D.C. Machines

1. field excitation of 20A in a certain alternator results in an armature current of 400A in short circuit and a terminal voltage of 2000V on open circuit. The magnitude of the internal voltage drop within the machine at a load current of 200A is

  1. V
  2. 0V
  3. 00V
  4. 000V

Correct answer: (D)

2. synchro has

  1. a 3-phase winding on rotor and a single-phase winding on stator.
  2. a 3-phase winding on stator and a commutator winding on rotor.
  3. a 3-phase winding on stator and a single-phase winding on rotor.
  4. a single-phase winding on stator and a commutator winding on rotor.

Correct answer: (C)
a 3-phase winding on stator and a single-phase winding on rotor.

3. synchronous motor connected to infinite busbars has at constant full-load, 100% excitation and unity pf. On changing the excitation only, the armature current will have

  1. leading pf with under-excitation
  2. leading pf with over excitation
  3. lagging pf with over excitation
  4. no change of pf

Correct answer: (B)
leading pf with over excitation

4. The most economical method of finding no losses of a large dc shunt motor is

  1. Hopkinson's test
  2. Swinburn's test
  3. Retardation test
  4. Field's test

Correct answer: (B)
Swinburn's test

5. When load on a synchronous motor is increased its armature current is increased provided it is

  1. normally excited
  2. over excited
  3. under exciter
  4. all of the above

Correct answer: (D)
all of the above

6. A 220V, 15kW,1000rpm shunt motor with armature resistance of 0.25Ω, has a rated line current of 68A and a rated field current of 2.2A. The change in field flux required to obtain a speed of 1600 rpm while drawing a line current of 52.8A and a field current of 1.8A is

  1. 18.18% increase
  2. 18.18% decrease
  3. 36.36% increase
  4. 36.36% decrease

Correct answer: (D)
36.36% decrease

7. A balanced three-phase, 50 Hz voltage is applied to a 3 phase, 4 pole, induction motor. When the motor is delivering rated output, the slip is found to be 0.05. The speed of the rotor m.m.f. relative to the rotor structure is

  1. 1500 r.p.m.
  2. 1425 r.p.m.
  3. 25 r.p.m.
  4. 75 r.p.m.

Correct answer: (D)
75 r.p.m.

8. A ceiling fan uses

  1. split-phase motor.
  2. capacitor start and capacitor run motor.
  3. universal motor.
  4. capacitor start motor.

Correct answer: (D)
capacitor start motor.

9. A dc motor can be looked upon as dc generator with the power flow

  1. Reduced
  2. Reversed
  3. Increased
  4. Modified

Correct answer: (B)

10. A DC series motor is best for driving

  1. Lathes
  2. Cranes and hoists
  3. Shears and punches
  4. Machine tools

Correct answer: (B)
Cranes and hoists

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