Physics Dc Gen.
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Welcome to Lectures on D.C.Generators
03/23/09
1
DIRECT CURRENT MACHINES DC machine Construction •
The major advantages of DC machines are: easy speed and torque regulation.
•
The stator of the DC motor has poles, which are excited by DC current to produce magnetic fields.
•
The rotor has a ring-shaped laminated iron-core with slots.
•
Coils with several turns are placed in the slots. The distance between the two legs of the coil is about 180 electric degrees.
03/23/09
Field Rotor
S
N
Brush
Stator with with poles
2
DC Generators. •
Basic operation of the generator – As the loop rotates, the magnetic flux through it changes with time – This induces an emf and a current in the external circuit – The ends of the loop are connected to slip rings that rotate with the loop – Connections to the external circuit are made by stationary brushes in contact with the slip rings 3
DC Generators, cont • The output voltage always has the same polarity • The current is a pulsating current • To produce a steady current, many loops and commutators around the axis of rotation are used – The multiple outputs are superimposed and the output is almost free of fluctuations 4
DC Generators If we increase the number of turns and arrange them as shown, we can smooth out the variations in voltage. In the old days, autos used DC generators to recharge the battery.
5
DC Generators Replace the brushes with slip rings and you get a DC generator. Notice the voltage is not steady. The coil is shown at the top and bottom of the shaft.
6
D. C. Generator Principle of Operation: When a conductor rotates in a magnetic field, dynamically induced e.m.f is produced in it according to Faraday’s law of electromagnetic induction.
03/23/09
7
Generated EMF Let
φ = Flux/Pole in Wb Z = Total No. of Armature Conductors = No. of Slots × No. of Conductors per Slot P = No. of Generator poles A = No. of Parallel Paths in Armature N = Armature rotation in rpm E = EMF induced in any parallel path in Armature Generated EMF E g = EMF generated in any one of the parallel paths i.e E
03/23/09
8
dφ volt dt Now, Flux cut/conductor in 1 revolution, dφ = φP Wb No. of Revolutions/sec = N/60
Average EMF Generated/Conductor =
60 ∴ Time for 1 revolution, dt = sec N
Hence, according to Faraday’s Laws of electromagnetic induction,
dφ EMF Generated/conductor = volt dt φP φPN = = volt 60/N 60 03/23/09
9
For Wave wound Generator No. of Parallel Paths = 2 No. of Conductors (in series) in 1 path = Z/2 φPN Z φZPN EMF Generated/path = × volt = volt 60 2 120
03/23/09
10
For Lap wound Generator No. of Parallel Paths = P No. of Conductors (in series) in 1 path = Z/P φPN Z φZN EMF Generated/path = × volt = volt 60 P 60
03/23/09
11
In General, Generated EMF
φZN P Eg = volt 60 A where, A = 2 for wave winding = P for lap winding
03/23/09
12
Types of D.C. Generators
According to the way in which their fields are excited, generators are classified into : i) Separately excited D.C. Generator ii) Self- excited D.C. Generator
03/23/09
13
i)
Separately excited D.C. Generators are those whose field magnets are energized from an independent source of D.C. Current If
I
Brush +
L O A D
Eg Field Winding Brush
V
-
R
V = E g − IR a − Brush Drops Ra = Armature Resistance 03/23/09
14
ii) Self excited D.C. Generators are those whose field magnets are energized by the currents produced by the generators themselves. There are 3 types of self-excited D.C. Generators named according to the manner in which their field coils are connected to the armature. These are : • • •
Shunt wound D.C. Generator Series wound D.C. Generator Compound wound D.C. Generator
03/23/09
15
i)
Shunt wound D.C. Generator I
Ish Ia Shunt Field Rsh
+ Ra
Eg
L O A D
V
-
Rsh – Shunt Field Resistance Ra - Armature Resistance
I a = I + I sh
V = E g − I a R a − Brush Drops V = I sh R sh 03/23/09
16
ii) Series wound D.C. Generator Ia=Ise=I + Eg
Rse
Series Field L O A D
V
V = E g − I a ( R a + R se ) − Brush Drops
03/23/09
17
iii) Compound wound D.C. Generator I
Series Field Ish Ia Shunt Field
Eg
L O A D
V
Short Shunt
03/23/09
18
I sh
I
Series Field Ia Shunt Field Eg
L O A D
V
Long Shunt
03/23/09
19
Portable D. C. Generator
03/23/09
20
03/23/09
1
DIRECT CURRENT MACHINES DC machine Construction •
The major advantages of DC machines are: easy speed and torque regulation.
•
The stator of the DC motor has poles, which are excited by DC current to produce magnetic fields.
•
The rotor has a ring-shaped laminated iron-core with slots.
•
Coils with several turns are placed in the slots. The distance between the two legs of the coil is about 180 electric degrees.
03/23/09
Field Rotor
S
N
Brush
Stator with with poles
2
DC Generators. •
Basic operation of the generator – As the loop rotates, the magnetic flux through it changes with time – This induces an emf and a current in the external circuit – The ends of the loop are connected to slip rings that rotate with the loop – Connections to the external circuit are made by stationary brushes in contact with the slip rings 3
DC Generators, cont • The output voltage always has the same polarity • The current is a pulsating current • To produce a steady current, many loops and commutators around the axis of rotation are used – The multiple outputs are superimposed and the output is almost free of fluctuations 4
DC Generators If we increase the number of turns and arrange them as shown, we can smooth out the variations in voltage. In the old days, autos used DC generators to recharge the battery.
5
DC Generators Replace the brushes with slip rings and you get a DC generator. Notice the voltage is not steady. The coil is shown at the top and bottom of the shaft.
6
D. C. Generator Principle of Operation: When a conductor rotates in a magnetic field, dynamically induced e.m.f is produced in it according to Faraday’s law of electromagnetic induction.
03/23/09
7
Generated EMF Let
φ = Flux/Pole in Wb Z = Total No. of Armature Conductors = No. of Slots × No. of Conductors per Slot P = No. of Generator poles A = No. of Parallel Paths in Armature N = Armature rotation in rpm E = EMF induced in any parallel path in Armature Generated EMF E g = EMF generated in any one of the parallel paths i.e E
03/23/09
8
dφ volt dt Now, Flux cut/conductor in 1 revolution, dφ = φP Wb No. of Revolutions/sec = N/60
Average EMF Generated/Conductor =
60 ∴ Time for 1 revolution, dt = sec N
Hence, according to Faraday’s Laws of electromagnetic induction,
dφ EMF Generated/conductor = volt dt φP φPN = = volt 60/N 60 03/23/09
9
For Wave wound Generator No. of Parallel Paths = 2 No. of Conductors (in series) in 1 path = Z/2 φPN Z φZPN EMF Generated/path = × volt = volt 60 2 120
03/23/09
10
For Lap wound Generator No. of Parallel Paths = P No. of Conductors (in series) in 1 path = Z/P φPN Z φZN EMF Generated/path = × volt = volt 60 P 60
03/23/09
11
In General, Generated EMF
φZN P Eg = volt 60 A where, A = 2 for wave winding = P for lap winding
03/23/09
12
Types of D.C. Generators
According to the way in which their fields are excited, generators are classified into : i) Separately excited D.C. Generator ii) Self- excited D.C. Generator
03/23/09
13
i)
Separately excited D.C. Generators are those whose field magnets are energized from an independent source of D.C. Current If
I
Brush +
L O A D
Eg Field Winding Brush
V
-
R
V = E g − IR a − Brush Drops Ra = Armature Resistance 03/23/09
14
ii) Self excited D.C. Generators are those whose field magnets are energized by the currents produced by the generators themselves. There are 3 types of self-excited D.C. Generators named according to the manner in which their field coils are connected to the armature. These are : • • •
Shunt wound D.C. Generator Series wound D.C. Generator Compound wound D.C. Generator
03/23/09
15
i)
Shunt wound D.C. Generator I
Ish Ia Shunt Field Rsh
+ Ra
Eg
L O A D
V
-
Rsh – Shunt Field Resistance Ra - Armature Resistance
I a = I + I sh
V = E g − I a R a − Brush Drops V = I sh R sh 03/23/09
16
ii) Series wound D.C. Generator Ia=Ise=I + Eg
Rse
Series Field L O A D
V
V = E g − I a ( R a + R se ) − Brush Drops
03/23/09
17
iii) Compound wound D.C. Generator I
Series Field Ish Ia Shunt Field
Eg
L O A D
V
Short Shunt
03/23/09
18
I sh
I
Series Field Ia Shunt Field Eg
L O A D
V
Long Shunt
03/23/09
19
Portable D. C. Generator
03/23/09
20
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