9
DC Generator
OBJECTIVE
OBJECTIVE--TYPE QUESTIONS
1. The main purpose of placing armature windings on the rotor in a dc machine
is for
(a) commutation
(b) generation of voltage
(c) energy conversion
(d) all the above
2. In a dc machine, the space distribution of air gap flux density wave at no load
is
(a) triangular
(b) sinusoidal
(c) flat-topped
(d) peaky
3. A commutator in a dc machine can convert
(a) dc to ac
(b) sinusoidal to dc
(c) ac to dc
(d) both ways (ac to dc and dc to ac)
4. With inter pole, the space angle between field mmf and armature mmf in a dc
machine is around
(a) 60°
(b) 90°
(c) 88°
(d) 80°
5. In a dc generator, the effect of armature mmf on the main-field mmf distribution is to
(a) result no distortion
(b) distort the main field mmf
(c) to shift the zero crossing of the resultant flux density waveform opposite
to the direction of rotation
(d) to shift the zero crossing of the resultant flux density waveform in the
direction of rotation.
6. The necessary conditions required for the self-excitation of the dc generator
are
(a) residual magnetism in the field coil
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Basic Electrical Engineering
(b) the resistance in the field circuit must be less than critical field resistance
(c) the field coil must be connected to the armature so that the residual flux
aids the flux produced by the field current
(d) all the above
In a dc generator the polarity of the interpoles is
(a) same as that of the main poles in advance of them
(b) same as that of the main poles but behind them
(c) opposite to the main poles in advance of them
(d) a and b
The rising external characteristic of a series generator can be used in
(a) dc distribution as booster to counter act the voltage drop in a feeder
(b) dc distribution as balancer
(c) dc distribution as amplydyne
(d) dc distribution as booster and balencer
The main factor for reduction in terminal voltage in a dc shunt generator with
load is
(a) armature resistance drop
(b) armature reaction
(c) reduction in field current with load (d)
b and c
The object of using compensating winding in a dc machine is to neutralise the
(a) armature reaction in the inter pole region
(b) cross-magnetising armature reaction
(c) armature reaction under the pole shoe
(d) demagnetising armature mmf completely
Is there a counter torque produced in a dc generator when running?
(a) Yes
(b) No
(c) Yes, but it is opposite to the torque of the prime mover
(d) Yes, but it is in the same sense as the torque produced by the prime mover
The polarity of the residual voltage generated in a d.c. shunt generator appears
to be reversed under dead short circuit of the armature because of
(a) armature mmf
(b) armature mmf neutralising field mmf
(c) iron losses
(d) none of the above
In Swinburn’s test, the main assumption is
(a) the stray losses remain variable
(b) the stray losses remain constant from no local to full load
(c) copper losses are negligible (d) B and C
Is there generator action in a dc motor when, running
(a) Yes, back emf is generated which opposes the applied voltage
(b) Yes, the back emf is generated which aids the applied voltage
(c) No, but back emf generated which is less than the applied voltage
(d) No, but back emf generated which is more than the applied voltage
If a in a dc machine, the generated voltage is 230 V and terminal voltage is 220
on load, then the machine work as
(a) generator
(b) motor
(c) neither generator nor motor
(d) none of them
When a constant terminal voltage is required at the load terminals, the following type of dc generator is preferred:
(a) Separately excited
(b) Shunt excited
(c) Level compounding
(d) Series excited
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DC Generator
9.3
17. For sparkless commutation, the total emf induced in the coil undergoing commutation must be
(a) very high
(b) low
(c) moderate
(d) zero
18. The nature of emf induced in a coil passing through magnetic neutral axis is
(a) transformer emf
(b) dynamic emf
(c) a and b
(d) zero
19. A dc generator is run with different modes of field excitation to keep the no
load voltage constant. The short circuit current is minimum when it is
(a) separately excited
(b) shunt
(c) commutative compounding
(d) differential compounding
20. A 230 V, 1000 rpm, dc shunt generator is running at 900 rpm on no load. The
inherent field resistance is 300 W. The voltage across the terminals of the
machine when an external resistance of 500 W is added is
(a) zero
(b) 207 V
(c) 230 V
(d) 250 V
21. Which of the following dc generator has rising external characteristic?
(a) Series
(b) Shunt
(c) Differential compounding
(d) Separately excited
PROBLEMS
9.1 The following table gives the open-circuit voltages for different field currents of a shunt generator driven at a constant speed.
Open circuit
120
(emf) (volt)
Field current
0.5
(Amps)
240
334
400
444
470
1.0
1.5
2.0
2.5
3.0
(a) Derive the characteristic showing the relation between induced emf
on open circuit and field resistance.
(b) Find the critical field resistance from the derived characteristic
Solution: Plot the O.C.C. using the data as shown in Fig. 9.58. Draw different
field resistance lines like OP, OQ, etc. voltage at P = 495 V, and Rf =
495
= 123.7 W. For every point Rf and the correspondslope of OP =
4
ing voltages are calculated.
Table 9.1
S.No.
1
2
3
4
5
6
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Operating
powL
P
Q
R
S
T
U
Rf
W
123.7
156.7
178
200
221.7
240
Generated
voltage
Vol H
495
470
445
400
332.5
240
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