DC MACHINES 9

 

DC MACHINES MCQ 9

 

 1. In a dc machine without interpoles to get improved commutation , the brush shift angle must be
a) varied with change in load
b) kept constant
c) 0 degree
d) none of these
ans. a

2. Compared to the air-gap under the field-poles, the inter-pole air-gap is made
a) larger for both the generators and the motors.
b) smaller for both the generators and larger for the motor.
c) smaller for generator and larger for the motors.
d) larger for generators and smaller for the motors.
ans. a

3. Compared to an uncompensated dc machine , the interpole as required in a compensated dc machine is
a) larger for the generators and smaller for the motors.

b) smaller for the generators and larger for the motors

c) smaller for both the generators and the motors.

d) larger for both the generators and the motors.
ans. c

4. Due to magnetic saturation , the flux per pole in a dc machine without brush shift
a) decrease the generators and increases in the motors with load.
b) decreases in both the generators and the motors with load.
c) increases in both the generators and the motors with load.
d) increases in the generators and decreases in the motors with load.
ans. b

5. The core losses in a dc machine occur due to
a) hysteresis only
b) eddy current only
c) armature current
d) both hysteresis and eddy currents.
ans. b

6. The core losses in a dc machine occur in
a) the armature only
b) the pole faces only
c) the yoke only
d) both the armature and the pole faces.
ans. d

7. In a dc machine the armature reaction and the inductance of the commutating coils result in
a) over-commutation
b) sinusoidal commutation
c) linear commutation
d) under commutation
ans. d

8. Under commutation gives rise to
a) sparking at the leading edge of the brush
b) sparking at the trailing edge of the brush
c) no sparking at all
d) sparking at the middle of the brush
ans. b

9. For a dc generator if the brushes are given , a small amount forward shift the effect of armature reaction is
a) totally demagnetizing
b) totally magnetizing
c) partly demagnetizing and partly cross magnetizing
d) totally cross-magnetizing
ans. c

10. In a dc motor if the brushes are given backward shift , then
a) commutation is worsened and speed decreases
b) commutation is improved and speed decreases
c) commutation is unaffected and speed increases
d) commutation is improved and speed increases
ans. d

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