Electrical Instrument
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To Be A World Class Maritime Academy
Electrical Instrument/ sul
March 2008
1
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Electrical Instrument/ sul
March 2008
2
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Electrical Instrument/ sul
March 2008
3
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Construction • •
•
A deflecting or operating device. A is produced by the current or voltage mechanical force which causes the pointer to deflect from its zero position. A controlling device. The controlling force acts in opposition to the deflecting force and ensures that the deflection shown on the meter is always the same for a given measured quantity. There are two main types of controlling device – spring control and gravity control. A damping device. The damping force ensures that the pointer comes to rest in its final position quickly and without undue oscillation. There are three main types of damping used eddy-current damping, air-friction damping and fluid-friction damping.
Electrical Instrument/ sul
March 2008
4
To Be A World Class Maritime Academy
Scale •
There are basically two types of scale – linear and non-linear. • A linear scale is shown in Figure 6.1(a), where the divisions or graduations are evenly spaced. The voltmeter shown has a range 0–100 V, i.e. a full-scale deflection (f.s.d.) of 100 V. • A non-linear scale is shown in Figure 6.1(b). The scale is cramped at the beginning and the graduations are uneven throughout the range. The ammeter shown has a f.s.d. of 10 A.
Electrical Instrument/ sul
March 2008
5
To Be A World Class Maritime Academy
Moving-iron instrument •
•
An attraction type of moving-iron instrument is shown diagrammatically in Figure 6.2(a). When current flows in the solenoid, a pivoted soft-iron disc is attracted towards the solenoid and the movement causes a pointer to move across a scale. In the repulsion type moving-iron instrument shown diagrammatically in Figure 6.2(b), two pieces of iron are placed inside the solenoid, one being fixed, and the other attached to the spindle carrying the pointer.
Electrical Instrument/ sul
March 2008
6
To Be A World Class Maritime Academy
Moving-iron instrument
Electrical Instrument/ sul
March 2008
7
To Be A World Class Maritime Academy
Ammeter and voltmeter • An ammeter, which measures current, has a low resistance (ideally zero) and must be connected in series with the circuit. • A voltmeter which measure p.d., has a high resistance (ideally infinite) and must be connected in parallel with the part of the circuit whose p.d. is required.
Electrical Instrument/ sul
March 2008
8
To Be A World Class Maritime Academy
Ammeter and voltmeter •
• •
•
When an ammeter is required to measure currents of larger magnitude, a proportion of the current is diverted through a low-value resistance connected in parallel with the meter. Such a diverting resistor is called a shunt. From Figure 6.3(a), VPQ = VRS. Hence Iara = IsRs The milliammeter is converted into a voltmeter by connecting a high value resistance (called a multiplier) in series with it as shown in Figure 6.3(b). From Figure 10.4(b), V = Va + VM = Ira + IRM
Electrical Instrument/ sul
March 2008
9
To Be A World Class Maritime Academy
Ohmmeter •
The ohmmeter An ohmmeter is an instrument for measuring electrical resistance. A simple ohmmeter circuit is shown in Figure 6.4. • Unlike the ammeter or voltmeter, the ohmmeter circuit does not receive the energy necessary for its operation from the circuit under test. • In the ohmmeter this energy is supplied by a self-contained source of voltage, such as a battery.
Electrical Instrument/ sul
March 2008
10
To Be A World Class Maritime Academy
Ohmmeter • Initially, terminals XX are short-circuited and R adjusted to give f.s.d. on the milliammeter. If current I is at a maximum value and voltage E is constant, then resistance R = E/I is at a minimum value. Thus f.s.d. on the milliammeter is made zero on the resistance scale. • When terminals XX are open circuited no current flows and R(= E/O) is infinity. • The milliammeter can thus be calibrated directly in ohms. When calibrated, an unknown resistance is placed between terminals XX and its value determined from the position of the pointer on the scale
Electrical Instrument/ sul
March 2008
11
To Be A World Class Maritime Academy
Multimeters also called universal instruments or multirange instruments. • Instruments are manufactured that combine a moving-coil meter with a number of shunts and series multipliers, to provide a range of readings on a single scale graduated to read current and voltage. • If a battery is incorporated then resistance can also be measured. • Often such instruments can be used in a.c. as well as d.c. circuits when a rectifier is incorporated in the instrument. Electrical Instrument/ sul
March 2008
12
To Be A World Class Maritime Academy
Instrument ‘loading’ effect • Some measuring instruments depend for their operation on power taken from the circuit in which measurements are being made. Depending on the ‘loading’ effect of the instrument (i.e. the current taken to enable it to operate), the prevailing circuit conditions may change. The resistance of voltmeters may be calculated since each have a stated • sensitivity (or ‘figure of merit’), often stated in ‘k_ per volt’ of f.s.d. A voltmeter should have as high a resistance as possible (_ ideally infinite). In a.c. circuits the impedance of the instrument varies with frequency and thus the loading effect of the instrument can change.
Electrical Instrument/ sul
March 2008
13
To Be A World Class Maritime Academy
Electrical Instrument/ sul
March 2008
14
To Be A World Class Maritime Academy
Electrical Instrument/ sul
March 2008
15
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Electrical Instrument/ sul
March 2008
16
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Electrical Instrument/ sul
March 2008
17
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Electrical Instrument/ sul
March 2008
18
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Electrical Instrument/ sul
March 2008
19
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Electrical Instrument/ sul
March 2008
20
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Electrical Instrument/ sul
March 2008
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March 2008
23
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Electrical Instrument/ sul
March 2008
24
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Electrical Instrument/ sul
March 2008
25
To Be A World Class Maritime Academy
Electrical Instrument/ sul
March 2008
26
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March 2008
27
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March 2008
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Electrical Instrument/ sul
March 2008
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Electrical Instrument/ sul
March 2008
30
To Be A World Class Maritime Academy
Electrical Instrument/ sul
March 2008
31
To Be A World Class Maritime Academy
Electrical Instrument/ sul
March 2008
32
To Be A World Class Maritime Academy
Electrical Instrument/ sul
March 2008
33
To Be A World Class Maritime Academy
Electrical Instrument/ sul
March 2008
34
To Be A World Class Maritime Academy
Electrical Instrument/ sul
March 2008
35
To Be A World Class Maritime Academy
Electrical Instrument/ sul
March 2008
36
To Be A World Class Maritime Academy
Electrical Instrument/ sul
March 2008
37
To Be A World Class Maritime Academy
Electrical Instrument/ sul
March 2008
38
To Be A World Class Maritime Academy
Electrical Instrument/ sul
March 2008
39
To Be A World Class Maritime Academy
Electrical Instrument/ sul
March 2008
40
To Be A World Class Maritime Academy
Electrical Instrument/ sul
March 2008
41
To Be A World Class Maritime Academy
Electrical Instrument/ sul
March 2008
42
To Be A World Class Maritime Academy
Electrical Instrument/ sul
March 2008
43
To Be A World Class Maritime Academy
Electrical Instrument/ sul
March 2008
44
To Be A World Class Maritime Academy
Electrical Instrument/ sul
March 2008
45
To Be A World Class Maritime Academy
Electrical Instrument/ sul
March 2008
46
To Be A World Class Maritime Academy
Electrical Instrument/ sul
March 2008
47
To Be A World Class Maritime Academy
Electrical Instrument/ sul
March 2008
48
To Be A World Class Maritime Academy
Electrical Instrument/ sul
March 2008
49
To Be A World Class Maritime Academy
Electrical Instrument/ sul
March 2008
50
To Be A World Class Maritime Academy
Electrical Instrument/ sul
March 2008
51
To Be A World Class Maritime Academy
Electrical Instrument/ sul
March 2008
52
Electrical Instrument/ sul
March 2008
1
To Be A World Class Maritime Academy
Electrical Instrument/ sul
March 2008
2
To Be A World Class Maritime Academy
Electrical Instrument/ sul
March 2008
3
To Be A World Class Maritime Academy
Construction • •
•
A deflecting or operating device. A is produced by the current or voltage mechanical force which causes the pointer to deflect from its zero position. A controlling device. The controlling force acts in opposition to the deflecting force and ensures that the deflection shown on the meter is always the same for a given measured quantity. There are two main types of controlling device – spring control and gravity control. A damping device. The damping force ensures that the pointer comes to rest in its final position quickly and without undue oscillation. There are three main types of damping used eddy-current damping, air-friction damping and fluid-friction damping.
Electrical Instrument/ sul
March 2008
4
To Be A World Class Maritime Academy
Scale •
There are basically two types of scale – linear and non-linear. • A linear scale is shown in Figure 6.1(a), where the divisions or graduations are evenly spaced. The voltmeter shown has a range 0–100 V, i.e. a full-scale deflection (f.s.d.) of 100 V. • A non-linear scale is shown in Figure 6.1(b). The scale is cramped at the beginning and the graduations are uneven throughout the range. The ammeter shown has a f.s.d. of 10 A.
Electrical Instrument/ sul
March 2008
5
To Be A World Class Maritime Academy
Moving-iron instrument •
•
An attraction type of moving-iron instrument is shown diagrammatically in Figure 6.2(a). When current flows in the solenoid, a pivoted soft-iron disc is attracted towards the solenoid and the movement causes a pointer to move across a scale. In the repulsion type moving-iron instrument shown diagrammatically in Figure 6.2(b), two pieces of iron are placed inside the solenoid, one being fixed, and the other attached to the spindle carrying the pointer.
Electrical Instrument/ sul
March 2008
6
To Be A World Class Maritime Academy
Moving-iron instrument
Electrical Instrument/ sul
March 2008
7
To Be A World Class Maritime Academy
Ammeter and voltmeter • An ammeter, which measures current, has a low resistance (ideally zero) and must be connected in series with the circuit. • A voltmeter which measure p.d., has a high resistance (ideally infinite) and must be connected in parallel with the part of the circuit whose p.d. is required.
Electrical Instrument/ sul
March 2008
8
To Be A World Class Maritime Academy
Ammeter and voltmeter •
• •
•
When an ammeter is required to measure currents of larger magnitude, a proportion of the current is diverted through a low-value resistance connected in parallel with the meter. Such a diverting resistor is called a shunt. From Figure 6.3(a), VPQ = VRS. Hence Iara = IsRs The milliammeter is converted into a voltmeter by connecting a high value resistance (called a multiplier) in series with it as shown in Figure 6.3(b). From Figure 10.4(b), V = Va + VM = Ira + IRM
Electrical Instrument/ sul
March 2008
9
To Be A World Class Maritime Academy
Ohmmeter •
The ohmmeter An ohmmeter is an instrument for measuring electrical resistance. A simple ohmmeter circuit is shown in Figure 6.4. • Unlike the ammeter or voltmeter, the ohmmeter circuit does not receive the energy necessary for its operation from the circuit under test. • In the ohmmeter this energy is supplied by a self-contained source of voltage, such as a battery.
Electrical Instrument/ sul
March 2008
10
To Be A World Class Maritime Academy
Ohmmeter • Initially, terminals XX are short-circuited and R adjusted to give f.s.d. on the milliammeter. If current I is at a maximum value and voltage E is constant, then resistance R = E/I is at a minimum value. Thus f.s.d. on the milliammeter is made zero on the resistance scale. • When terminals XX are open circuited no current flows and R(= E/O) is infinity. • The milliammeter can thus be calibrated directly in ohms. When calibrated, an unknown resistance is placed between terminals XX and its value determined from the position of the pointer on the scale
Electrical Instrument/ sul
March 2008
11
To Be A World Class Maritime Academy
Multimeters also called universal instruments or multirange instruments. • Instruments are manufactured that combine a moving-coil meter with a number of shunts and series multipliers, to provide a range of readings on a single scale graduated to read current and voltage. • If a battery is incorporated then resistance can also be measured. • Often such instruments can be used in a.c. as well as d.c. circuits when a rectifier is incorporated in the instrument. Electrical Instrument/ sul
March 2008
12
To Be A World Class Maritime Academy
Instrument ‘loading’ effect • Some measuring instruments depend for their operation on power taken from the circuit in which measurements are being made. Depending on the ‘loading’ effect of the instrument (i.e. the current taken to enable it to operate), the prevailing circuit conditions may change. The resistance of voltmeters may be calculated since each have a stated • sensitivity (or ‘figure of merit’), often stated in ‘k_ per volt’ of f.s.d. A voltmeter should have as high a resistance as possible (_ ideally infinite). In a.c. circuits the impedance of the instrument varies with frequency and thus the loading effect of the instrument can change.
Electrical Instrument/ sul
March 2008
13
To Be A World Class Maritime Academy
Electrical Instrument/ sul
March 2008
14
To Be A World Class Maritime Academy
Electrical Instrument/ sul
March 2008
15
To Be A World Class Maritime Academy
Electrical Instrument/ sul
March 2008
16
To Be A World Class Maritime Academy
Electrical Instrument/ sul
March 2008
17
To Be A World Class Maritime Academy
Electrical Instrument/ sul
March 2008
18
To Be A World Class Maritime Academy
Electrical Instrument/ sul
March 2008
19
To Be A World Class Maritime Academy
Electrical Instrument/ sul
March 2008
20
To Be A World Class Maritime Academy
Electrical Instrument/ sul
March 2008
21
To Be A World Class Maritime Academy
Electrical Instrument/ sul
March 2008
22
To Be A World Class Maritime Academy
Electrical Instrument/ sul
March 2008
23
To Be A World Class Maritime Academy
Electrical Instrument/ sul
March 2008
24
To Be A World Class Maritime Academy
Electrical Instrument/ sul
March 2008
25
To Be A World Class Maritime Academy
Electrical Instrument/ sul
March 2008
26
To Be A World Class Maritime Academy
Electrical Instrument/ sul
March 2008
27
To Be A World Class Maritime Academy
Electrical Instrument/ sul
March 2008
28
To Be A World Class Maritime Academy
Electrical Instrument/ sul
March 2008
29
To Be A World Class Maritime Academy
Electrical Instrument/ sul
March 2008
30
To Be A World Class Maritime Academy
Electrical Instrument/ sul
March 2008
31
To Be A World Class Maritime Academy
Electrical Instrument/ sul
March 2008
32
To Be A World Class Maritime Academy
Electrical Instrument/ sul
March 2008
33
To Be A World Class Maritime Academy
Electrical Instrument/ sul
March 2008
34
To Be A World Class Maritime Academy
Electrical Instrument/ sul
March 2008
35
To Be A World Class Maritime Academy
Electrical Instrument/ sul
March 2008
36
To Be A World Class Maritime Academy
Electrical Instrument/ sul
March 2008
37
To Be A World Class Maritime Academy
Electrical Instrument/ sul
March 2008
38
To Be A World Class Maritime Academy
Electrical Instrument/ sul
March 2008
39
To Be A World Class Maritime Academy
Electrical Instrument/ sul
March 2008
40
To Be A World Class Maritime Academy
Electrical Instrument/ sul
March 2008
41
To Be A World Class Maritime Academy
Electrical Instrument/ sul
March 2008
42
To Be A World Class Maritime Academy
Electrical Instrument/ sul
March 2008
43
To Be A World Class Maritime Academy
Electrical Instrument/ sul
March 2008
44
To Be A World Class Maritime Academy
Electrical Instrument/ sul
March 2008
45
To Be A World Class Maritime Academy
Electrical Instrument/ sul
March 2008
46
To Be A World Class Maritime Academy
Electrical Instrument/ sul
March 2008
47
To Be A World Class Maritime Academy
Electrical Instrument/ sul
March 2008
48
To Be A World Class Maritime Academy
Electrical Instrument/ sul
March 2008
49
To Be A World Class Maritime Academy
Electrical Instrument/ sul
March 2008
50
To Be A World Class Maritime Academy
Electrical Instrument/ sul
March 2008
51
To Be A World Class Maritime Academy
Electrical Instrument/ sul
March 2008
52
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