Tuesday, September 1, 2020

Chapter 6: DC Machines- Page 1

Chapter 6 : DC Machines

1. The essential condition for stable parallel operation of two dc generators having similar characteristics is that they should have

A. Same percentage regulation
B. Same no-load and full-load speeds .
C. Same kilowatt output ratings
D. Drooping voltage characteristics

... Answer is (D)



2. A DC generator can be termed as a

A. Prime mover
B. Power pump
C. Rotating amplifier
D. None of the above

... Answer is (A)


3. A DC generator generates

A. DC and ac voltage
B. Pulsating DC

DC voltage
D. AC voltage

... Answer is (D)



4. Which condition out of the five given below is not required for the build-up of a shunt generator?

A. If excited on open circuit, the shunt field resistance should be more than the critical resistance
B. Armature speed should be more than the critical speed

There must be sufficient residual magnetism in the poles
D. For the given direction of rotation, shunt field coils should be corectly connected to the armature

  ... Answer is (B)


5. A simplex lap-wound 10-pole dc generator has 800 armature conductors. If each pole face covers 75% of pitch, compensating conductors required per pole face are
A. 80
B. 6
C. 60
D. 8
Answer is (C)


6. In a dc generator, the brushes on the commutator are shifted from geometrical neutral positions to A.obtain highest generator voltage
B. avoid sparking.
C. obtain highest generator efficiency
D. all the above

  ... Answer is (B)



7. The efficiency of a d.c. shunt generator is maximum when the

A. Stray losses are equal to the copper losses
B. Armature copper losses are equal to constant losses.

Magnetic losses are equal to the mechanical losses
D. Field copper losses are equal to the constant losses

... Answer is (B)


8. The main factor which leads to unstable parallel operation of flat and overcome pound generators is

A. their rising voltage characteristics
B. unequal speed regulation of their prime movers -

unequal number of turns in their series field windings
D. unequal series field resistances

... Answer is (A)


9. A dc shunt generator develops a fault in the form of an open armature coil. As a temporary measure, you should

A. bridge the two commutator bars across which open coil is connected
B. change the armature it self
C. Disconnect the open coil from the armature
D. Use new brushes which can span 3 commutator segments
  ... Answer is (A)


10. If field resistance of a dc shunt generator is increased beyond its critical value, the generator

A.output voltage will exceed its nameplate rating
B. will not build up
C. may burn out if loaded to its nameplate rating
D. power output may exceed its nameplate rating

  ... Answer is (B)



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