive or combination of R and L
5.
McMurray Bedford full bridge inverter uses A.
natural commutation
B.
auxiliary commutation
C. D.
6.
any of the above
Which converter is used in HVDC transmission? A.
3 pulse
B.
6 pulse
C. D.
7.
complementary commutation
12 pulse
24 pulse
Assertion (A): The turn on time of SCR is about 1 to 4 ms. Reason (R): The turn off time of SCR is about 10 to 300 ms. A. B.
8.
Both A and R are correct and R is correct explanation of A Both A and R correct but R is not correct explanation of A
C.
A is correct but R is wrong
D.
A is wrong but R is correct
An n pulse rectifier is fed by a source having an inductance L. If load current is I 0, the voltage drop due to overlap is A.
B.
C.
D.
10. A single phase cycloconverter feeds a resistance heating load. The heating power is provided by A.
only the fundamental component of output wave
B.
only higher harmonics in the output wave
C. D.
both fundamental and higher harmonics in the output wave
either (a) or (b)
11. A 3 phase fully controlled bridge converter is fed by a 3 phase system having phase voltage v = Vm sin ω t. The firing angle is . The dc output voltage is A.
B.
C.
D.
12. Which of these commutation methods uses an external pulse for commutation? A.
Class A
B.
Class C
C. D.
Class E
Class F
13. In a single phase full wave regulator feeding an R-L load, the power factor angle of load is θ. The range of control of firing angle is 0≤
≤
θ≤
≤
C.
0≤
≤ ( - θ)
D.
θ≤
≤ ( + θ)
A.
B.
14. A separately excited dc motor is run from a dc source through a chopper. At 600 rpm and rated torque, the duty cycle of chopper is 0.8. At 600 rpm and half the rated torque, the duty cycle is likely to be A. C.
0.9 0.75
B.
0.8
D.
0.6
15. A relaxation oscillator uses A. B.
tunnel diode UJT
C.
both tunnel diode and UJT
D.
PIN diode
16. In a 3 phase bridge inverter with 180° mode of operation the numbers of thyristors conducting at one time are A. C.
1 3
B.
2
D.
4
17. A single phase full converter can operate in A.
4 quadrants
B.
3 quadrants
C. D.
2 quadrants
1 quadrant
18. In a 3 phase semiconverter, the freewheeling diode comes into operation only if firing angle A.
is zero
B.
is 60°
C. D.
is more than 60°
is more than 90°
19. In the circuit of figure, the load current when thyristor is off
A.
is constant
B.
increases exponentially
C. D.
decreases exponentially
zero
20. A full bridge inverter is fed by a battery of 30 V. The rms value of output voltage is A.
15 V
B.
152 V
C. D.
30 V
302 V
21. In brushless excitation system of modern alternators, the main exciter is A. B.
dc generator ac generator
C.
either dc or ac generator
D.
induction generator
22. The two-transistor model of a thyristor consists of two transistors : A.
one n-p-n and other p-n-p
B.
both p-n-p
C.
both n-p-n
D.
one n-p-n and other UJT
23. In a thyristor, the forward breakover voltage A.
is constant
B.
may be constant or may depend on gate current
C. D.
decreases as gate current is increased
increases as gate current is increased
24. The gate lead, in a thyristor is welded to : A. B.
p layer to which anode is connected n layer nearest to the anode terminal
C.
p layer nearest to the cathode terminal
D.
outside n layer
25. In a 3 phase full converter, the firing angle is less than 60°. The instantaneous output voltage A.
will have positive part only
B.
will have both positive and negative parts
C.
may have both positive and negative parts
D.
will have negative part if load is inductive
26. In a stud mounted SCR, the stud is A.
in thermal contact with heat sink
B.
insulated from heat sink
C. D.
in electrical and thermal contact with heat sink
in either electrical or thermal contact with heat sink
27. In pulse width modulation of chopper A.
T is kept constant and Ton is varied
B.
Ton is kept constant T is varied
C.
both T and Ton are varied
D.
either T or Ton is varied
28. In a full bridge inverter, the waveshape of load current depends on A.
relative values of load parameters R, L and C
B.
relative values of load parameters R and L
C.
relative values of load parameters L and C
D.
gating frequency
29. The characteristics of which device has peak point and valley point A. B.
SCR PUT
C.
SCS
D.
Shockley diode
30. In a step down chopper using time ratio control, the input voltage is V and duty cycle is . The load is purely resistive having resistance R. The average and rms load currents are A.
B.
C.
D.
31. In a thyristors are i = kva. The value of a is A.
about 0.5
B.
about 2
C. D.
about 5
about 20
32. In an ordinary thyristor A.
turn on and turn off times are equal
B.
turn on time is more than turn off time
C. D.
turn off time is more than turn on time
turn on time may be equal, less than or more than turn off time
33. Thyristors connected in series need : A.
static equalizing circuit
B.
dynamic equalizing circuit
C. D.
both static and dynamic equalizing circuits
none of the above
34. The symbol in figure is for
A.
SUS
B.
SBS
C.
SCS
D.
None of the above
35. If 4 quadrant operation is required we need A.
dual converter
B.
full converter
C.
semi converter
D.
none of the above
36. A single phase half wave converter is feeding a resistive load. Output dc power is P dcand total power delivered to load is P ac then A.
P dc = P ac for all values of firing angles
B.
P dc = P ac if firing angle is less than 180°
C.
P dc < P ac for all values of firing angles
D.
P dc < P ac if firing angle is less than 180°
37. Which of the following devices exhibit negative resistance characteristics? A.
UJT
B.
PUT
C. D.
Both UJT and PUT
UJT, PUT and SBS
38. In integral cycle control of ac regulators, the period of integral cycle control, i.e., (m +n) T should be A. B.
less than mechanical or thermal time constant of the system
more than the mechanical or thermal time constant of the system
C.
more or less than the mechanical or thermal time constant
D.
very high
39. All other parameters remaining the same, a 3 phase bridge inverter with 180° mode of operation A. B.
has the same peak thyristor current as in 120° mode of operation has higher peak thyristor current than in 120° mode of operation
C.
has lower peak thyristor current than in 120° mode of operation
D.
any of the above
40. As compared to BJT, MOSFET has A.
lower switching losses and higher conduction losses
B.
higher switching losses and higher conduction losses
C.
higher switching losses and lower conduction losses
D.
lower switching losses and lower conduction losses
41. If a step down chopper is fed by a dc voltage V with Ton and Toff are on and off periods then A.
is average output voltage
B.
rms output voltage
C.
average or rms output voltage depending on load
D.
neither of the above
42. A voltage VBB is applied across the terminals of a UJT. The emitter voltage at peak point is A.
hVBB
B.
(h + 1) VBB
C. D.
hVBB + VD
VD
43. A single phase bridge type cycloconverter uses eight thyristors. A.
True
B.
False
44. In the circuit of figure, the waveshape of load current when thyristor conducting is
A.
constant
B.
increasing exponentially
C.
decreasing exponentially
D.
either (b) or (c)
45. In the basic series inverter circuit, the source supplie s load A.
only in positive half cycle
B.
both in positive and negative half cycles
C.
only in part of positive half cycle
D.
in positive half cycle and part of negative half cycle
46. A current source inverter has simple commutation circuit. A.
True
B.
False
47. In load commutated chopper, the commutating element A.
is capacitor
B.
is capacitor and inductor
C.
is capacitor, inductor and auxiliary thyristor
D.
any of the above depending on load
48. In a half bridge inverter each thyristor is gated at a frequency f. The output frequency is A. B.
2f f
C.
0.5 f
D.
0.25 f
49. Assertion (A): A transistor has lower voltage drop during conduction as compared to thyristor Reason (R): Transistors are manufactured in very high voltage and current ratings. A.
Both A and R are correct and R is correct explanation of A
B.
Both A and R correct but R is not correct explanation of A
C. D.
A is correct but R is wrong
A is wrong but R is correct
50. Assertion (A): Typical value of thermal resistance from source to sink of an SCR is about 0.3°
C/W Reason (R): Heat sinks of thyristors are generally made of aluminium. A. B.
Both A and R are correct and R is correct explanation of A Both A and R correct but R is not correct explanation of A
C.
A is correct but R is wrong
D.
A is wrong but R is correct
SECTION 5 1.
Multiphase choppers are used in A. B.
2.
low power circuits high power circuit
C.
both low and high power circuits
D.
medium power circuits
In a single phase converter using a centre tapped transformer and with a highly inductive load, the firing angles is . Each thyristor conducts for a period A.
B.
C.
-
D.
3.
In a parallel inverter A. B.
each thyristor is turned on twice during each cycle
each thyristor is turned on once during each cycle
4.
C.
each thyristor is turned on either once or twice in each load cycle
D.
each thyristor may be turned on upto four times in each load cycle
A step up chopper can give an output voltage A.
5.
B.
lower than input voltage
C.
both higher and lower input voltage
D.
all of the above
The specified maximum junction temperature of thyristors is about A. C.
6.
50°C 120°C
B.
80°C
D.
200°C
A bridge type single phase cycloconverter changes the frequency f to f/3. Then one half wave of output contains A. B.
7.
higher than input voltage
3 full waves of input 3 half waves of input
C.
6 full waves of input
D.
6 half waves of input
A half bridge inverter A.
needs a 3 wire dc supply
B.
may use a two wire or 3 wire dc supply
C.
has higher output voltage if fed from 3 wire dc supply
D.
8.
A 3 phase series inverter has A. B.
9.
has higher efficiency if fed from 3 wire dc supply
12 thyristors 6 thyristors
C.
3 thyristors
D.
3 or 6 thyristors
In figure, thyristor Th is in off state. When thyristor is turned on, the peak thyristor current can be
A.
5A
B.
10 A
C. D.
15 A
20 A
10. HVDC systems are preferable for only long distances. A.
True
B.
False
11. Surge current rating of an SCR is A. B.
the maximum repetitive surge current in which the SCR can withstand the maximum non-repetitive surge current in which the SCR can withstand
C.
the maximum repetitive or non repetitive surge current in which the SCR can withstand
D.
the maximum surge current in which the SCR can withstand if both surge current and steady current exist together
12. Which of the following devices has metal-silicon junction? A. B.
General purpose power diode Schottky diode
C.
SCR
D.
MOSFET
13. In a single phase full wave converter (M-2 connection) feeding on R-L load the input voltage is v = Vm sin ω t. The expression for rms output voltage is
A.
B.
C.
D.
14. A dc chopper is feeding an RLE load. The condition for maximum ripple is
=1
B.
= 0.8
C.
= 0.5
D.
< 0.5
A.
15. In a series inverter A. B.
the commutating elements L and C are in parallel the commutating elements L and C are in series with load
C.
the commutating L and C may be in series or parallel with the load
D.
none of the above
16. When a power diode is conducting, the voltage drop across it is about A.
IV
B.
5V
C.
20 V
D.
50 V
17. In thermal ohm's law, temperature and power loss are analogous to A. B.
V and I respectively of ohm's law I and V respectively of ohm's law
C.
R and V respectively of ohm's law
D.
I and R respectively of ohm's law
18. A string of series connected thyristors needs equalising circuit to A.
ensure proper firing
B.
ensure proper current division
C. D.
ensure proper voltage division
none of the above
19. A surge voltage A.
has very high magnitude and very long duration
B.
has very high magnitude and very small duration
C. D.
has very high magnitude and very small duration and can be positive or negative
can be only positive
20. The minimum gate current which can turn on SCR is called : A.
trigger current
B.
holding current
C.
junction current
D.
breakover current
21. The total number of leads in SUS, SBS and SCS respectively are A.
3, 3, 4,
B.
3, 3, 3
C.
2, 3, 3
D.
3, 3, 5
22. Which of these devices can conduct in both directions? A. C.
SCR SBS
B.
SUS
D.
UJT
23. Assertion (A): A half wave controlled rectifier has poorer efficiency and higher ripple factor than a full wave controlled rectifier. Reason (R): The use of a freewheeling diode in half wave controlled rectifier circuit improves the waveform of load current and circuit power factor. A. B.
Both A and R are correct and R is correct explanation of A Both A and R correct but R is not correct explanation of A
C.
A is correct but R is wrong
D.
A is wrong but R is correct
24. A 3 phase dual converter consists of A.
two 3 phase semiconverters connected in parallel
B.
two 3 phase semiconverters connected in antiparallel
C. D.
two 3 phase full converters connected in antiparallel
two 3 phase full converters connected in parallel
25. Which inverter circuit uses coupled inductors? A.
Modified McMurray full bridge inverter
B.
McMurray Bedford half bridge inverter
C.
Modified McMurray Bedford half bridge inverter
D.
Both (b) and (c)
26. When an SCR is in forward blocking mode, the applied voltage A.
appears across only one junction
B.
is equally shared by two junctions
C.
is equally shared by three junctions
D.
may appear across one or more junctions
27. In sinusoidal PWM for inverters the width of different pulses are not the same. A.
True
B.
False
28. A class D chopper A.
can operate in first quadrant only
B.
can operate in second quadrant only
C. D.
can operate in either first or fourth quadrant
can operate in all the quadrants
29. The symbol in figure is for
A.
UJT
B.
C.
SCS
D.
30. The efficiency of a chopper circuit is about A.
80% or more
PUT
SBS
B.
around 50%
C.
around 20%
D.
around 5%
31. If the frequency is less than the damped frequency in a series - inverter A. B.
the distortion of output voltage waveform is low the distortion of output voltage waveform is high
C.
the distortion of output voltage may be high or low depending on the load impedance
D.
none of the above
32. The output frequency of a cycloconverter can be changed by changing the firing angle. A.
True
B.
False
33. Mc Murray full bridge inverter uses A.
4 thyristors and 2 diodes
B.
4 thyristors and 4 diodes
C. D.
8 thyristors and 4 diodes
8 thyristors and 8 diodes
34. Assertion (A): A DIAC has four layers but only two terminals. Reason (R): A DIAC can conduct in both directions. A. B. C.
Both A and R are correct and R is correct explanation of A Both A and R correct but R is not correct explanation of A
A is correct but R is wrong
D.
A is wrong but R is correct
35. In a 3 phase full converter, the insta ntaneous output voltage can have negative part if A.
firing angle is less than 60°
B.
firing angle is less than 90°
C. D.
firing angle is more than 60°
firing angle is more than 90°
36. In a single phase full wave ac regulator using two thyristors only, the gate circuits of the two thyristors A.
must be isolated
B.
may or may not be isolated
C.
should be preferably, isolated
D.
should not be isolated
37. The circuit in figure is for
A.
resonant commutation
B.
line commutation
C. D.
auxiliary commutation complementary commutation
38. A fully controlled bridge converter is feeding an RLE load. For some circuit conditions firing angle is 47°. If connections of battery are reversed, the firing angle is likely to be A.
47°
B.
less than 47°
C. D.
more than 47°
either more or less than 47°
39. In a single phase half wave converter feeding resistive load and full wave converter (M - 2 connection) feeding a resistive load the circuit conditions are similar. The firing angle is . Then the waveshapes of voltages across thyristors A.
are similar
B.
are not similar
C.
may be similar or dissimilar
D.
are similar only for some values of
40. In a 3 phase full converter feeding a highly inductive load, the average load current is 150 A. The peak current through thyristor is A.
150 A
B.
75 A
C.
50 A
D.
300 A
41. A 3 phase bridge inverter is fed by 400 V battery. The load is star connected and has a resistance of 10 Ω per phase. The peak value of thyristor current is A.
B.
C.
D.
42. The main utility of SCR is due to A. B.
gate triggering
high temperature triggering
C.
D.
triggering high breakdown voltage
43. In a single phase full wave ac regulator the firing angles during both half cycles are equal. A.
True
B.
False
44. In a 3 phase full wave regulator feeding a star connected resistance load, firing angle is 75°. Then the number of thyristors conducting at one time is A.
1
B.
C.
3
D.
2
4
45. The symbol in figure is for
A.
SUS
B.
SBS
C. D.
Shockley diode
UJT
46. The turn on characteristics of a power transistor can be assumed to be linear, as shown in the below figure . During turn on time T, voltage reduces linearly from V to 0 and current rises linearly from 0 to I. The energy loss during time T is
A.
B.
C.
D.
VIT
47. The circuit in figure is for
A. B.
resonant commutation self commutation
C.
load commutation
D.
auxiliary commutation
48. A single phase half bridge inverter required to feed R-L loads needs A.
2 thyristors
B.
4 thyristors
C. D.
2 thyristors and two diodes
4 thyristors and 4 diodes
49. Typical values of A. B.
1 to 10 25 to 500
C.
0.1 to 1
D.
1000 to 2000
for an SCR are in the range of
50. When a dc chopper feeds an RLE load, the load current, during steady state operation A. B.
remains constant varies between maximum and minimum values
C.
may remain constant or vary
D.
is constant if R is constant
SECTION 6 1.
In a single phase full wave ac regulator feeding R-L load, the gate pulses A.
are always of short duration
B.
may be of short duration
C.
D.
2.
3.
should be rectangular with width ( - )
cannot be rectangular with width ( - )
In a semi converter feeding a resistive load, A.
the waveshapes of input voltage and input current are always similar
B.
the waveshapes of input voltage and input current can never be similar
C.
the waveshapes of input voltage and input current are similar only if firing angle is zero
D.
the waveshapes of input voltage and input current are similar only if firing angle is high
In a single phase semi-converter the number of thyristors is A.
16
B.
C.
4
D.
8 2
4.
A dc chopper is feeding an RLE load. The current can become discontinuous if A.
B.
C.
E is high
D.
5.
none of the above
A.
Class A
B.
Class B
C.
Class C Class D
A single phase half wave rectifier is feeding a resistive load. As firing angle is increased A. B.
7.
is low
Which of these commutation methods uses two leads?
D.
6.
is high
dc output voltage and rms output voltage decrease in the same proportion dc output voltage and rms output voltage decrease
C.
dc output voltage and rms output voltage increase
D.
dc output voltage and rms output voltage increase in the same proportion
A cycloconverter for single phase operation has A.
one positive group and one negative group converters
B.
two positive group and two negative group converters
C.
three positive group and three negative group converters
D.
any of the above
8.
A chopper is a high efficiency circuit. A.
9.
True
B.
False
The difference between resonant commutation and self commutation is A.
both these methods are based on entirely different principles
B.
in resonant commutation, the commutating elements carry load current but in self commutation, the commutating elements do not carry load current
C.
resonant commutation uses L and C but self commutation uses only capacitor
D.
none of the above
10. If 0.5 T is the time period of oscillations and Toff is the time between turn off of one thyristor and turn on of second thyristor, the frequency of output f of a series inverter is A.
B.
C.
D.
11. In a series inverter A.
the commutating elements carry full load curre nt
B.
the commutating elements carry upto 50% of load current
C.
the commutating elements do not carry load current
D.
any of the above
12. A NAND gate has 3 inputs and one output. The number of thyristors required for this gate are A.
1
B.
2
C. D.
3
3 or 4
13. Which of these devices is used in relaxation oscillator? A. C.
UJT
DIAC
B.
SCR
D.
TRIAC
14. In auxiliary commutated chopper fed by voltage V and having L and C commutating elements, the peak value of capacitor current is A.
B.
C.
D.
15. In a single phase semiconverter feeding a highly inductive load, the load current is A. B.
sinusoidal nearly constant
C.
absolutely constant
D.
rectangular
16. Two thyristors of the same rating and s pecifications A.
will have equal on state voltage drops
B.
will have equal forward blocking voltage for the same current
C.
will have different on state voltage drops may have equal or different on state voltage drops and equal or different forward
D.
blocking voltage's for the same current
17. A 3 phase induction motor is controlled by voltage and frequency control such that the ratio V/f is constant. At 50 Hz, the starting torque is 100 N-m. At 25 Hz, the starting torque A. B.
will be less than 100 N-m will be more than 100 N-m
C.
may be more or less than 100 N-m depending on motor parameters
D.
very small
18. In a three phase to single phase full wave bridge cycloconverter, the number of thyristors is A. C.
36 12
B.
18
D.
8
19. A single phase half wave converter is feeding-a resistor load. The input voltage is v = Vm sin ω t. If firing angle is zero, the output dc voltage Vdc and output rms voltage Vrmsrespectively A.
B.
C.
D.
20. In a cycloconverter it is possible to vary A.
only output frequency
B.
only output voltage
C.
both output voltage and frequency but not simultaneously
D.
both output voltage and output frequency simultaneously
21. A single phase full bridge inverter for R-L loads needs A. B.
4 thyristors 4 thyristors and 4 diodes
C.
4 thyristors and 2 diodes
D.
8 thyristors
22. In the basic series inverter circuit, the source current has A. B.
low harmonic content high harmonic content
C.
low or high harmonic content depending on firing angle
D.
zero harmonic content
23. Thyristors connected in series need static and dynamic stabilizing circuits. A.
True
B.
False
24. In a controlled rectifier a freewheeling diode is necessary if the load is A.
inductive
B.
resistive
C.
capacitive
D.
any of the above
25. For an illumination control circuit it is advisable to use a matched TRIAC -DIAC pair. A.
True
B.
False
26. A single phase converter is fed by a source having an inductance L. If load current is I 0, the decrease in output voltage due to overlap is A.
B.
C.
D.
27. Assertion (A): An ac regulator uses natural commutation Reason (R): An ac regulator can be used for static on load tap changing of transformer. A. B.
Both A and R are correct and R is correct explanation of A Both A and R correct but R is not correct explanation of A
C.
A is correct but R is wrong
D.
A is wrong but R is correct
28. When a UJT is used as a relaxation oscillator the current should b e A.
less than Ip
B.
more than Iv
C. D.
more than Ip but less than Iv
negative
29. In auxiliary commutated chopper, the charging and discharging time of capacitor A. B.
are constant depend on load current
C.
may be constant or may depend on load current
D.
none of the above
30. Which chopper circuit uses saturable reactor? A.
Auxiliary commutated
B.
Jones chopper
C. D.
Morgan chopper
Load commutated
31. A thyristor circuit is feeding an R-L load. The turn on time can be reduced by A.
decreasing R
B.
decreasing L
C.
increasing L
D.
decreasing R and L together
32. A single phase semiconverter is supplying RLE load and has a freewheeling diode across the load. The firing angle is . The freewheeling diode conducts for A.
° in
B.
( -
C.
° in
D.
( +
each half cycle
)° in each cycle one full cycle
)° in each half cycle
33. In a full bridge inverter fed by a battery of voltage V, the rms value of fundamental component of output voltage is A.
B.
C.
D.
34. In a single phase full wave regulator feeding a resistive load, the average current over one full cycle A.
is always zero
B.
cannot be zero
C.
may be sometimes zero
D.
is always positive
35. Two single phase semi-converters are connected in series to form a series converter. The input is v = Vm sin ω t and 1, 2 are the firing angles. If 1 = 0, the dc output voltage is A.
(1 + cos
2)
B.
C.
D.
(2 + cos
2)
(3 + cos
2)
(4 + cos
2)
36. The total number of leads in MOS controlled thyristor is A. B.
2 3
C.
4
D.
2 or 3
37. Assertion (A): Chopper always requires forced commutation Reason (R): Morgan chopper uses a saturable reactor. A. B.
Both A and R are correct and R is correct explanation of A Both A and R correct but R is not correct explanation of A
C.
A is correct but R is wrong
D.
A is wrong but R is correct
38. In a power diode the reverse recovery time is the time from the instant the forward current is zero to the instant when reverse recovery current has decayed to A.
50% of peak reverse current
B.
25% of peak reverse current
C.
10% of peak reverse current
D.
zero value
39. Assertion (A): McMurray Bredford half bridge inverter uses complementary commutation Reason (R): Three phase inverter can have two modes of operation. A. B.
Both A and R are correct and R is correct explanation of A Both A and R correct but R is not correct explanation of A
C.
A is correct but R is wrong
D.
A is wrong but R is correct
40. A capacitor C farads is charged to a voltage V. It discharges through a thyristor, a diode a nd an inductance L. The peak current in the circuit is A.
VCL
B.
C.
D.
41. Which of these devices have four layers? A. B.
UJT SCR
C.
TRIAC
D.
SCR and TRIAC
42. When UJT is used for thyristor gate signal, the output of UJT is A.
sine wave
B. C. D.
square wave saw tooth wave
either (a) or (c)
43. The stud in a stud mounted SCR acts as A.
anode
B.
cathode
C.
gate
D.
anode or cathode
44. In a single phase full wave regulator feeding R-L load, short gate pulse are not suitable. A.
True
B.
False
45. Half bridge and full bridge inverters of one type use different commutation methods. A.
True
B.
False
46. In rectifier circuits, the rms value of ac component of output voltage is equal to A.
Vrms
B.
0.5 Vrms
C.
(Vrms - Vdc)0.5
D.
(Vrms2 - Vdc2)0.5
47. A UJT exhibits negative resistance characteristics when A.
Current is less than valley current
B. C. D.
Current is less than peak current Current is more than peak current but less than valley current
None of the above
48. A dc chopper feeds an RLE load and has a duty cycle of 1. The load current will be such tha t A.
B.
Imax > Imin
C.
Imin > 0
D.
Imin < 0
49. In rectifier circuits, ripple factor RF and form factor FF are related as A.
RF = (FF2 - 1)0.5
B.
RF = FF
C.
RF = FF - 1
D.
RF = (FF - 1)0.5
50. If we connect two silicon unilateral switches in anti- parallel we get A.
SCR
B.
C.
SCS
D.
SBS
UJT
SECTION 7 1.
Assertion (A): SUS is a PUT and avalanche diode connected between gate and cathode Reason (R): SBS has two SUS inbuilt together and connected in anti-parallel A. B.
2.
A is correct but R is wrong
D.
A is wrong but R is correct
In a single phase full wave controlled rectifier using centre tap transformer, the voltage across each half of secondary is Vm sin ω t. The peak inverse voltage is 2 Vm
B.
Vm
C.
0.5 Vm
D.
0.25 Vm
Which statement is correct? A.
BJT and MOSFET are current controlled devices
B.
BJT is voltage controlled and MOSFET is current controlled device
C.
BJT and MOSFET are voltage controlled devices
D.
4.
Both A and R correct but R is not correct explanation of A
C.
A.
3.
Both A and R are correct and R is correct explanation of A
BJT is current controlled and MOSFET is voltage controlled device
In a three phase series inverter, the firing frequency is A. B.
three times output frequency six times output frequency
5.
C.
twice the output frequency
D.
equal to output frequency
A 3 phase fully controlled converter is a A. B.
6.
7.
6 pulse converter
C.
2 pulse converter
D.
12 pulse converte
In a half bridge inverter the gate pulses of the two thyristors have a phase displacement of A.
60°
B.
C.
120°
D.
90° 180°
A fully controlled bridge converter can operate in quadrants A.
1
B.
1 and 2
C.
1 and 3
D.
8.
3 pulse converter
1 and 4
A class E chopper A.
can operate in second quadrant only
B.
can operate in first or third quadrant
C. D.
can operate in all the four quadrants
can operate in either second or third quadrant
9.
A cycloconverter uses A.
natural commutation
B.
forced commutation
C. D.
either natural or forced commutation
both natural and forced commutation together
11. The commutation method in an inverter, is A.
line commutation
B.
forced commutation
C. D.
either (a) or (b)
none of the above
12. Inverters find applications in A.
HVDC transmission
B.
UPS
C.
variable speed ac drives
D.
all of the above
13. When a thyristor is conducting, the voltage drop across it is about A.
1V
B.
10 V
C.
100 V
D.
0.1 V
14. The use of free wheeling diode in controlled rectifier improves the waveshape of load current. A.
True
B.
False
15. As the firing of a thyristor is delayed, the output voltage of a controlled rectifier decreases. A.
True
B.
False
16. Assertion (A): A GTO can be turned on and turned off by gate pulses Reason (R): A GTO can be turned off by negative gate pulse. A. B.
Both A and R are correct and R is correct explanation of A Both A and R correct but R is not correct explanation of A
C.
A is correct but R is wrong
D.
A is wrong but R is correct
17. In a half bridge inverter circuit fed by a battery of voltage V, the rms value of fundamental component of output voltage is A.
2V
B.
C.
D.
18. In a full bridge single phase inverter feeding RLC load A.
one thyristor and one diode conduct simultaneously
B.
two thyristors conduct simultaneously
C. D.
two thyristors or two diodes conduct simultaneously
four thyristors conduct simultaneously
19. It is possible to use a freewheeling diode in A.
single phase half wave controlled rectifier as well as in single phase full wave controlled rectifier (M - 2 connection)
B.
single phase half wave controlled rectifier but not in single phase full wave controlled rectifier (M - 2 connection)
C.
single phase full wave controlled rectifier (M - 2 connection) but not in single phase half wave controlled rectifier
D.
all controlled rectifier circuits
20. Assertion (A): When the anode current of an SCR starts increasing during turn on process, the anode cathode voltage drop starts decreasing. Reason (R): During turn on the reverse biased junction in SCR breaksdown. A.
Both A and R are correct and R is correct explanation of A
B.
Both A and R correct but R is not correct explanation of A
C.
A is correct but R is wrong
D.
A is wrong but R is correct
21. In a certain inverter circuit each thyristor carries 26.67 A for 60°, 13.33 A for 120° and zero A for 180° of the cycle. The rms current of thyristor is A. B.
20 A 13.33 A
C.
6.67 A
D.
3.33 A
22. The factor η for a UJT is about A. C.
0.1 0.6
B.
0.2
D.
0.9
23. The symbol in figure is for
A.
UJT
B.
PUT
C.
Shockley diode
D.
TRIAC
24. In a single phase full converter fed by a source having inductance, the number of thyristors conducting during overlap is A.
1
B.
C.
3
D.
2 4
25. The cut in voltages for germanium and silicon diodes respectively as A.
0.3 V and 0.7 V
B.
0.7 V and 0.3 V
C.
0.6 V and 0.9 V
D.
0.5 V and 0.7 V
26. An SCR is triggered at 80° in the positive half cycle, only. The rms anode current is 25 A. If the firing angle is changed to 160°, the rms current is likely to be A.
25 A
B.
12.5 A
C.
less than 25 A
D.
less than 12.5 A
27. The output wave of a cycloconverter consists of a number of segments derived from the input wave. A.
True
B.
False
28. A class A chopper has an input voltage of 200 V. Ton = 1.5 ms and Toff = 4.5 ms. The duty cycle, average output voltage and ripple factor respectively are A. B.
0.5, 100 V and 1 0.25, 50 V and 3
C.
0.25, 50 V and 1
D.
0.5, 100 V and 3
29. In a parallel inverter the turn off time tq of thyristor and values of commutating elements L and C are related as A.
B.
C.
D.
tq =
LC
tq =
LC
tq =
LC
tq =
LC
31. Which of the following devices has two gates? A.
SCR
B.
C.
SUS
D.
SCS
SBS
32. In auxiliary commutated chopper, the peak value of thyristor current is A.
equal to peak value of load current
B.
equal to peak value of capacitor current
C. D.
sum of peak value of load current and capacitor current
none of the above
33. The symbol in figure is for
A. B.
SUS SBS
C.
SCS
D.
Shockley diode
34. If peak value of input voltage is Vm and firing angle is
, the average output voltage for single
phase half wave and single phase full wave regulators are A.
B.
C.
D.
35. The static equalisation circuit for thyristors connected in series is A. B.
one resistor in series with each thyristor one resistor in parallel with each thyristor
C.
one resistor and one capacitor in parallel with each thyristor
D.
none of the above
36. A cycloconverter is a group of controlled rectifiers. A.
True
B.
False
37. A single phase full wave regulator feeds a resistive load. The firing angle of thyristors is 90°. The input power factor is A. B.
1 0.707 lagging
C.
0.5 lagging
D.
0.2 lagging
38. The leads in a TRIAC are denoted by A. B.
anode cathode, gate T1 T2, gate
C.
T1, T2, anode
D.
T1, T2, T3
39. A step down cycloconverter has an input frequency f. If it uses natural commutation, the output frequency A.
must be a sub-multiple of input frequency
B.
can be any frequency
C.
must be an even sub-multiple of input frequency
D.
must be an odd sub-multiple of input frequency
40. In a single phase full wave regulator, the rms current in ea ch thyristors is Ir The run load current is equal to A. B. C.
2Ir 2 Ir
Ir
D.
41. Lead mounting for SCR is used for A. B.
rating less than mA rating about 1 A or less
C.
rating of 20 A or less than 20 A
D.
rating 100 A or less
42. A load commutated chopper circuit has
A.
one thyristor
B.
two thyristors
C. D.
four thyristors
six thyristors
43. Assertion (A): The output voltage of an inverter can be controlled by PWM technique Reason (R): In a current source inverter the input current is constant. A. B.
Both A and R are correct and R is correct explanation of A Both A and R correct but R is not correct explanation of A
C.
A is correct but R is wrong
D.
A is wrong but R is correct
44. A TRIAC is equivalent to A. B.
two thyristors in series two thyristors in parallel
C.
one thyristor and one diode
D.
one thyristor and one transistor
45. Which of the following commutation methods is the overall circuit under damped? A.
Class A
B.
Class B
C.
Class C
D.
Both class A and Class B
46. In a half bridge inverter, the freewheeling diodes are needed A.
even when load is resistive
B.
only when load is inductive
C. D.
only when load is inductive or capacitive
for all types of loads
47. When a thyristor is conducting, the voltage drop across it A.
is absolutely constant
B.
decreases with increase in load current
C. D.
increases slightly with increase in load current
none of the above
48. The advantage of modified series inverter with two inductors and two capacitors is A.
the intermittent operation of battery is avoided
B.
efficiency of circuit is more
C.
the output frequency is low
D.
all of the above
49. An SUS is A.
exactly similar to SCR but with higher rating
B.
similar to UJT
C. D.
PUT and avalanche diode connected in anti parallel
SCR and avalanche diode connected in parallel
50. Which chopper circuit operates in quadrants 1 and 4? A. C.
A D
B.
B
D.
E
SECTION 8 1.
In a 3 phase bridge inverter, the gating signals for the three phases have a phase difference of A.
2.
120°
B.
60°
C.
240°
D.
any of the above
Assertion (A): The circuit of figure thyristor will conduct for 180° during positive half cycle if it is continuously fired.
Reason (R): A thyristor can conduct only when it is forward biased. A.
Both A and R are correct and R is correct explanation of A
B.
Both A and R correct but R is not correct explanation of A
C.
A is correct but R is wrong
D.
A is wrong but R is correct
3.
The total number of leads in SCR, DIAC and TRIAC respectively are A.
4.
B.
2, 3, 3
C.
3, 3, 2
D.
3, 2, 4
A single phase semiconverter is feeding a highly inductive load and has a freewheeling diode across the load. The waveshape of input current is A. B.
5.
3, 2, 3
sinusoidal rectangular
C.
sinusoidal or rectangular
D.
triangular
In the below figure, the load is highly inductive. The firing angle is The periods of conduction of Th 1 and Th 2 respectively are
A.
0 to
and to 2
B.
to
and +
to
+
to
-
C.
D.
to 2
and +
and +
to 2 +
to 2 -
for each of the thyristors.
6.
A crowbar circuit is generally needed for protection under fault conditions. A.
True
B.
False
7. Forward
8.
A.
the maximum rate of rise of anode voltage which will trigger the SCR
B.
the maximum rate of rise of anode voltage which will not trigger the SCR if gate signal is applied
C.
the maximum rate of rise of anode voltage which will not trigger the SCR if gate signal is not applied
D.
either (b) or (c)
The number of leads in a DIAC are A. C.
9.
rating of an SCR is
2
4
B.
3
D.
6
The condition for underdamped oscillations in a series RLC circuit is A.
B.
C.
D.
10. The method of triggering SCS is A.
application of positive pulse at cathode gate or negative pulse at an ode gate
B.
application of pulse at anode gate
C.
application of pulse at cathode gate
D.
application of low voltage at anode
11. The number of thyristors in a single phase fully controlled bridge rectifier is A. C.
16 4
B.
8
D.
2
12. In a bridge inverter A. B.
an output transformer is essential an output transformer is not essential
C.
an output transformer cannot be connected
D.
an output transformer, if used, can improve the efficiency of the circuit
13. An ideal rectifier should have 100% efficiency, zero ripple factor, zero harmonic factor and unity displacement A.
factor
B.
100% efficiency, zero tipple factor, zero harmonic factor and zero displacement factor
C.
100% efficiency, zero ripple factor, unity harmonic factor and unity displacement factor
D.
100% efficiency, unity ripple factor, zero harmonic factor and unity displacement factor
14. When thyristors are connected in parallel, the current distribution may become non-uniform due to A.
inductive effect of current carrying conductors
B.
capacitive effect of current carrying conductors
C.
both inductive and capacitive effects
D.
none of the above
15. A single phase fully controlled bridge converter is fed by a voltage V m sin ω t. The average and rms values of output voltage are A.
B.
C.
D.
cos
cos
and Vm
and 2 Vm
16. In a single phase half wave ac regulator feeding a resistive load, the rms load current A.
is always positive
B.
may be positive or negative
C.
is always negative
D.
can be even zero
17. Which of the following is correct for a BJT? A.
B.
C.
D.
18. In a bridge inverter with single pulse width modulation A.
the output frequency is equal to frequency of refe rence signal
B.
the output frequency is equal to frequency of control signal
C.
either (a) or (b)
D.
neither (a) nor (b)
19. In a modified series inverter with two inductors and two capacitors, the load current A.
has 2 parallel paths
B.
has only one parallel path
C.
can have three parallel paths
D.
has four parallel paths
20. The number of layers in a Shockley diode are A.
2
B.
3
C. D.
4
2 or 3
21. In a modified series inverter A. B.
the maximum output frequency is less than frequency of damped o scillations the output frequency can be even higher than the frequency of damped oscillations
C.
the output frequency cannot exceed the frequency of damped oscillation s
D.
any of the above
22. Assertion (A): Light triggering is very suitable for HVDC transmission. Reason (R): Light triggering has the advantage of complete electrical isolation of gate circuit. A.
Both A and R are correct and R is correct explanation of A
B.
Both A and R correct but R is not correct explanation of A
C.
A is correct but R is wrong
D.
A is wrong but R is correct
23. Class A and class B chopper A.
operate in first quadrant
B.
operate in second quadrant
C. D.
class A chopper operates in first quadrant while class B operates in second quadrant
may operate in either first or second quadrant
24. To turn off an SCR, it is necessary to reduce its current to less than : A. B.
trigger current holding current
C.
breakover current
D.
none of the above
25. When a single phase dual converter is operating in circulating current mode A. B.
both converters are working as rectifiers simultaneously one converter is always working as rectifier and the other as inverter
C.
only one converter is working at one time
D.
both converters are either working as rectifiers or as inverters simultaneously
26. The load circuit of a single phase full bridge inverter A.
must be underdamped
B.
must be overdamped
C.
may be underdamped or overdamped
D.
may be any type of load
27. When an SCR is reverse biased A.
two junctions are reverse biased and one junction is forward biased.
B.
all the three junctions are reverse biased.
C.
one junction is reverse biased and two junctions are forward biased.
D.
any of the above depending on the magnitude of reverse bias.
28. A dc series motor is coupled to a load whose torque is proportional to speed. It is fed by a single phase ac supply through a full converter. When speed is 500 rpm, firing angle is 45°. It is desired to raise the speed. It will be necessary to A.
decrease the firing angle
B.
increase the firing angle
C.
either decrease or increase the firing angle depending on the required speed
D.
decrease the load torque
29. In Jone chopper, the commutating element A.
is a capacitor
B.
is a capacitor and an inductor
C. D.
is a capacitor and an auto transformer
none of the above
30. A LASCR has A.
4 semiconductor layers and 3 junctions
B.
3 semiconductor layers and 2 junctions
C.
2 semiconductor layers and 2 junctions
D.
any of these depending on the rating
31. A fully controlled bridge converter is feeding an RLE load. The load current is I 0. The input voltage is Vm sin ω t. The KVL equation to find firing angle is
A.
B.
C.
D.
cos
cos
cos
cos
= E + I0R
= E + I0R
= E - I0R
= E - I0R
32. A single phase cycloconverter with centre tapped input transformer requires A. B.
2 thyristors 4 thyristors
C.
8 thyristors
D.
either 2 or 4 thyristors
33. The duty cycle of a step down chopper is A.
B.
C.
D.
34. In a 3 phase bridge inverter with 120° mode of operation the. number of thyristors conducting at one time are A.
1
B.
C.
3
D.
2
4
35. A rectifier is fed by a transformer. The rms voltage and current at secondary of transfo rmer are Vs and Is. Output dc power is Vdc Idc and output ac power is (Vrms ) (Irms), Transformer utilisation factor TUF is A.
B.
C.
D.
36. Assertion (A): A class C chopper can operate in first and second quadrants Reason (R): A class B chopper operates only in second quadrant. A. B.
Both A and R are correct and R is correct explanation of A Both A and R correct but R is not correct explanation of A
C.
A is correct but R is wrong
D.
A is wrong but R is correct
37. A single phase dual converter consists of A. B.
two fully controlled converters connected in parallel two fully controlled converters connected in antiparallel
C.
two fully controlled semiconverters connected in antiparallel
D.
two fully controlled semiconverters connected in parallel
38. The snubber circuit consists of A.
a capacitor connected in series with thyristor
B.
a capacitor connected in parallel with thyristor
C. D.
a resistor and capacitor in series connected across thyristor
none of the above
39. The effect of source inductance in a 3 phase fully controlled bridge converter is to A.
decrease the output voltage
B.
increase the output voltage
C.
decrease the output voltage and increase the output current
D.
increase the output voltage and decrease the output current
40. In disc mounting of SCR A. B.
SCR is clamped on one side of disc shaped heat sink SCR is clamped between two disc shaped heat sinks
C.
The SCR has low current rating
D.
None of the above
41. As compared to a p-n junction diode A.
a schottky diode has lower cut in voltage and lower reverse saturation current
B.
a schottky diode has higher cut in voltage and higher reverse saturation current
C. D.
a Schottky diode has lower cut in voltage and higher reverse saturation current
a schottky diode has higher cut in voltage and lower reverse saturation current
42. From which instant is the turn off time of a thyristor defined A. B.
when anode voltage becomes zero when anode current becomes zero
C.
when both anode voltage and current become zero
D.
none of the above
43. In a single phase cycloconverter with centre tapped transformer A. B.
all thyristors carry currents for equal duration the durations of currents in all thyristors may or may not be equal
C.
the durations of currents in all thyristors cannot be equal
D.
the duration of currents in all thyristors must be equal for satisfactory operation
44. If Ar and Ac are the amplitudes of reference and control signals, the modulation index Mis A.
B.
C.
D.
45. Thyristors can be used for control of A.
dc separately excited motor
B.
dc shunt motor
C.
dc series motor
D.
all of the above
46. Assertion (A): Thyristors can be used in controlled heating, excitation systems of alternators, speed control of motors and HVDC Reason (R): A static var system using thyristors is very commonly used in high voltage ac systems. A.
Both A and R are correct and R is correct e xplanation of A
B.
Both A and R correct but R is not correct explanation of A
C.
A is correct but R is wrong
D.
A is wrong but R is correct
47. In a dual converter operated in circulating current mode, the circulating current flows A.
when load current is zero
B.
when load current is non-zero
C. D.
even when load current is zero
none of the above
48. For a single phase half wave converter feeding a resistive load, the firing angle in below figure is for
A.
output voltage
B.
input voltage
C.
voltage drop across thyristor
D.
none of the above
. The waveshape
49. A class B chopper operates in third quadrant. A.
True
B.
False
50. In a full bridge circuit supplying an RLC under damped circuit A.
the waveshapes of output voltage and load current are square
B.
the waveshapes of output voltage and load current are sinusoidal
C.
the waveshapes of output voltage is square but the waveshape of load current is nearly sinusoidal
D.
none of the above
SECTION 9 1.
In a semi converter feeding a resistive load A.
2.
the waveshapes of output voltage and output current are exactly similar
B.
the waveshapes of output voltage and output current can not be similar
C.
the waveshapes of output voltage and output current are similar only for low values of firing angle
D.
the waveshapes of output voltage and output current may be similar or dissimilar
In an n stage high frequency series inverter having a time period T, each stage is active for a time interval of A.
B.
C.
D.
T
3.
In a 3 phase semiconverter, the output voltage becomes discontinuous for A. B.
4.
firing angle > 30° firing angle > 60°
C.
firing angle > 90°
D.
firing angle > 120°
A single phase fully controlled bridge converter is feeding an RLE load. Input voltage is V m sin ω t and firing angle is . If average current is I0, then
A.
cos
B.
cos
C.
cos
D.
5.
cos
= RI0 + E
= RE 0 + E
= E 0 - RI0
= E - RI0
In integral cycle control of ac regulator, the input power factor is equal to A.
duty cycle
B.
2
C.
D.
6.
( )1/3
McMurrary full bridge inverter uses A. B.
auxiliary commutation
complementary commutation
7.
C.
natural commutation
D.
any of the above
A fully controlled bridge converter feeds a highly inductive load such that the load current can be assumed to be constant. The wave shape of input current is A.
dc
B.
sinusoidal
C. D.
8.
none of the above
A static excitation system uses A.
9.
rectangular
transformer and thyristors
B.
transformer and ac exciter
C.
dc exciter and ac exciter
D.
transformer and dc exciter
The application of gate signal decreases the break over voltage of on SCR. A.
True
B.
False
10. The power factor at the input terminals of a cyclo - converter is generally A. B.
low and leading low and lagging
C.
high and leading
D.
high and lagging
11. Ripple factor RF and form factor FF are related as A. B.
RF = FF2 - 1 RF = (FF2 - 1)0.5
C.
RF = FF - 1
D.
RF = (FF - 1)0.5
12. Modified Mc Murray full bridge inverter uses A.
4 thyristors and 2 diodes
B.
4 thyristors and 4 diodes
C.
8 thyristors and 4 diodes
D.
8 thyristors and 8 diodes
13. Assertion (A): Gate triggering is the most commonly used method for triggering of an SCR. Reason (R): Even a small gate current is sufficient to turn on an SCR. A.
Both A and R are correct and R is correct explanation of A
B.
Both A and R correct but R is not correct explanation of A
C.
A is correct but R is wrong
D.
A is wrong but R is correct
- END – Gambate ECE 5B!! ^^
A cycloconverter can be A.
step down
B.
step up
C.
step down or step up
D.
none of the above
The output frequency can be more or less than input frequency. 2.
In the below figure the average load current is 15 A. The rms value of transformer secondary current is
A.
15 A
B.
10.61 A
C.
7.5 A
D.
14.14 A
RMS secondary current =
3.
= 15/1.414 = 10.608 A.
In a 3 phase fully controlled converter the firing frequency is A.
3 times the line frequency
B.
6 times the line frequency
C.
9 times the line frequency
D.
12 times the line frequency
It is a 6 pulse converter. 4.
Two thyristor of same rating and same specifications A.
will have equal turn on and turn off periods
B.
will have equal turn on but unequal turn off periods
C.
may have equal or unequal turn on and turn off periods
D.
will have unequal turn on and turn off periods
Turn on and turn off times of similar SCRs may not be same. 5.
6.
The amount of dielectric heating is inversely proportional to frequency. A.
True
B.
False
A semiconverter feeding an RLE load operates in A.
first quadrant only
B.
first and second quadrants
C.
first and fourth quadrants
D.
first and third quadrants
Semiconvertor operates only in first quadrant. 7.
The phenomenon of overlap in converter operation due to A.
source resistance
B.
source inductance
C.
both source resistance and source inductance
D.
high value of firing angle
Source inductance delays turn off and hence causes overlap. 8.
A thyristor has a maximum allowable junction temperature of 120°C and the ambient temperature is 40°C. If thermal resistance is 1.6° C/W, the maximum allowable internal power dissipation is A.
20 W
B.
50 W
C.
92 W
D.
128 W
. 9.
For a 12 pulse operation, the two 6 pulse units are fed by A.
Y - Y and Y - Y transformers
B.
Δ - Δ and Δ - Δ transformers
C.
Y - Y and Y - Δ transformers
D.
Y - Δ and Y - Δ transformers
30° phase difference is necessary. 10. In a single phase full wave converter (M - 2 connection) feeding a highly inductive load, the firing angle for each thyristor is a in the respective half cycle. The period of conduction of each thyristor is A.
-
B.
C.
+
D.
- 2
Since conduction is from
to
+ ,
period of conduction is
in each half cycle.
11. An SCR is triggered at 40° in the positive half cycle only. The average anode current is 50 A. If the firing angle is changed to 80°, the average anode current is likely to be A.
50 A
B.
25 A
C.
less than 50 A but more than 25 A
D.
less than 25 A
. 12. The characteristics of a non-linear resistance is i = kv4. If i becomes 100 times, vbecomes A.
about 100 times
B.
about 10 times
C.
about 3 times
D.
about twice
v ∝ (i)1/4 ∝ (100)1/4 ≈ 3. 13. In a step down chopper using pulse width modulation, Ton = 3 x 10 -3 and Toff = 1 x 10 -3s. The chopping frequency is A.
333.33
B.
250
C.
500
D.
1000
. 14. Thyristors are suitable for dc circuit breakers but not for ac circuit breakers. A.
True
B.
False
They are suitable for both dc and ac circuit breakers.
15. Assertion (A): Inverters and choppers use fast switching thyristors Reason (R): Fast switching SCR has low turn off time. A.
Both A and R are correct and R is correct explanation of A
B.
Both A and R correct but R is not correct explanation of A
C.
A is correct but R is wrong
D.
A is wrong but R is correct
Low turn off time enables fast switching. 16. The value of capacitor C for dynamic equalising circuit of series connected thyristors is determined by A. B.
forward characteristics of thyristors reverse recovery characteristics of thyristors
C.
both forward and reverse recovery characteristics of thyristors
D.
none of the above
Value of C is proportional to maximum difference between reverse charge of SCRs in series.
17. A chopper A. B.
converts constant voltage dc into ac and then into variable vo ltage dc converts constant voltage dc into variable voltage dc directly
C.
converts ac of one frequency into ac of another frequency
D.
converts ac to dc
A chopper is a dc-ac converter. It converts a fixed dc voltage into a variable dc voltage.
18. Assertion (A): If the duty cycle of chopper is low the current would not become discontinuous Reason (R): If the time constant L/R of the load in chopper is low the current may become discontinuous. A.
Both A and R are correct and R is correct explanation of A
B.
Both A and R correct but R is not correct explanation of A
C. D.
A is correct but R is wrong
A is wrong but R is correct
If duty cycle is low, current may become discontinuous.
19. In a single phase full wave regulator, the firing angles in the positive and negative half cycles are generally A.
equal
B.
different
C.
equal or different
D.
different but sometimes equal
Firing angles are kept equal to get symmetrical output voltage.
20. An electric heater is controlled by thyristors through on-off control. If = 0.4, the heating is A. B.
16% maximum 40% of maximum
C.
60% of maximum
D.
84% of m ximum
Vl = V
. Therefore Vl2 - V2
(a). Heating is proportional to
.
21. A two winding transformer is feeding a single phase half wave rectifier circuit. The load is purely resistive. The rms value of transformer secondary current is I s and rms value of load current in Irms. Then A.
Is = Irms irrespective of the value of firing angle
B.
Is = 0.5 Irms irrespective of the value of firing angle
C.
Is = Irms if firing angle is less than 30°
D.
Is = Irms if firing angle is more than 30°
.
22. The terminals of a power BJT are called A.
emitter, base, collector
B.
emitter, base, drain
C.
source, base, drain
D.
source, base, collector
In a BJT terminals are emitter, base and collector irrespective of rating.
23. In single phase half wave regulator, the average current over one full cycle A.
is always positive
B.
may be positive or negative
C.
may be negative
D.
is always negative
The input current during positive half cycle is less than that during negative half cycle.
24. The dynamic equalising circuit consists of a series combination of capacitor C and resistor RC across each thyristor. This resistance RC (along with parallel connected diode) A. B.
limits charging current limits discharging current
C.
limits both charging and discharging currents
D.
none of the above
Discharge current flows through Re .
25. A single phase semiconverter is feeding a highly inductive load and has freewheeling diode across the load. The waveshapes of output voltage and output current A. B.
are similar are not similar
C.
may be similar or dissimilar
D.
are similar only if firing angle is zero
When freewheeling diode conducts output current exists but output voltage is zero.
26. Current derating may be necessary when thyristors are operating in A. B.
series parallel
C.
series or parallel
D.
none of the above
Since current does not divide equally, current derating is necessary.
27. If duty cycle is
and the load is a resistance R, the rms value of output voltage is
A.
B.
V
2V
C.
D.
RMS output voltage VL =
V.
28. For a BJT
= 0.98, then β =
A.
0.02
B.
C.
4.9
D.
0.5 48
29. Assertion (A): Power transistor has lower switching time than SCR Reason (R): Power transistor has greater switching time than SCR A.
Both A and R are correct and R is correct explanation of A
B.
Both A and R correct but R is not correct explanation of A
C.
A is correct but R is wrong
D.
A is wrong but R is correct
Power transistor does not have high
rating.
30. A 3 kV circuit uses SCR of 800 V rating. If derating is 25%, the number of SCRs in series is A.
4
B.
C.
6
D.
5
8
.
31. In a circuit using a full wave converter (M - 2 connection) the PIV of each thyristor is 400 V. For the same output voltage and fully controlled bridge converter, PIV will be A.
400 V
B.
800 V
C. D.
200 V
2002 V
In bridge connection PIV is half of that in M-2 connection.
32. A dc separately excited motor has constant field current. The armature is fed from a s ingle phase supply through a full converter. When = 0, speed is 500 rpm. If = 45°, the speed is likely to be A.
about 350 rpm
B.
about 250 rpm
C.
about 175 rpm
D.
about 125 rpm
Average output voltage ∝ cos
and therefore reduced to 0.707 of the value for = 0. Speed is
nearly proportional to voltage.
33. In a multiphase chopper, all choppers operate together. A.
True
B.
False
They operate in a sequence.
34. The terminals of a power MOSFET are called A. B.
emitter, base, collector source, gate, drain
C.
source, base, drain
D.
emitter, gate, drain
In a MOSFET terminals are source, gate, drain irrespective of rating.
35. Assertion (A): The surge current which an SCR can withstand is much higher than rms on state current. Reason (R): The duration of surge current is very small. A.
Both A and R are correct and R is correct explanation of A
B.
Both A and R correct but R is not correct explanation of A
C.
A is correct but R is wrong
D.
A is wrong but R is correct
Since duration of surge current is small, the associated energy is small.
36. A single phase half wave controlled rectifier circuit has a free wheeling diode. The load is a combination of R and L. The firing angle is . The period of conduction of SCR and free wheeling diode respectively are
< ωt <
and <
ω t < (2 +
B.
< ωt <
and <
ω t < 2
C.
< ω t < ( +
D.
< ωt <
A.
) and (
and <
+
)
) < ω t < 2
ω t < (2 -
)
SCR conducts from to
and free wheeling diode from to (2
+
).
37. Assertion (A): A transistor requires a continuous base signal for conduction but a thyristor requires a gate pulse.Reason (R): Transistor find widespread application in power electronic circuits. A.
Both A and R are correct and R is correct explanation of A
B.
Both A and R correct but R is not correct explanation of A
C. D.
A is correct but R is wrong
A is wrong but R is correct
Transistors are used but not very widely.
38. In a thyristor the gate current is increased, then A.
anode current will increase
B.
anode current will decrease
C. D.
anode current will remain constant
anode current may increase or decrease
Gate current does not affect the magnitude of anode current.
39. Thyristors are not suitable for logic circuits. A.
True
B.
False
Thyristors can be used for logic circuits.
40. A single phase half wave rectifier is feeding a resistive load. Input voltage v = Vm sin ω t. The output dc voltage is Vdc and output rms voltage is Vrms . If firing angle is 180°, Vdcand Vrms respectively are A.
0 and 0
B.
C.
0 and - Vm
D.
Put = 180° in Eqs Otherwise also = 180° means no conduction. Hence zero output voltage.
41. The number of leads in an SCR are : A.
2
B.
C.
4
D.
3
5
Anode, cathode and gate.
42. In a single phase full wave converter (M-2 connection) feeding on R - L load, the input voltage is v = Vm sin ω t. The expression for dc output voltage is
A.
B.
C.
D.
Due to inductive load the period of conduction is
to +
in each half cycle.
43. A thyristor has a turn on time of 6 μs. If the anode circuit is inductive, the turn on time will be A.
6 μs
B.
less than 6μs
C. D.
more than 6 μs
either 6 μs or less
Inductive load increases the turn on time.
44. The total number of leads in reverse conducting thyris tor is A.
2
B.
C.
1
D.
3
4
45. Assertion (A): A TRIAC is a bidirectional SCR. Reason (R): A TRIAC is a four layers 3 terminal device and can conduct in both directions. A.
Both A and R are correct and R is correct explanation of A
B.
Both A and R correct but R is not correct explanation of A
C.
A is correct but R is wrong
D.
A is wrong but R is correct
Since a TRIAC can conduct in both directions it is bi directional SC R.
46. If Vdc1 is the dc output voltage in half wave controlled rectifier circuit feeding resistive load and Vdc2 is the dc output voltage in full wave controlled rectifier circuit (M - 2 connection) feeding resistive load, then A.
Vdc1 = 0.5 Vdc2 for all values of firing angle
B.
Vdc1 = Vdc2 for all values of firing angle
C.
Vdc1 ≤ 0.5 Vdc2 for all values of firing angle
D.
Vdc1 ≥ 0.5 Vdc2 for all values of firing angle
and Vdc =
(1 + cos
).
47. A semi converter operates in two quadrants. A.
True
B.
False
It operates only in quadrant.
48. Figure shows a chopper feeding RLE load, The free wheeling diode conducts when
A. B.
thyristor is on thyristor is off
C.
both when thyristor is on and thyristor is off
D.
partly when thyristor is off and partly when thyristor is on
FD is forward biased only when thyristor is off.
49. Assertion (A): An ideal rectifier should have unity harmonic factor Reason (R): An ideal rectifier should have unity displacement factor. A.
Both A and R are correct and R is correct explanation of A
B.
Both A and R correct but R is not correct explanation of A
C.
A is correct but R is wrong
D.
A is wrong but R is correct
Ideal rectifier should have zero harmonic factor and unity displacement factor.
50. A single phase half wave rectifier is feeding a resistive load. When V. When = 30°, the dc output voltage will be about A. B.
127 V 120 V
C.
110 V
D.
100 V
Using Eq
= 0.933.
= 0, dc output voltage is 127
Hence Vdc for 30° firing angle = 0.933 x 127 = ≈ 120 V.
SECTION 2 1.
A dc circuit breaker must use A.
natural commutation
B.
forced commutation
C.
both natural and forced commutation
D.
either natural or forced commutation
Answer & Explanation
Answer: Option B Explanation: Natural commutation is possible only in ac circuits.
2.
In a single phase full wave regulator feeding R-L load, the minimum firing angle can be zero. A.
True
B.
False
Answer & Explanation
Answer: Option B Explanation: Range of control of is 0 ≤
3.
≤
.
An RC snubber circuit is used to protect a thyristor against : A.
false triggering
B.
failure to turn on
C.
switching transients
D.
failure to commutate
Answer & Explanation
Answer: Option C Explanation:
Especially high
4.
during switching.
A DIAC has two terminals, anode and cathode. A.
True
B.
False
Answer & Explanation
Answer: Option B Explanation: They are designated as T1 and T2 because diac can conduct in both directions.
5.
A single phase semiconverter has a freewheeling diode. If the firing angle is
and the load is
purely resistive, the periods of conduction and freewheeling respectively are A.
-
and 0
B.
-
and
C.
+
and 0
D.
+
and
Answer & Explanation
Answer: Option A Explanation: Freewheeling diode conducts only when load is inductive. If load is purely resistive free wheeling diode does not come into operation.
6.
A dc separately excited motor has a constant field current. The armature is fed from a single phase supply through a semi-converter. When = 0, speed is 600 rpm. If = 90°, the speed is likely to be A.
about 300 rpm
B.
about 150 rpm
C.
about 75 rpm
D.
about zero
Answer & Explanation
Answer: Option A Explanation: Average output voltage is reduced to 50% if = 90°. Hence speed is also reduced to 50%.
7.
A static induction thyristor A.
is a majority carrier device
B.
is a minority carrier device
C.
either a majority carrier or minority carrier device
D.
has a total number of 2 leads
Answer & Explanation
Answer: Option B Explanation: It can be turned on by positive gate pulse and turned off by negative gate pulse. It is minority carrier device. It can be manufactured in high current and high voltage ratings and its on state voltage drops is low.
8.
A single phase step up cycloconverter changes 50 Hz to 100 Hz. Then one half wave of input will give rise to
A.
one half wave of output
B.
one full wave of output
C.
two full waves of output
D.
either (b) or (c)
Answer & Explanation
Answer: Option B Explanation: Since output frequency is 2f, one half wave of input must equal one full wave of output.
9.
Which of these triggering methods for series connected SCRs use LASCR in the gate circuit? A.
Resistance triggering
B.
Simultaneous triggering
C.
Sequential triggering
D.
Optical triggering
Answer & Explanation
Answer: Option D Explanation: Optical triggering uses LASCR.
10. A 3 phase ac regulator uses A.
3 thyristors
B.
15 thyristors
C.
9 thyristors
D.
12 thyristors
Answer & Explanation
Answer: Option B Explanation: Two thyristors for each phase.
11. The average load current supplied by a thyristor depends on A.
firing angle
B.
firing frequency
C.
magnitude of gate current
D.
all of the above
Answer & Explanation
Answer: Option A Explanation: The firing angle controls the period of conduction and hence average load current.
12. A dc chopper is feeding an RLE load. The maximum steady state ripple is A.
B.
C.
D.
Answer & Explanation
Answer: Option A Explanation:
(Imax - Imin)max =
.
13. In a 3 phase full wave regulator feeding a star connected resistance load the input voltage is 400 V line to line. The firing angle is 160°. The line to line output voltage would be A.
400 V
B.
about 100 V
C.
about 20 V
D.
zero
Answer & Explanation
Answer: Option D Explanation: In a three-phase full wave regulator, output voltage is zero if ≥ 150°.
14. A thyristor is reverse biased. A positive gate pulse is applied. The thyristor A.
will be turned on
B.
may or may not turn on
C.
will not turn on
D.
will turn on after sometime
Answer & Explanation
Answer: Option C
Explanation: Thyristor can turn on only when it is forward biased.
15. A single phase series converter is used for A.
high voltage output
B.
high current output
C.
high voltage and high current output
D.
improving the circuit efficiency
Answer & Explanation
Answer: Option A Explanation: Series connection increases the voltage.
16. A single phase half wave controlled rectifier circuit has an R-L load. A freewheeling diode is also in the circuit. When freewheeling diode is conducting the SCR A.
is forward biased
B.
is reverse biased
C.
may be forward biased or reverse biased
D.
forward biased initially but reverse biased afterwards
Answer & Explanation
Answer: Option B Explanation: Free wheeling diode is forward biased only when SCR is reverse biased.
17. An SBS can conduct is one direction only. A.
True
B.
False
Answer & Explanation
Answer: Option B Explanation: It is a bilateral switch.
18. A 750 A circuit uses thyristor of current rating 175 A. If derating is 25%, the number of thyristors in parallel is A.
3
B.
4
C.
6
D.
10
Answer & Explanation
Answer: Option C Explanation:
.
19. Assertion (A): An inverter does not require forced commutation Reason (R): A series inverter is a forced commutation inverter. A.
Both A and R are correct and R is correct explanation of A
B.
Both A and R correct but R is not correct explanation of A
C.
A is correct but R is wrong
D.
A is wrong but R is correct
Answer & Explanation
Answer: Option D Explanation: Some inverters require forced commutation.
20. In a single phase full wave converter (M - 2 connection), the rms transformer voltage is 300 V across each half of secondary. The PIV for each thyris tor is A.
300 V
B.
C.
6002 V
D. Answer & Explanation
Answer: Option C Explanation: PIV is twice the peak voltage across each half of secondary.
21. The number of doped regions in a SCR is A.
2
B.
3
C.
4
D.
5
Answer & Explanation
Answer: Option C Explanation: p = n = p = n.
22. All inverters use forced commutation. A.
True
B.
False
Answer & Explanation
Answer: Option B Explanation: Line commutated inverters are also used especially in HVDC.
23. In 180° mode of operation of a 3 phase bridge inverter, two thyristors conduct at one time. A.
True
B.
False
Answer & Explanation
Answer: Option B Explanation: In 180° mode three thyristors conduct simultaneously.
24. In a single phase full wave ac regulator the rms value of load current is 20 A load is purely resistive. Then A.
average thyristor current will be less than 20 A but rms thyristor current will be more than 20 A
B.
both the average and rms thyristor currents will be less than 20 A
C.
both the average and rms thyristor curre nts will be more than 20 A
D.
rms thyristor current will be 20 A and average thyristor current will be 10 A
Answer & Explanation
Answer: Option B Explanation:
Each thyristor conducts for 50% of time.
25. A reactor is used in a dual converter operating in non-circulating current mode. A.
True
B.
False
Answer & Explanation
Answer: Option B Explanation: Reactor is used only for circulating current mode.
26. Which of these commutation methods uses an auxiliary SCR? A.
Class A
B.
Class B
C.
Class C
D.
Class D
Answer & Explanation
Answer: Option C Explanation: Class C commutation uses an auxiliary SCR and it is also called auxiliary commutation.
27. A thyristor needs protection against A.
B.
high
high
C.
D.
both high
and high
either high
or high
Answer & Explanation
Answer: Option C Explanation:
Both high
and high
can destroy SCR.
28. Two semiconverters are connected in series to form a single phase series converter. The input v = Vm sin ω t and 1 = 2 are the firing angles of the two converters. If 1 = 2= 0, the dc output voltage is A.
B.
C.
D. Answer & Explanation
Answer: Option C Explanation:
Output voltage is
for each.
29. In integral cycle control the period of on-off control should be more than mechanical time constant of load. A.
True
B.
False
Answer & Explanation
Answer: Option B Explanation: It should be less than mechanical time constant.
30. A single phase half wave converter is feeding a resistive load. Just before triggering, the voltage across thyristor is A.
zero
B.
about 1 V
C.
same as input voltage
D.
twice the input voltage
Answer & Explanation
Answer: Option C Explanation: Before triggering the whole of applied voltage is across SCR and load current is zero.
31. In a 3 phase full wave regulator feeding resistance load connected in star, the possible range of firing angle is A.
0≤
≤ 180°
B.
0≤
≤ 150°
C.
0≤
≤ 120°
D.
0≤
≤ 90°
Answer & Explanation
Answer: Option B Explanation: Since output voltage is zero if ≥ 150°, the possible range of firing angles is 0 ≤
≤ 150°.
32. Assertion (A): A dual converter may be circulating current type or non circulating current type. Reason (R): In circulating current dual converter only one converter can be operated at one time. A.
Both A and R are correct and R is correct explanation of A
B.
Both A and R correct but R is not correct explanation of A
C.
A is correct but R is wrong
D.
A is wrong but R is correct
Answer & Explanation
Answer: Option C Explanation: In circulating current dual converter both converters operate together.
33. Current chopping leads to A.
over voltage
B.
over current
C.
both over voltage and over current
D.
either over voltage or over current
Answer & Explanation
Answer: Option A
Explanation:
e=L
. In current chopping
is high. Hence e is high.
34. A 3 phase bridge inverter is fed by 400 V battery. The load is star connected and has a resistance of 10 Ω per phase. The peak value of load current is A.
40 A
B.
20 A
C.
10 A
D.
5A
Answer & Explanation
Answer: Option B Explanation: Two resistances are in series at any time.
35. In an SCR, the anode current is controlled by A.
gate current only
B.
external circuit only
C.
both gate current and external circuit
D.
none of the above
Answer & Explanation
Answer: Option B Explanation: Once SCR has been triggered the gate has no control. Only external resistance controls the anode current.
36. The rated current of a thyristor is 40 A. The holding current will be about : A.
20 A
B.
1A
C.
100 mA
D.
10 mA
Answer & Explanation
Answer: Option D Explanation: The holding current is much less then 1% of rated current.
37. A cycloconverter is A.
ac-dc converter
B.
dc-ac converter
C.
dc-dc converter
D.
ac-ac converter
Answer & Explanation
Answer: Option D Explanation: It changes ac at frequency f into ac at a frequency different from f.
38. The number of leads in a power semiconductor diode are A.
2
B.
3
C.
4
D.
2 or 3
Answer & Explanation
Answer: Option A Explanation: A diode has 2 leads irrespective of its rating.
39. If ton and toff are the turn on and turn off times of a thyristor, then A.
ton and toff are almost equal
B.
ton >> toff
C.
toff >> ton
D.
ton is about twice toff
Answer & Explanation
Answer: Option C Explanation: Turn on time is less than 3 μs while turn off time is between 10 -300 μs.
40. Assertion (A): The gate current of an SCR is always in the form of pulses. Reason (R): The gate losses, in pulse triggering, are low. A.
Both A and R are correct and R is correct explanation of A
B.
Both A and R correct but R is not correct explanation of A
C.
A is correct but R is wrong
D.
A is wrong but R is correct
Answer & Explanation
Answer: Option A Explanation: Gate current in the form of pulses is commonly used because gate losses are low.
41. UJT can be used for A.
firing an SCR
B.
turning off an SCR
C.
both firing and turning off an SCR
D.
none of the above
Answer & Explanation
Answer: Option A Explanation: UJT relaxation oscillator is used to provide gate pulses to SCR for triggering.
42. In a series inverter supplying a load resistance R, the commutating e lements L and C should be such that A.
B.
C.
D. Answer & Explanation
Answer: Option B Explanation: Load should be under damped.
43. In a full wave converter (M - 2 connection) feeding an RLE load, the peak thyristor current is 15 A. For fully controlled bridge circuit and same load current, the peak thyristor current will be A.
30 A
B.
152 A
C.
15 A
D. Answer & Explanation
Answer: Option C Explanation: In both the connections the output voltage and peak thyristor current are the same. Both are full wave rectifiers.
44. Induction heating systems require low frequency inverters. A.
True
Answer & Explanation
Answer: Option B Explanation: It uses high frequency inverter.
B.
False
45. A single phase half wave controlled rectifier circuit is feeding on RL load. The circuit has a freewheeling diode. The input voltage is v = Vm sin ω t. If I0 is the load current during the conduction of freewheeling diode, the KVL equation during the conduction of free wheeling diode is A.
B.
C.
D. Answer & Explanation
Answer: Option B Explanation: Load is short-circuited by free wheeling diode.
46. HVDC transmission systems generally use A.
12 pulse converters
B.
6 pulse converters
C.
3 pulse converters
D.
either 12 or 6 pulse converters
Answer & Explanation
Answer: Option A Explanation: To reduce ripple, 12 pulse converters are used in HVDC.
47. A single phase semiconverter is feeding a highly inductive load. The circuit has a freewheeling
diode across the load and firing angle is
. During
one cycle of input voltage, the total duration of
conduction of freewheeling diode is A.
B.
2
C.
4
D.
0.5
Answer & Explanation
Answer: Option B Explanation: Freewheeling diode conducts for ° in each half cycle.
48. During forward blocking state of SCR, the voltage and current respectively are A.
high and high
B.
low and low
C.
high and low
D.
low and high
Answer & Explanation
Answer: Option C Explanation: Voltage is high but current is nearly zero.
49. In a 3 phase full converter the average load current is 150 A. The average thyristor current is A.
150 A
B.
75A
C.
50 A
D.
25 A
Answer & Explanation
Answer: Option C Explanation: Each thyristor conducts for one -third of one cycle.
50. The two modes of operation of three phase bridge inverters are known as A.
60° mode and 120° mode
B.
90° mode and 120° mode
C.
120° mode and 180° mode
D.
120° mode and 240° mode
Answer & Explanation
Answer: Option C Explanation: In 180° mode each thyristor conducts for 180° and in 120° mode each thyristor conducts for 120°.
SECTION 3 1.
In a dual converter both converters work as rectifiers. A.
True
Answer & Explanation
Answer: Option B Explanation:
B.
False
One operators as rectifier and the other as inverter.
2.
A single phase full wave regulator feeds R-L load. The best gating signal is A.
short duration pulses
B.
long duration pulses
C.
pulse train
D.
either (a) or (b)
Answer & Explanation
Answer: Option C Explanation: To ensure proper triggering pulse train in necessary because load is R - L.
3.
A 3 phase induction motor is fed by a 3 phase ac regulator to change the stator voltage. The variation in speed will be more if A.
load torque is constant
B.
load torque increases with increase in speed
C.
load torque decreases with increase in speed
D.
either (a) or (b)
Answer & Explanation
Answer: Option B Explanation: If load torque is constant, the change in speed is very small.
4.
A cycloconverter cannot give a frequency higher than input frequency. A.
True
B.
False
Answer & Explanation
Answer: Option B Explanation: Step up cycloconverter can give a frequency higher than input frequency.
5.
A thyristor has internal power dissipation of 40W and is operated at an ambient temperature of 20°C. If thermal resistance is 1.6° C/W, the junction temperature is A.
114°C
B.
164°C
C.
94° C
D.
84° C
Answer & Explanation
Answer: Option D Explanation:
.
6.
In a single phase dual converter, 1, and A.
1 - 2 =
B.
1 + 2 =
C.
1 - 2 =
D.
1 + 2 =
2 are firing
angles of the two converters. Then
Answer & Explanation
Answer: Option B Explanation: Since one converter is rectifying and the other is inverting Vdc1 = -Vdc2 or
7.
1 + 2 = .
The number of gates in an SCR is A.
1
B.
3
C.
4
D.
6
Answer & Explanation
Answer: Option A Explanation: No answer description available for this question. Let us discuss.
8.
The waveshape of output voltage of half bridge inverter is A.
sinusoidal
B.
square
C.
triangular
D.
either (a) or (b)
Answer & Explanation
Answer: Option B Explanation:
No answer description available for this question. Let us discuss.
9.
A 3 phase induction motor is controlled by voltage and frequency control such that the ratio V/f is constant. At 50 Hz, the speed is 1440 rpm. At 25 Hz, the speed is likely to be A.
about 1350 rpm
B.
about 720 rpm
C.
about 360 rpm
D.
either (b) or (c)
Answer & Explanation
Answer: Option B Explanation: In this case synchronous speed at 50 Hz is 1500 rpm. Therefore synchronous speed at 25 Hz is 750 rpm. Since slip is about 3 - 5%, speed is about 720 rpm.
10. The gate current required to turn on an SCR is A.
few amperes
B.
a few mA
C.
almost equal to anode current
D.
about 50% of anode current
Answer & Explanation
Answer: Option B Explanation: Only a very small gate current is required to trigger SCR into conduction.
11. A single phase half wave converter is used to charge a battery of 50 V. The thyristor is continuously fired by dc signal. Input voltage is v = Vm sin ω t. The thyristor will conduct A.
from 0 to 180°
B.
when v > 50 V
C.
from 0 to 90°
D.
When v < 50 V
Answer & Explanation
Answer: Option B Explanation: Thyristor can conduct only when it is forward biased. This occurs only when v > 50 V.
12. The symbol in figure is for
A.
DIAC
B.
P-N junction diode
C.
Schottky diode
D.
None of the above
Answer & Explanation
Answer: Option C Explanation: No answer description available for this question. Let us discuss.
13. In a 3 phase semiconverter, the firing frequency is A.
equal to line frequency
B.
3 times the line frequency
C.
6 times the line frequency
D.
12 times the line frequency
Answer & Explanation
Answer: Option B Explanation: Since the circuit has 3 thyristors, firing frequency is three times the line frequency.
14. Present day HVDC converters are all A.
3 pulse converters
B.
6 pulse converters
C.
12 pulse converters
D.
either 6 pulse or 12 converters
Answer & Explanation
Answer: Option C Explanation: To reduce ripple 12 pulse converters are used.
15. A single phase full converter gives maximum and minimum output voltage a t firing angles of A.
0 and 180° respectively
B.
180° and 0° respectively
C.
0 and 90° respectively
D.
90° and 0° respectively
Answer & Explanation
Answer: Option A Explanation: Output voltage is maximum if
= 0 and is minimum if
= 180°.
16. A thyristor has its gate current zero. To start conduction the forward voltage must exceed A.
about 0.5 V
B.
forward breakover voltage
C.
on state voltage
D.
peak working off state forward voltage
Answer & Explanation
Answer: Option B Explanation: With zero gate current, conduction can be started only if forward voltage exceeds breakover voltage.
17. Assertion (A): Snubber circuit has an RC series circuit in parallel with SCR Reason (R): Voltage across a capacitor can not change instantaneously. A.
Both A and R are correct and R is correct explanation of A
B.
Both A and R correct but R is not correct explanation of A
C.
A is correct but R is wrong
D.
A is wrong but R is correct
Answer & Explanation
Answer: Option A Explanation:
Since voltage across capacitor cannot change instantaneously it protects SCR against high
18. In dielectric heating, the thyristor circuits consist of A.
rectifier-chopper combination
B.
controlled rectifier
C.
ac regulator
D.
rectifier-inverter combination
Answer & Explanation
Answer: Option D Explanation: Input ac is rectified and then converted to high frequency ac.
19. On load tap changer using thyristors can have a maximum of two taps. A.
True
Answer & Explanation
Answer: Option B Explanation: Any number of taps are possible.
B.
False
.
20. Assertion (A): A cycloconverter may be line commutated or forced commutated Reason (R): A step up cycloconverter requires forced commutation. A.
Both A and R are correct and R is correct explanation of A
B.
Both A and R correct but R is not correct explanation of A
C.
A is correct but R is wrong
D.
A is wrong but R is correct
Answer & Explanation
Answer: Option A
21. In the below figure the rms voltage across each half of secondary is 200 V. The peak inverse voltage across each of thyristor is
A.
200 V
B.
2(200) V
C.
2(400) V
D.
800 V
Answer & Explanation
Answer: Option C Explanation: PIV = 2(2 x 200).
22. Assertion (A): A GTO circuit is more compact than SCR circuit Reason (R): A GTO can be turned off by negative gate pulse. A.
Both A and R are correct and R is correct explanation of A
B.
Both A and R correct but R is not correct explanation of A
C.
A is correct but R is wrong
D.
A is wrong but R is correct
Answer & Explanation
Answer: Option A Explanation: Since commutation circuit is not required, GTO circuit is compact.
23. In a single phase full wave ac regulator, the firing angles in the two half cycles A.
are always equal
B.
are sometimes equal
C.
are never equal
D.
may be equal or unequal
Answer & Explanation
Answer: Option A Explanation: No answer description available for this question. Let us discuss.
24. In a single phase full bridge converter (B - 2 connection), the input voltage is v = Vm sin ω t. The output dc voltage for resistive load
A.
B.
C.
D.
Answer & Explanation
Answer: Option C Explanation: SCRs conduct from to
in each half cycle.
25. As regards single phase dual converter A.
the change in operation from one quadrant to the other is faster in non-circulating current mode
B.
the change in operation from one quadrant to the other is faster in circulating current mode
C.
the time response to change in operation from one quadrant to other is the same in both the modes
D.
the time response to change in operation from one quadrant to the other may be faster in any of the two modes
Answer & Explanation
Answer: Option B
26. To protect an SCR against high di/dt we connect A.
a capacitance in series with SCR
B.
a capacitance in parallel with SCR
C.
an inductance in parallel with SCR
D.
an inductance in series with SCR
Answer & Explanation
Answer: Option D Explanation: current through inductance cannot change instantaneously.
27. In a parallel inverter A.
the commutating capacitor C is across the primary of transformer and the load is across the secondary
B.
the commutating capacitor and load are connected across the secondary of transformer
C.
the commutating capacitor C and load are connected in series and the combination is across the secondary of transformer
D.
none of the above
Answer & Explanation
Answer: Option A
28. A silicon controlled switch has : A.
2 leads
B.
3 leads
C.
4 leads
D.
5 leads
Answer & Explanation
Answer: Option C Explanation: Anode, cathode, anode gate and cathode gate.
29. A dc chopper V = 500 V feeds an electric fraction dc series motor. The chopping frequency is 400 Hz. The series inductance required to limit the maximum current swing to 10 A is A.
20 mH
B.
about 31 mH
C.
about 51 mH
D.
about 92 mH
Answer & Explanation
Answer: Option B Explanation:
.
30. A single phase fully controlled bridge rectifier is fed by a source having inductance. The output voltage during overlap period is A.
Vm
B.
0.5 Vm
C.
zero
D.
0.5 Vm - voltage drop across inductance
Answer & Explanation
Answer: Option C Explanation: During the period of overlap all the four thyristors conduct and load is effectively short-circuited. Output voltage is zero.
31. In a semi converter feeding a resistive load, the firing angle is A.
B.
( -
C.
( +
D.
( - 0.5
. Each thyristor conducts for
radians
) radians ) radians ) radians
Answer & Explanation
Answer: Option B Explanation: The conduction is from
to
.
32. Between which points does UJT exhibit negative resistance characteristics A.
between peak and valley points
B.
before peak point
C.
after peak point
D.
after valley point
Answer & Explanation
Answer: Option A
33. A thyristor has maximum allowable junction temperature of 110° C and the ambient temperature is 40°C, If the maximum internal power dissipation is 50 W, the thermal resistance is A.
2° C/W
B.
1.4° C/W
C.
0.7°C/W
D.
2.2° C/W
Answer & Explanation
Answer: Option B Explanation:
.
34. A single phase electric heater uses phase co ntrolled thyristors. The firing angle is 90°. The degree of heating is A.
25% of maximum
B.
50% of maximum
C.
75% of maximum
D.
none of the above
Answer & Explanation
Answer: Option B Explanation: Conduction occurs only during 50% of wave.
35. Which of these commutation methods is used in AC -DC converters? A.
Class A
B.
Class C
C.
Class D
D.
Class F
Answer & Explanation
Answer: Option D Explanation: Class F is natural commutation AC = DC converter use natural (or line) commutation.
36. When thyristors are connected in series and parallel, it may be necessary to have : A.
current derating
B.
voltage derating
C.
both current and voltage derating
D.
none of the above
Answer & Explanation
Answer: Option C Explanation: Thyristors in series do not share the voltage equally and require voltage derating. Thyristors in parallel need current derating.
37. A single phase half wave regulator uses A.
2 diodes and 2 thyristors
B.
2 thyristors
C.
1 diode and 1 thyristor
D.
2 diodes
Answer & Explanation
Answer: Option C Explanation: Regulation is possible only in positive half cycle.
38. A Shockley diode can be used A.
in place of SCR
B.
as a trigger switch for SCR
C.
in place of UJT
D.
both in place of SCR and as a trigger switch for SCR
Answer & Explanation
Answer: Option B Explanation: It is a four layered PNPN device without a gate terminal. Its characteristics is similar to that of SCR with zero gate current. It is acting as a trigger switch for Thyristor.
39. An SCR has a turn on time of 4 μs. The gate pulse should have a duration of about A.
2 μs
B.
5 μs
C.
20 μs
D.
50 μs
Answer & Explanation
Answer: Option B Explanation: Duration of gate pulse should be slightly more than turn time of SCR.
40. In a multiphase chopper, the different choppers operate in A.
series and all simultaneously
B.
parallel and one at a time
C.
parallel and simultaneously
D.
either (b) or (c)
Answer & Explanation
Answer: Option B Explanation: Only one chopper operates at one time.
41. In a 3 phase semiconverter, the dc output voltage for a certain firing angle is 50% of the maximum possible value. The firing a ngle is A.
30°
B.
60°
C.
90°
D.
120°
Answer & Explanation
Answer: Option C Explanation: Vdc ∝ (1 + cos
). Maximum possible output voltage occurs when = 0.
42. A high frequency series inverter consists of
A.
a number of series inverters in series
B.
a number of series inverters in parallel
C.
a number of parallel inverters in parallel
D.
a number of parallel inverters in series
Answer & Explanation
Answer: Option B Explanation: A high frequency series inverter has many series inverters in parallel. Only one inverter operates at one time and they operate in fixed sequence.
43. In an SCR, the anode current consists of A.
only holes
B.
only electrons
C.
both electrons and holes
D.
either electrons or holes
Answer & Explanation
Answer: Option C Explanation: Both electrons and holes contribute to current.
44. In the below figure, the average value of load current is 15 A. The load is highly indu ctive. The peak value of thyristor current is
A.
15 A
B.
152 A
C.
D.
30 A
Answer & Explanation
Answer: Option A Explanation: Because of highly inductive load, output current is nearly constant.
45. A 3 phase semiconverter when supplying a certain load has a rectification efficiency of 0.6 and transformer utilisation factor of 0.35. When a 3 phase full converter is supplying the same load A.
rectification efficiency will be more than 0.6 and TUF will be more than 0.35
B.
rectification efficiency will be more than 0.6 but TUF will be less than 0.35
C.
rectification efficiency will be less than 0.6 and TUF will be less than 0.35
D.
rectification efficiency will be less than 0.6 but TUF will be more than 0.35
Answer & Explanation
Answer: Option A
46. Assertion (A): A high dv/dt may cause triggering of a thyristor. Reason (R): A high dv/dt results in charging current in the thyristor. A.
Both A and R are correct and R is correct explanation of A
B.
Both A and R correct but R is not correct explanation of A
C.
A is correct but R is wrong
D.
A is wrong but R is correct
Answer & Explanation
Answer: Option A Explanation: The charging current may cause triggering.
47. Assertion (A): When an SCR is conducting, the voltage drop across is about 1 V. Reason (R): When an SCR is conducting, the junction temperature is about 200°C. A.
Both A and R are correct and R is correct explanation of A
B.
Both A and R correct but R is not correct explanation of A
C.
A is correct but R is wrong
D.
A is wrong but R is correct
Answer & Explanation
Answer: Option C Explanation: Junction temperature is about 120°C. Hence R is wrong.
48. When the core of a saturable reactor is saturated, the inductance is high.
A.
True
B.
False
Answer & Explanation
Answer: Option B Explanation: Since core is saturated, change of flux is small and inductance is low .
49. In controlled heating circuits, thyristors act as A.
rectifier
B.
inverter
C.
ac regulator
D.
chopper
Answer & Explanation
Answer: Option C Explanation: The output voltage is controlled ac.
50. In a fully controlled bridge converter feeding a resistive load the waveshapes of output voltage and output current A.
are always similar
B.
can never be similar
C.
may be similar or dissimilar
D.
are similar only if
Answer & Explanation
=0
Answer: Option A Explanation: For resistive load output current is always proportional to output voltage.
SECTION 4 1.
A chopper cannot be line commutated. A.
2.
B.
False
DC circuits only AC circuits only
C.
both AC and DC circuits
D.
none of the above
In a thyristor the gate lead is connected to the n layer near the p layer to which anode is connected. A.
4.
B.
Natural commutation can be used in : A.
3.
True
True
B.
False
A single phase half wave rectifier circuit has a free wheeling diode. The free wheeling diode will conduct only if A.
load is purely resistive
B.
load is purely inductive
C.
load is combination of R and L
D.
load is purely inductive or combination of R and L
5.
McMurray Bedford full bridge inverter uses A.
natural commutation
B.
auxiliary commutation
C. D.
6.
any of the above
Which converter is used in HVDC transmission? A.
3 pulse
B.
6 pulse
C. D.
7.
complementary commutation
12 pulse
24 pulse
Assertion (A): The turn on time of SCR is about 1 to 4 ms. Reason (R): The turn off time of SCR is about 10 to 300 ms. A. B.
8.
Both A and R are correct and R is correct explanation of A Both A and R correct but R is not correct explanation of A
C.
A is correct but R is wrong
D.
A is wrong but R is correct
An n pulse rectifier is fed by a source having an inductance L. If load current is I 0, the voltage drop due to overlap is A.
B.
C.
D.
10. A single phase cycloconverter feeds a resistance heating load. The heating power is provided by A.
only the fundamental component of output wave
B.
only higher harmonics in the output wave
C. D.
both fundamental and higher harmonics in the output wave
either (a) or (b)
11. A 3 phase fully controlled bridge converter is fed by a 3 phase system having phase voltage v = Vm sin ω t. The firing angle is . The dc output voltage is A.
B.
C.
D.
12. Which of these commutation methods uses an external pulse for commutation? A.
Class A
B.
Class C
C. D.
Class E
Class F
13. In a single phase full wave regulator feeding an R-L load, the power factor angle of load is θ. The range of control of firing angle is 0≤
≤
θ≤
≤
C.
0≤
≤ ( - θ)
D.
θ≤
≤ ( + θ)
A.
B.
14. A separately excited dc motor is run from a dc source through a chopper. At 600 rpm and rated torque, the duty cycle of chopper is 0.8. At 600 rpm and half the rated torque, the duty cycle is likely to be A. C.
0.9 0.75
B.
0.8
D.
0.6
15. A relaxation oscillator uses A. B.
tunnel diode UJT
C.
both tunnel diode and UJT
D.
PIN diode
16. In a 3 phase bridge inverter with 180° mode of operation the numbers of thyristors conducting at one time are A. C.
1 3
B.
2
D.
4
17. A single phase full converter can operate in A.
4 quadrants
B.
3 quadrants
C. D.
2 quadrants
1 quadrant
18. In a 3 phase semiconverter, the freewheeling diode comes into operation only if firing angle A.
is zero
B.
is 60°
C. D.
is more than 60°
is more than 90°
19. In the circuit of figure, the load current when thyristor is off
A.
is constant
B.
increases exponentially
C. D.
decreases exponentially
zero
20. A full bridge inverter is fed by a battery of 30 V. The rms value of output voltage is A.
15 V
B.
152 V
C. D.
30 V
302 V
21. In brushless excitation system of modern alternators, the main exciter is A. B.
dc generator ac generator
C.
either dc or ac generator
D.
induction generator
22. The two-transistor model of a thyristor consists of two transistors : A.
one n-p-n and other p-n-p
B.
both p-n-p
C.
both n-p-n
D.
one n-p-n and other UJT
23. In a thyristor, the forward breakover voltage A.
is constant
B.
may be constant or may depend on gate current
C. D.
decreases as gate current is increased
increases as gate current is increased
24. The gate lead, in a thyristor is welded to : A. B.
p layer to which anode is connected n layer nearest to the anode terminal
C.
p layer nearest to the cathode terminal
D.
outside n layer
25. In a 3 phase full converter, the firing angle is less than 60°. The instantaneous output voltage A.
will have positive part only
B.
will have both positive and negative parts
C.
may have both positive and negative parts
D.
will have negative part if load is inductive
26. In a stud mounted SCR, the stud is A.
in thermal contact with heat sink
B.
insulated from heat sink
C. D.
in electrical and thermal contact with heat sink
in either electrical or thermal contact with heat sink
27. In pulse width modulation of chopper A.
T is kept constant and Ton is varied
B.
Ton is kept constant T is varied
C.
both T and Ton are varied
D.
either T or Ton is varied
28. In a full bridge inverter, the waveshape of load current depends on A.
relative values of load parameters R, L and C
B.
relative values of load parameters R and L
C.
relative values of load parameters L and C
D.
gating frequency
29. The characteristics of which device has peak point and valley point A. B.
SCR PUT
C.
SCS
D.
Shockley diode
30. In a step down chopper using time ratio control, the input voltage is V and duty cycle is . The load is purely resistive having resistance R. The average and rms load currents are A.
B.
C.
D.
31. In a thyristors are i = kva. The value of a is A.
about 0.5
B.
about 2
C. D.
about 5
about 20
32. In an ordinary thyristor A.
turn on and turn off times are equal
B.
turn on time is more than turn off time
C. D.
turn off time is more than turn on time
turn on time may be equal, less than or more than turn off time
33. Thyristors connected in series need : A.
static equalizing circuit
B.
dynamic equalizing circuit
C. D.
both static and dynamic equalizing circuits
none of the above
34. The symbol in figure is for
A.
SUS
B.
SBS
C.
SCS
D.
None of the above
35. If 4 quadrant operation is required we need A.
dual converter
B.
full converter
C.
semi converter
D.
none of the above
36. A single phase half wave converter is feeding a resistive load. Output dc power is P dcand total power delivered to load is P ac then A.
P dc = P ac for all values of firing angles
B.
P dc = P ac if firing angle is less than 180°
C.
P dc < P ac for all values of firing angles
D.
P dc < P ac if firing angle is less than 180°
37. Which of the following devices exhibit negative resistance characteristics? A.
UJT
B.
PUT
C. D.
Both UJT and PUT
UJT, PUT and SBS
38. In integral cycle control of ac regulators, the period of integral cycle control, i.e., (m +n) T should be A. B.
less than mechanical or thermal time constant of the system
more than the mechanical or thermal time constant of the system
C.
more or less than the mechanical or thermal time constant
D.
very high
39. All other parameters remaining the same, a 3 phase bridge inverter with 180° mode of operation A. B.
has the same peak thyristor current as in 120° mode of operation has higher peak thyristor current than in 120° mode of operation
C.
has lower peak thyristor current than in 120° mode of operation
D.
any of the above
40. As compared to BJT, MOSFET has A.
lower switching losses and higher conduction losses
B.
higher switching losses and higher conduction losses
C.
higher switching losses and lower conduction losses
D.
lower switching losses and lower conduction losses
41. If a step down chopper is fed by a dc voltage V with Ton and Toff are on and off periods then A.
is average output voltage
B.
rms output voltage
C.
average or rms output voltage depending on load
D.
neither of the above
42. A voltage VBB is applied across the terminals of a UJT. The emitter voltage at peak point is A.
hVBB
B.
(h + 1) VBB
C. D.
hVBB + VD
VD
43. A single phase bridge type cycloconverter uses eight thyristors. A.
True
B.
False
44. In the circuit of figure, the waveshape of load current when thyristor conducting is
A.
constant
B.
increasing exponentially
C.
decreasing exponentially
D.
either (b) or (c)
45. In the basic series inverter circuit, the source supplie s load A.
only in positive half cycle
B.
both in positive and negative half cycles
C.
only in part of positive half cycle
D.
in positive half cycle and part of negative half cycle
46. A current source inverter has simple commutation circuit. A.
True
B.
False
47. In load commutated chopper, the commutating element A.
is capacitor
B.
is capacitor and inductor
C.
is capacitor, inductor and auxiliary thyristor
D.
any of the above depending on load
48. In a half bridge inverter each thyristor is gated at a frequency f. The output frequency is A. B.
2f f
C.
0.5 f
D.
0.25 f
49. Assertion (A): A transistor has lower voltage drop during conduction as compared to thyristor Reason (R): Transistors are manufactured in very high voltage and current ratings. A.
Both A and R are correct and R is correct explanation of A
B.
Both A and R correct but R is not correct explanation of A
C. D.
A is correct but R is wrong
A is wrong but R is correct
50. Assertion (A): Typical value of thermal resistance from source to sink of an SCR is about 0.3°
C/W Reason (R): Heat sinks of thyristors are generally made of aluminium. A. B.
Both A and R are correct and R is correct explanation of A Both A and R correct but R is not correct explanation of A
C.
A is correct but R is wrong
D.
A is wrong but R is correct
SECTION 5 1.
Multiphase choppers are used in A. B.
2.
low power circuits high power circuit
C.
both low and high power circuits
D.
medium power circuits
In a single phase converter using a centre tapped transformer and with a highly inductive load, the firing angles is . Each thyristor conducts for a period A.
B.
C.
-
D.
3.
In a parallel inverter A. B.
each thyristor is turned on twice during each cycle
each thyristor is turned on once during each cycle
4.
C.
each thyristor is turned on either once or twice in each load cycle
D.
each thyristor may be turned on upto four times in each load cycle
A step up chopper can give an output voltage A.
5.
B.
lower than input voltage
C.
both higher and lower input voltage
D.
all of the above
The specified maximum junction temperature of thyristors is about A. C.
6.
50°C 120°C
B.
80°C
D.
200°C
A bridge type single phase cycloconverter changes the frequency f to f/3. Then one half wave of output contains A. B.
7.
higher than input voltage
3 full waves of input 3 half waves of input
C.
6 full waves of input
D.
6 half waves of input
A half bridge inverter A.
needs a 3 wire dc supply
B.
may use a two wire or 3 wire dc supply
C.
has higher output voltage if fed from 3 wire dc supply
D.
8.
A 3 phase series inverter has A. B.
9.
has higher efficiency if fed from 3 wire dc supply
12 thyristors 6 thyristors
C.
3 thyristors
D.
3 or 6 thyristors
In figure, thyristor Th is in off state. When thyristor is turned on, the peak thyristor current can be
A.
5A
B.
10 A
C. D.
15 A
20 A
10. HVDC systems are preferable for only long distances. A.
True
B.
False
11. Surge current rating of an SCR is A. B.
the maximum repetitive surge current in which the SCR can withstand the maximum non-repetitive surge current in which the SCR can withstand
C.
the maximum repetitive or non repetitive surge current in which the SCR can withstand
D.
the maximum surge current in which the SCR can withstand if both surge current and steady current exist together
12. Which of the following devices has metal-silicon junction? A. B.
General purpose power diode Schottky diode
C.
SCR
D.
MOSFET
13. In a single phase full wave converter (M-2 connection) feeding on R-L load the input voltage is v = Vm sin ω t. The expression for rms output voltage is
A.
B.
C.
D.
14. A dc chopper is feeding an RLE load. The condition for maximum ripple is
=1
B.
= 0.8
C.
= 0.5
D.
< 0.5
A.
15. In a series inverter A. B.
the commutating elements L and C are in parallel the commutating elements L and C are in series with load
C.
the commutating L and C may be in series or parallel with the load
D.
none of the above
16. When a power diode is conducting, the voltage drop across it is about A.
IV
B.
5V
C.
20 V
D.
50 V
17. In thermal ohm's law, temperature and power loss are analogous to A. B.
V and I respectively of ohm's law I and V respectively of ohm's law
C.
R and V respectively of ohm's law
D.
I and R respectively of ohm's law
18. A string of series connected thyristors needs equalising circuit to A.
ensure proper firing
B.
ensure proper current division
C. D.
ensure proper voltage division
none of the above
19. A surge voltage A.
has very high magnitude and very long duration
B.
has very high magnitude and very small duration
C. D.
has very high magnitude and very small duration and can be positive or negative
can be only positive
20. The minimum gate current which can turn on SCR is called : A.
trigger current
B.
holding current
C.
junction current
D.
breakover current
21. The total number of leads in SUS, SBS and SCS respectively are A.
3, 3, 4,
B.
3, 3, 3
C.
2, 3, 3
D.
3, 3, 5
22. Which of these devices can conduct in both directions? A. C.
SCR SBS
B.
SUS
D.
UJT
23. Assertion (A): A half wave controlled rectifier has poorer efficiency and higher ripple factor than a full wave controlled rectifier. Reason (R): The use of a freewheeling diode in half wave controlled rectifier circuit improves the waveform of load current and circuit power factor. A. B.
Both A and R are correct and R is correct explanation of A Both A and R correct but R is not correct explanation of A
C.
A is correct but R is wrong
D.
A is wrong but R is correct
24. A 3 phase dual converter consists of A.
two 3 phase semiconverters connected in parallel
B.
two 3 phase semiconverters connected in antiparallel
C. D.
two 3 phase full converters connected in antiparallel
two 3 phase full converters connected in parallel
25. Which inverter circuit uses coupled inductors? A.
Modified McMurray full bridge inverter
B.
McMurray Bedford half bridge inverter
C.
Modified McMurray Bedford half bridge inverter
D.
Both (b) and (c)
26. When an SCR is in forward blocking mode, the applied voltage A.
appears across only one junction
B.
is equally shared by two junctions
C.
is equally shared by three junctions
D.
may appear across one or more junctions
27. In sinusoidal PWM for inverters the width of different pulses are not the same. A.
True
B.
False
28. A class D chopper A.
can operate in first quadrant only
B.
can operate in second quadrant only
C. D.
can operate in either first or fourth quadrant
can operate in all the quadrants
29. The symbol in figure is for
A.
UJT
B.
C.
SCS
D.
30. The efficiency of a chopper circuit is about A.
80% or more
PUT
SBS
B.
around 50%
C.
around 20%
D.
around 5%
31. If the frequency is less than the damped frequency in a series - inverter A. B.
the distortion of output voltage waveform is low the distortion of output voltage waveform is high
C.
the distortion of output voltage may be high or low depending on the load impedance
D.
none of the above
32. The output frequency of a cycloconverter can be changed by changing the firing angle. A.
True
B.
False
33. Mc Murray full bridge inverter uses A.
4 thyristors and 2 diodes
B.
4 thyristors and 4 diodes
C. D.
8 thyristors and 4 diodes
8 thyristors and 8 diodes
34. Assertion (A): A DIAC has four layers but only two terminals. Reason (R): A DIAC can conduct in both directions. A. B. C.
Both A and R are correct and R is correct explanation of A Both A and R correct but R is not correct explanation of A
A is correct but R is wrong
D.
A is wrong but R is correct
35. In a 3 phase full converter, the insta ntaneous output voltage can have negative part if A.
firing angle is less than 60°
B.
firing angle is less than 90°
C. D.
firing angle is more than 60°
firing angle is more than 90°
36. In a single phase full wave ac regulator using two thyristors only, the gate circuits of the two thyristors A.
must be isolated
B.
may or may not be isolated
C.
should be preferably, isolated
D.
should not be isolated
37. The circuit in figure is for
A.
resonant commutation
B.
line commutation
C. D.
auxiliary commutation complementary commutation
38. A fully controlled bridge converter is feeding an RLE load. For some circuit conditions firing angle is 47°. If connections of battery are reversed, the firing angle is likely to be A.
47°
B.
less than 47°
C. D.
more than 47°
either more or less than 47°
39. In a single phase half wave converter feeding resistive load and full wave converter (M - 2 connection) feeding a resistive load the circuit conditions are similar. The firing angle is . Then the waveshapes of voltages across thyristors A.
are similar
B.
are not similar
C.
may be similar or dissimilar
D.
are similar only for some values of
40. In a 3 phase full converter feeding a highly inductive load, the average load current is 150 A. The peak current through thyristor is A.
150 A
B.
75 A
C.
50 A
D.
300 A
41. A 3 phase bridge inverter is fed by 400 V battery. The load is star connected and has a resistance of 10 Ω per phase. The peak value of thyristor current is A.
B.
C.
D.
42. The main utility of SCR is due to A. B.
gate triggering
high temperature triggering
C.
D.
triggering high breakdown voltage
43. In a single phase full wave ac regulator the firing angles during both half cycles are equal. A.
True
B.
False
44. In a 3 phase full wave regulator feeding a star connected resistance load, firing angle is 75°. Then the number of thyristors conducting at one time is A.
1
B.
C.
3
D.
2
4
45. The symbol in figure is for
A.
SUS
B.
SBS
C. D.
Shockley diode
UJT
46. The turn on characteristics of a power transistor can be assumed to be linear, as shown in the below figure . During turn on time T, voltage reduces linearly from V to 0 and current rises linearly from 0 to I. The energy loss during time T is
A.
B.
C.
D.
VIT
47. The circuit in figure is for
A. B.
resonant commutation self commutation
C.
load commutation
D.
auxiliary commutation
48. A single phase half bridge inverter required to feed R-L loads needs A.
2 thyristors
B.
4 thyristors
C. D.
2 thyristors and two diodes
4 thyristors and 4 diodes
49. Typical values of A. B.
1 to 10 25 to 500
C.
0.1 to 1
D.
1000 to 2000
for an SCR are in the range of
50. When a dc chopper feeds an RLE load, the load current, during steady state operation A. B.
remains constant varies between maximum and minimum values
C.
may remain constant or vary
D.
is constant if R is constant
SECTION 6 1.
In a single phase full wave ac regulator feeding R-L load, the gate pulses A.
are always of short duration
B.
may be of short duration
C.
D.
2.
3.
should be rectangular with width ( - )
cannot be rectangular with width ( - )
In a semi converter feeding a resistive load, A.
the waveshapes of input voltage and input current are always similar
B.
the waveshapes of input voltage and input current can never be similar
C.
the waveshapes of input voltage and input current are similar only if firing angle is zero
D.
the waveshapes of input voltage and input current are similar only if firing angle is high
In a single phase semi-converter the number of thyristors is A.
16
B.
C.
4
D.
8 2
4.
A dc chopper is feeding an RLE load. The current can become discontinuous if A.
B.
C.
E is high
D.
5.
none of the above
A.
Class A
B.
Class B
C.
Class C Class D
A single phase half wave rectifier is feeding a resistive load. As firing angle is increased A. B.
7.
is low
Which of these commutation methods uses two leads?
D.
6.
is high
dc output voltage and rms output voltage decrease in the same proportion dc output voltage and rms output voltage decrease
C.
dc output voltage and rms output voltage increase
D.
dc output voltage and rms output voltage increase in the same proportion
A cycloconverter for single phase operation has A.
one positive group and one negative group converters
B.
two positive group and two negative group converters
C.
three positive group and three negative group converters
D.
any of the above
8.
A chopper is a high efficiency circuit. A.
9.
True
B.
False
The difference between resonant commutation and self commutation is A.
both these methods are based on entirely different principles
B.
in resonant commutation, the commutating elements carry load current but in self commutation, the commutating elements do not carry load current
C.
resonant commutation uses L and C but self commutation uses only capacitor
D.
none of the above
10. If 0.5 T is the time period of oscillations and Toff is the time between turn off of one thyristor and turn on of second thyristor, the frequency of output f of a series inverter is A.
B.
C.
D.
11. In a series inverter A.
the commutating elements carry full load curre nt
B.
the commutating elements carry upto 50% of load current
C.
the commutating elements do not carry load current
D.
any of the above
12. A NAND gate has 3 inputs and one output. The number of thyristors required for this gate are A.
1
B.
2
C. D.
3
3 or 4
13. Which of these devices is used in relaxation oscillator? A. C.
UJT
DIAC
B.
SCR
D.
TRIAC
14. In auxiliary commutated chopper fed by voltage V and having L and C commutating elements, the peak value of capacitor current is A.
B.
C.
D.
15. In a single phase semiconverter feeding a highly inductive load, the load current is A. B.
sinusoidal nearly constant
C.
absolutely constant
D.
rectangular
16. Two thyristors of the same rating and s pecifications A.
will have equal on state voltage drops
B.
will have equal forward blocking voltage for the same current
C.
will have different on state voltage drops may have equal or different on state voltage drops and equal or different forward
D.
blocking voltage's for the same current
17. A 3 phase induction motor is controlled by voltage and frequency control such that the ratio V/f is constant. At 50 Hz, the starting torque is 100 N-m. At 25 Hz, the starting torque A. B.
will be less than 100 N-m will be more than 100 N-m
C.
may be more or less than 100 N-m depending on motor parameters
D.
very small
18. In a three phase to single phase full wave bridge cycloconverter, the number of thyristors is A. C.
36 12
B.
18
D.
8
19. A single phase half wave converter is feeding-a resistor load. The input voltage is v = Vm sin ω t. If firing angle is zero, the output dc voltage Vdc and output rms voltage Vrmsrespectively A.
B.
C.
D.
20. In a cycloconverter it is possible to vary A.
only output frequency
B.
only output voltage
C.
both output voltage and frequency but not simultaneously
D.
both output voltage and output frequency simultaneously
21. A single phase full bridge inverter for R-L loads needs A. B.
4 thyristors 4 thyristors and 4 diodes
C.
4 thyristors and 2 diodes
D.
8 thyristors
22. In the basic series inverter circuit, the source current has A. B.
low harmonic content high harmonic content
C.
low or high harmonic content depending on firing angle
D.
zero harmonic content
23. Thyristors connected in series need static and dynamic stabilizing circuits. A.
True
B.
False
24. In a controlled rectifier a freewheeling diode is necessary if the load is A.
inductive
B.
resistive
C.
capacitive
D.
any of the above
25. For an illumination control circuit it is advisable to use a matched TRIAC -DIAC pair. A.
True
B.
False
26. A single phase converter is fed by a source having an inductance L. If load current is I 0, the decrease in output voltage due to overlap is A.
B.
C.
D.
27. Assertion (A): An ac regulator uses natural commutation Reason (R): An ac regulator can be used for static on load tap changing of transformer. A. B.
Both A and R are correct and R is correct explanation of A Both A and R correct but R is not correct explanation of A
C.
A is correct but R is wrong
D.
A is wrong but R is correct
28. When a UJT is used as a relaxation oscillator the current should b e A.
less than Ip
B.
more than Iv
C. D.
more than Ip but less than Iv
negative
29. In auxiliary commutated chopper, the charging and discharging time of capacitor A. B.
are constant depend on load current
C.
may be constant or may depend on load current
D.
none of the above
30. Which chopper circuit uses saturable reactor? A.
Auxiliary commutated
B.
Jones chopper
C. D.
Morgan chopper
Load commutated
31. A thyristor circuit is feeding an R-L load. The turn on time can be reduced by A.
decreasing R
B.
decreasing L
C.
increasing L
D.
decreasing R and L together
32. A single phase semiconverter is supplying RLE load and has a freewheeling diode across the load. The firing angle is . The freewheeling diode conducts for A.
° in
B.
( -
C.
° in
D.
( +
each half cycle
)° in each cycle one full cycle
)° in each half cycle
33. In a full bridge inverter fed by a battery of voltage V, the rms value of fundamental component of output voltage is A.
B.
C.
D.
34. In a single phase full wave regulator feeding a resistive load, the average current over one full cycle A.
is always zero
B.
cannot be zero
C.
may be sometimes zero
D.
is always positive
35. Two single phase semi-converters are connected in series to form a series converter. The input is v = Vm sin ω t and 1, 2 are the firing angles. If 1 = 0, the dc output voltage is A.
(1 + cos
2)
B.
C.
D.
(2 + cos
2)
(3 + cos
2)
(4 + cos
2)
36. The total number of leads in MOS controlled thyristor is A. B.
2 3
C.
4
D.
2 or 3
37. Assertion (A): Chopper always requires forced commutation Reason (R): Morgan chopper uses a saturable reactor. A. B.
Both A and R are correct and R is correct explanation of A Both A and R correct but R is not correct explanation of A
C.
A is correct but R is wrong
D.
A is wrong but R is correct
38. In a power diode the reverse recovery time is the time from the instant the forward current is zero to the instant when reverse recovery current has decayed to A.
50% of peak reverse current
B.
25% of peak reverse current
C.
10% of peak reverse current
D.
zero value
39. Assertion (A): McMurray Bredford half bridge inverter uses complementary commutation Reason (R): Three phase inverter can have two modes of operation. A. B.
Both A and R are correct and R is correct explanation of A Both A and R correct but R is not correct explanation of A
C.
A is correct but R is wrong
D.
A is wrong but R is correct
40. A capacitor C farads is charged to a voltage V. It discharges through a thyristor, a diode a nd an inductance L. The peak current in the circuit is A.
VCL
B.
C.
D.
41. Which of these devices have four layers? A. B.
UJT SCR
C.
TRIAC
D.
SCR and TRIAC
42. When UJT is used for thyristor gate signal, the output of UJT is A.
sine wave
B. C. D.
square wave saw tooth wave
either (a) or (c)
43. The stud in a stud mounted SCR acts as A.
anode
B.
cathode
C.
gate
D.
anode or cathode
44. In a single phase full wave regulator feeding R-L load, short gate pulse are not suitable. A.
True
B.
False
45. Half bridge and full bridge inverters of one type use different commutation methods. A.
True
B.
False
46. In rectifier circuits, the rms value of ac component of output voltage is equal to A.
Vrms
B.
0.5 Vrms
C.
(Vrms - Vdc)0.5
D.
(Vrms2 - Vdc2)0.5
47. A UJT exhibits negative resistance characteristics when A.
Current is less than valley current
B. C. D.
Current is less than peak current Current is more than peak current but less than valley current
None of the above
48. A dc chopper feeds an RLE load and has a duty cycle of 1. The load current will be such tha t A.
B.
Imax > Imin
C.
Imin > 0
D.
Imin < 0
49. In rectifier circuits, ripple factor RF and form factor FF are related as A.
RF = (FF2 - 1)0.5
B.
RF = FF
C.
RF = FF - 1
D.
RF = (FF - 1)0.5
50. If we connect two silicon unilateral switches in anti- parallel we get A.
SCR
B.
C.
SCS
D.
SBS
UJT
SECTION 7 1.
Assertion (A): SUS is a PUT and avalanche diode connected between gate and cathode Reason (R): SBS has two SUS inbuilt together and connected in anti-parallel A. B.
2.
A is correct but R is wrong
D.
A is wrong but R is correct
In a single phase full wave controlled rectifier using centre tap transformer, the voltage across each half of secondary is Vm sin ω t. The peak inverse voltage is 2 Vm
B.
Vm
C.
0.5 Vm
D.
0.25 Vm
Which statement is correct? A.
BJT and MOSFET are current controlled devices
B.
BJT is voltage controlled and MOSFET is current controlled device
C.
BJT and MOSFET are voltage controlled devices
D.
4.
Both A and R correct but R is not correct explanation of A
C.
A.
3.
Both A and R are correct and R is correct explanation of A
BJT is current controlled and MOSFET is voltage controlled device
In a three phase series inverter, the firing frequency is A. B.
three times output frequency six times output frequency
5.
C.
twice the output frequency
D.
equal to output frequency
A 3 phase fully controlled converter is a A. B.
6.
7.
6 pulse converter
C.
2 pulse converter
D.
12 pulse converte
In a half bridge inverter the gate pulses of the two thyristors have a phase displacement of A.
60°
B.
C.
120°
D.
90° 180°
A fully controlled bridge converter can operate in quadrants A.
1
B.
1 and 2
C.
1 and 3
D.
8.
3 pulse converter
1 and 4
A class E chopper A.
can operate in second quadrant only
B.
can operate in first or third quadrant
C. D.
can operate in all the four quadrants
can operate in either second or third quadrant
9.
A cycloconverter uses A.
natural commutation
B.
forced commutation
C. D.
either natural or forced commutation
both natural and forced commutation together
11. The commutation method in an inverter, is A.
line commutation
B.
forced commutation
C. D.
either (a) or (b)
none of the above
12. Inverters find applications in A.
HVDC transmission
B.
UPS
C.
variable speed ac drives
D.
all of the above
13. When a thyristor is conducting, the voltage drop across it is about A.
1V
B.
10 V
C.
100 V
D.
0.1 V
14. The use of free wheeling diode in controlled rectifier improves the waveshape of load current. A.
True
B.
False
15. As the firing of a thyristor is delayed, the output voltage of a controlled rectifier decreases. A.
True
B.
False
16. Assertion (A): A GTO can be turned on and turned off by gate pulses Reason (R): A GTO can be turned off by negative gate pulse. A. B.
Both A and R are correct and R is correct explanation of A Both A and R correct but R is not correct explanation of A
C.
A is correct but R is wrong
D.
A is wrong but R is correct
17. In a half bridge inverter circuit fed by a battery of voltage V, the rms value of fundamental component of output voltage is A.
2V
B.
C.
D.
18. In a full bridge single phase inverter feeding RLC load A.
one thyristor and one diode conduct simultaneously
B.
two thyristors conduct simultaneously
C. D.
two thyristors or two diodes conduct simultaneously
four thyristors conduct simultaneously
19. It is possible to use a freewheeling diode in A.
single phase half wave controlled rectifier as well as in single phase full wave controlled rectifier (M - 2 connection)
B.
single phase half wave controlled rectifier but not in single phase full wave controlled rectifier (M - 2 connection)
C.
single phase full wave controlled rectifier (M - 2 connection) but not in single phase half wave controlled rectifier
D.
all controlled rectifier circuits
20. Assertion (A): When the anode current of an SCR starts increasing during turn on process, the anode cathode voltage drop starts decreasing. Reason (R): During turn on the reverse biased junction in SCR breaksdown. A.
Both A and R are correct and R is correct explanation of A
B.
Both A and R correct but R is not correct explanation of A
C.
A is correct but R is wrong
D.
A is wrong but R is correct
21. In a certain inverter circuit each thyristor carries 26.67 A for 60°, 13.33 A for 120° and zero A for 180° of the cycle. The rms current of thyristor is A. B.
20 A 13.33 A
C.
6.67 A
D.
3.33 A
22. The factor η for a UJT is about A. C.
0.1 0.6
B.
0.2
D.
0.9
23. The symbol in figure is for
A.
UJT
B.
PUT
C.
Shockley diode
D.
TRIAC
24. In a single phase full converter fed by a source having inductance, the number of thyristors conducting during overlap is A.
1
B.
C.
3
D.
2 4
25. The cut in voltages for germanium and silicon diodes respectively as A.
0.3 V and 0.7 V
B.
0.7 V and 0.3 V
C.
0.6 V and 0.9 V
D.
0.5 V and 0.7 V
26. An SCR is triggered at 80° in the positive half cycle, only. The rms anode current is 25 A. If the firing angle is changed to 160°, the rms current is likely to be A.
25 A
B.
12.5 A
C.
less than 25 A
D.
less than 12.5 A
27. The output wave of a cycloconverter consists of a number of segments derived from the input wave. A.
True
B.
False
28. A class A chopper has an input voltage of 200 V. Ton = 1.5 ms and Toff = 4.5 ms. The duty cycle, average output voltage and ripple factor respectively are A. B.
0.5, 100 V and 1 0.25, 50 V and 3
C.
0.25, 50 V and 1
D.
0.5, 100 V and 3
29. In a parallel inverter the turn off time tq of thyristor and values of commutating elements L and C are related as A.
B.
C.
D.
tq =
LC
tq =
LC
tq =
LC
tq =
LC
31. Which of the following devices has two gates? A.
SCR
B.
C.
SUS
D.
SCS
SBS
32. In auxiliary commutated chopper, the peak value of thyristor current is A.
equal to peak value of load current
B.
equal to peak value of capacitor current
C. D.
sum of peak value of load current and capacitor current
none of the above
33. The symbol in figure is for
A. B.
SUS SBS
C.
SCS
D.
Shockley diode
34. If peak value of input voltage is Vm and firing angle is
, the average output voltage for single
phase half wave and single phase full wave regulators are A.
B.
C.
D.
35. The static equalisation circuit for thyristors connected in series is A. B.
one resistor in series with each thyristor one resistor in parallel with each thyristor
C.
one resistor and one capacitor in parallel with each thyristor
D.
none of the above
36. A cycloconverter is a group of controlled rectifiers. A.
True
B.
False
37. A single phase full wave regulator feeds a resistive load. The firing angle of thyristors is 90°. The input power factor is A. B.
1 0.707 lagging
C.
0.5 lagging
D.
0.2 lagging
38. The leads in a TRIAC are denoted by A. B.
anode cathode, gate T1 T2, gate
C.
T1, T2, anode
D.
T1, T2, T3
39. A step down cycloconverter has an input frequency f. If it uses natural commutation, the output frequency A.
must be a sub-multiple of input frequency
B.
can be any frequency
C.
must be an even sub-multiple of input frequency
D.
must be an odd sub-multiple of input frequency
40. In a single phase full wave regulator, the rms current in ea ch thyristors is Ir The run load current is equal to A. B. C.
2Ir 2 Ir
Ir
D.
41. Lead mounting for SCR is used for A. B.
rating less than mA rating about 1 A or less
C.
rating of 20 A or less than 20 A
D.
rating 100 A or less
42. A load commutated chopper circuit has
A.
one thyristor
B.
two thyristors
C. D.
four thyristors
six thyristors
43. Assertion (A): The output voltage of an inverter can be controlled by PWM technique Reason (R): In a current source inverter the input current is constant. A. B.
Both A and R are correct and R is correct explanation of A Both A and R correct but R is not correct explanation of A
C.
A is correct but R is wrong
D.
A is wrong but R is correct
44. A TRIAC is equivalent to A. B.
two thyristors in series two thyristors in parallel
C.
one thyristor and one diode
D.
one thyristor and one transistor
45. Which of the following commutation methods is the overall circuit under damped? A.
Class A
B.
Class B
C.
Class C
D.
Both class A and Class B
46. In a half bridge inverter, the freewheeling diodes are needed A.
even when load is resistive
B.
only when load is inductive
C. D.
only when load is inductive or capacitive
for all types of loads
47. When a thyristor is conducting, the voltage drop across it A.
is absolutely constant
B.
decreases with increase in load current
C. D.
increases slightly with increase in load current
none of the above
48. The advantage of modified series inverter with two inductors and two capacitors is A.
the intermittent operation of battery is avoided
B.
efficiency of circuit is more
C.
the output frequency is low
D.
all of the above
49. An SUS is A.
exactly similar to SCR but with higher rating
B.
similar to UJT
C. D.
PUT and avalanche diode connected in anti parallel
SCR and avalanche diode connected in parallel
50. Which chopper circuit operates in quadrants 1 and 4? A. C.
A D
B.
B
D.
E
SECTION 8 1.
In a 3 phase bridge inverter, the gating signals for the three phases have a phase difference of A.
2.
120°
B.
60°
C.
240°
D.
any of the above
Assertion (A): The circuit of figure thyristor will conduct for 180° during positive half cycle if it is continuously fired.
Reason (R): A thyristor can conduct only when it is forward biased. A.
Both A and R are correct and R is correct explanation of A
B.
Both A and R correct but R is not correct explanation of A
C.
A is correct but R is wrong
D.
A is wrong but R is correct
3.
The total number of leads in SCR, DIAC and TRIAC respectively are A.
4.
B.
2, 3, 3
C.
3, 3, 2
D.
3, 2, 4
A single phase semiconverter is feeding a highly inductive load and has a freewheeling diode across the load. The waveshape of input current is A. B.
5.
3, 2, 3
sinusoidal rectangular
C.
sinusoidal or rectangular
D.
triangular
In the below figure, the load is highly inductive. The firing angle is The periods of conduction of Th 1 and Th 2 respectively are
A.
0 to
and to 2
B.
to
and +
to
+
to
-
C.
D.
to 2
and +
and +
to 2 +
to 2 -
for each of the thyristors.
6.
A crowbar circuit is generally needed for protection under fault conditions. A.
True
B.
False
7. Forward
8.
A.
the maximum rate of rise of anode voltage which will trigger the SCR
B.
the maximum rate of rise of anode voltage which will not trigger the SCR if gate signal is applied
C.
the maximum rate of rise of anode voltage which will not trigger the SCR if gate signal is not applied
D.
either (b) or (c)
The number of leads in a DIAC are A. C.
9.
rating of an SCR is
2
4
B.
3
D.
6
The condition for underdamped oscillations in a series RLC circuit is A.
B.
C.
D.
10. The method of triggering SCS is A.
application of positive pulse at cathode gate or negative pulse at an ode gate
B.
application of pulse at anode gate
C.
application of pulse at cathode gate
D.
application of low voltage at anode
11. The number of thyristors in a single phase fully controlled bridge rectifier is A. C.
16 4
B.
8
D.
2
12. In a bridge inverter A. B.
an output transformer is essential an output transformer is not essential
C.
an output transformer cannot be connected
D.
an output transformer, if used, can improve the efficiency of the circuit
13. An ideal rectifier should have 100% efficiency, zero ripple factor, zero harmonic factor and unity displacement A.
factor
B.
100% efficiency, zero tipple factor, zero harmonic factor and zero displacement factor
C.
100% efficiency, zero ripple factor, unity harmonic factor and unity displacement factor
D.
100% efficiency, unity ripple factor, zero harmonic factor and unity displacement factor
14. When thyristors are connected in parallel, the current distribution may become non-uniform due to A.
inductive effect of current carrying conductors
B.
capacitive effect of current carrying conductors
C.
both inductive and capacitive effects
D.
none of the above
15. A single phase fully controlled bridge converter is fed by a voltage V m sin ω t. The average and rms values of output voltage are A.
B.
C.
D.
cos
cos
and Vm
and 2 Vm
16. In a single phase half wave ac regulator feeding a resistive load, the rms load current A.
is always positive
B.
may be positive or negative
C.
is always negative
D.
can be even zero
17. Which of the following is correct for a BJT? A.
B.
C.
D.
18. In a bridge inverter with single pulse width modulation A.
the output frequency is equal to frequency of refe rence signal
B.
the output frequency is equal to frequency of control signal
C.
either (a) or (b)
D.
neither (a) nor (b)
19. In a modified series inverter with two inductors and two capacitors, the load current A.
has 2 parallel paths
B.
has only one parallel path
C.
can have three parallel paths
D.
has four parallel paths
20. The number of layers in a Shockley diode are A.
2
B.
3
C. D.
4
2 or 3
21. In a modified series inverter A. B.
the maximum output frequency is less than frequency of damped o scillations the output frequency can be even higher than the frequency of damped oscillations
C.
the output frequency cannot exceed the frequency of damped oscillation s
D.
any of the above
22. Assertion (A): Light triggering is very suitable for HVDC transmission. Reason (R): Light triggering has the advantage of complete electrical isolation of gate circuit. A.
Both A and R are correct and R is correct explanation of A
B.
Both A and R correct but R is not correct explanation of A
C.
A is correct but R is wrong
D.
A is wrong but R is correct
23. Class A and class B chopper A.
operate in first quadrant
B.
operate in second quadrant
C. D.
class A chopper operates in first quadrant while class B operates in second quadrant
may operate in either first or second quadrant
24. To turn off an SCR, it is necessary to reduce its current to less than : A. B.
trigger current holding current
C.
breakover current
D.
none of the above
25. When a single phase dual converter is operating in circulating current mode A. B.
both converters are working as rectifiers simultaneously one converter is always working as rectifier and the other as inverter
C.
only one converter is working at one time
D.
both converters are either working as rectifiers or as inverters simultaneously
26. The load circuit of a single phase full bridge inverter A.
must be underdamped
B.
must be overdamped
C.
may be underdamped or overdamped
D.
may be any type of load
27. When an SCR is reverse biased A.
two junctions are reverse biased and one junction is forward biased.
B.
all the three junctions are reverse biased.
C.
one junction is reverse biased and two junctions are forward biased.
D.
any of the above depending on the magnitude of reverse bias.
28. A dc series motor is coupled to a load whose torque is proportional to speed. It is fed by a single phase ac supply through a full converter. When speed is 500 rpm, firing angle is 45°. It is desired to raise the speed. It will be necessary to A.
decrease the firing angle
B.
increase the firing angle
C.
either decrease or increase the firing angle depending on the required speed
D.
decrease the load torque
29. In Jone chopper, the commutating element A.
is a capacitor
B.
is a capacitor and an inductor
C. D.
is a capacitor and an auto transformer
none of the above
30. A LASCR has A.
4 semiconductor layers and 3 junctions
B.
3 semiconductor layers and 2 junctions
C.
2 semiconductor layers and 2 junctions
D.
any of these depending on the rating
31. A fully controlled bridge converter is feeding an RLE load. The load current is I 0. The input voltage is Vm sin ω t. The KVL equation to find firing angle is
A.
B.
C.
D.
cos
cos
cos
cos
= E + I0R
= E + I0R
= E - I0R
= E - I0R
32. A single phase cycloconverter with centre tapped input transformer requires A. B.
2 thyristors 4 thyristors
C.
8 thyristors
D.
either 2 or 4 thyristors
33. The duty cycle of a step down chopper is A.
B.
C.
D.
34. In a 3 phase bridge inverter with 120° mode of operation the. number of thyristors conducting at one time are A.
1
B.
C.
3
D.
2
4
35. A rectifier is fed by a transformer. The rms voltage and current at secondary of transfo rmer are Vs and Is. Output dc power is Vdc Idc and output ac power is (Vrms ) (Irms), Transformer utilisation factor TUF is A.
B.
C.
D.
36. Assertion (A): A class C chopper can operate in first and second quadrants Reason (R): A class B chopper operates only in second quadrant. A. B.
Both A and R are correct and R is correct explanation of A Both A and R correct but R is not correct explanation of A
C.
A is correct but R is wrong
D.
A is wrong but R is correct
37. A single phase dual converter consists of A. B.
two fully controlled converters connected in parallel two fully controlled converters connected in antiparallel
C.
two fully controlled semiconverters connected in antiparallel
D.
two fully controlled semiconverters connected in parallel
38. The snubber circuit consists of A.
a capacitor connected in series with thyristor
B.
a capacitor connected in parallel with thyristor
C. D.
a resistor and capacitor in series connected across thyristor
none of the above
39. The effect of source inductance in a 3 phase fully controlled bridge converter is to A.
decrease the output voltage
B.
increase the output voltage
C.
decrease the output voltage and increase the output current
D.
increase the output voltage and decrease the output current
40. In disc mounting of SCR A. B.
SCR is clamped on one side of disc shaped heat sink SCR is clamped between two disc shaped heat sinks
C.
The SCR has low current rating
D.
None of the above
41. As compared to a p-n junction diode A.
a schottky diode has lower cut in voltage and lower reverse saturation current
B.
a schottky diode has higher cut in voltage and higher reverse saturation current
C. D.
a Schottky diode has lower cut in voltage and higher reverse saturation current
a schottky diode has higher cut in voltage and lower reverse saturation current
42. From which instant is the turn off time of a thyristor defined A. B.
when anode voltage becomes zero when anode current becomes zero
C.
when both anode voltage and current become zero
D.
none of the above
43. In a single phase cycloconverter with centre tapped transformer A. B.
all thyristors carry currents for equal duration the durations of currents in all thyristors may or may not be equal
C.
the durations of currents in all thyristors cannot be equal
D.
the duration of currents in all thyristors must be equal for satisfactory operation
44. If Ar and Ac are the amplitudes of reference and control signals, the modulation index Mis A.
B.
C.
D.
45. Thyristors can be used for control of A.
dc separately excited motor
B.
dc shunt motor
C.
dc series motor
D.
all of the above
46. Assertion (A): Thyristors can be used in controlled heating, excitation systems of alternators, speed control of motors and HVDC Reason (R): A static var system using thyristors is very commonly used in high voltage ac systems. A.
Both A and R are correct and R is correct e xplanation of A
B.
Both A and R correct but R is not correct explanation of A
C.
A is correct but R is wrong
D.
A is wrong but R is correct
47. In a dual converter operated in circulating current mode, the circulating current flows A.
when load current is zero
B.
when load current is non-zero
C. D.
even when load current is zero
none of the above
48. For a single phase half wave converter feeding a resistive load, the firing angle in below figure is for
A.
output voltage
B.
input voltage
C.
voltage drop across thyristor
D.
none of the above
. The waveshape
49. A class B chopper operates in third quadrant. A.
True
B.
False
50. In a full bridge circuit supplying an RLC under damped circuit A.
the waveshapes of output voltage and load current are square
B.
the waveshapes of output voltage and load current are sinusoidal
C.
the waveshapes of output voltage is square but the waveshape of load current is nearly sinusoidal
D.
none of the above
SECTION 9 1.
In a semi converter feeding a resistive load A.
2.
the waveshapes of output voltage and output current are exactly similar
B.
the waveshapes of output voltage and output current can not be similar
C.
the waveshapes of output voltage and output current are similar only for low values of firing angle
D.
the waveshapes of output voltage and output current may be similar or dissimilar
In an n stage high frequency series inverter having a time period T, each stage is active for a time interval of A.
B.
C.
D.
T
3.
In a 3 phase semiconverter, the output voltage becomes discontinuous for A. B.
4.
firing angle > 30° firing angle > 60°
C.
firing angle > 90°
D.
firing angle > 120°
A single phase fully controlled bridge converter is feeding an RLE load. Input voltage is V m sin ω t and firing angle is . If average current is I0, then
A.
cos
B.
cos
C.
cos
D.
5.
cos
= RI0 + E
= RE 0 + E
= E 0 - RI0
= E - RI0
In integral cycle control of ac regulator, the input power factor is equal to A.
duty cycle
B.
2
C.
D.
6.
( )1/3
McMurrary full bridge inverter uses A. B.
auxiliary commutation
complementary commutation
7.
C.
natural commutation
D.
any of the above
A fully controlled bridge converter feeds a highly inductive load such that the load current can be assumed to be constant. The wave shape of input current is A.
dc
B.
sinusoidal
C. D.
8.
none of the above
A static excitation system uses A.
9.
rectangular
transformer and thyristors
B.
transformer and ac exciter
C.
dc exciter and ac exciter
D.
transformer and dc exciter
The application of gate signal decreases the break over voltage of on SCR. A.
True
B.
False
10. The power factor at the input terminals of a cyclo - converter is generally A. B.
low and leading low and lagging
C.
high and leading
D.
high and lagging
11. Ripple factor RF and form factor FF are related as A. B.
RF = FF2 - 1 RF = (FF2 - 1)0.5
C.
RF = FF - 1
D.
RF = (FF - 1)0.5
12. Modified Mc Murray full bridge inverter uses A.
4 thyristors and 2 diodes
B.
4 thyristors and 4 diodes
C.
8 thyristors and 4 diodes
D.
8 thyristors and 8 diodes
13. Assertion (A): Gate triggering is the most commonly used method for triggering of an SCR. Reason (R): Even a small gate current is sufficient to turn on an SCR. A.
Both A and R are correct and R is correct explanation of A
B.
Both A and R correct but R is not correct explanation of A
C.
A is correct but R is wrong
D.
A is wrong but R is correct
- END – Gambate ECE 5B!! ^^
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