1. What will happen if the back e.m.f. of a D.C. motor vanishes suddenly?
(a) The motor will stop
(b) The motor will continue to run
(c) The armature may burn
(d) The motor will run noisy
Ans: c
2. In case of D.C. shunt motors the speed is dependent on back e.m.f. only because
(a) back e.m.f. is equal to armature drop
(b) armature drop is negligible
(c) flux is proportional to armature current
(d) flux is practically constant in D:C. shunt motors
Ans: d
3. In a D.C. shunt motor, under the conditions of maximum power, the current in the armature will be
(a) almost negligible
(b) rated full-load current
(c) less than full-load current
(d) more than full-load current
Ans: d
4. These days D.C. motors are widely used in
(a) pumping sets
(b) air compressors
(c) electric traction
(d) machine shops
Ans: c
5. By looking at which part of the motor, it can be easily confirmed that a particular motor is D.C. motor?
(a) Frame
(b) Shaft
(c) Commutator
(d) Stator
Ans: c
6. In which of the following applications D.C. series motor is invariably tried?
(a) Starter for a car
(b) Drive for a water pump
(c) Fan motor
(d) Motor operation in A.C. or D.C.
Ans: a
7. In D.C. machines fractional pitch winding is used
(a) to improve cooling
(b) to reduce copper losses
(c) to increase the generated e.m.f.
(d) to reduce the sparking
Ans: d
8. A three point starter is considered suitable for
(a) shunt motors
(b) shunt as well as compound motors
(c) shunt, compound and series motors
(d) all D.C. motors
Ans: b
9. In case-the conditions for maximum power for a D.C. motor are established, the efficiency of the motor will be
(a) 100%
(b) around 90%
(c) anywhere between 75% and 90%
(d) less than 50%
Ans d
:
10. The ratio of starting torque to full-load torque is least in case of
(a) series motors
(b) shunt motors
(c) compound motors
(d) none of the above
Ans: b
12. In D.C. motor which of the following can sustain the maximum temperature rise?
(a) Slip rings
(b) Commutator
(c) Field winding
(d) Armature winding
Ans: c
13. Which of the following law/rule can he used to determine the direction of rotation of
D.C. motor ?
(a) Lenz's law
(b) Faraday's law
(c) Coloumb's law
(d) Fleming's left-hand rule
Ans: d
14. Which of the following load normally needs starting torque more than the rated torque?
(a) Blowers
(b) Conveyors
(c) Air compressors
(d) Centrifugal pumps
Ans: b
15. The starting resistance of a D.C. motor is generally
(a) low
(b) around 500 Q
(c) 1000 Q
(d) infinitely large
Ans: a
16. The speed of a D.C. series motor is
(a) proportional to the armature current
(b) proportional to the square of the armature current
(c) proportional to field current
(d) inversely proportional to the armature current
Ans: d
17. In a D.C. series motor, if the armature current is reduced by 50%, the torque of the motor will be equal
to
(a) 100% of the previous value
(b) 50% of the previous value
(c) 25% of the previous value
(d) 10% of the previous value
(e) none of the above
Ans: c
18. The current drawn by the armature of D.C. motor is directly proportional to
(a) the torque required
(b) the speed of the motor
(c) the voltage across the terminals
(d) none of the above
Ans: a
19. The power mentioned on the name plate of an electric motor indicates
(a) the power drawn in kW
(b) the power drawn in kVA
(c) the gross power
(d) the output power available at the shaft
Ans: d
20. Which D.C. motor has got maximum self loading property?
(a) Series motor
(b) Shunt motor
(c) Cumulatively compounded 'motor
(d) Differentially compounded motor
Ans: d
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