Thursday, September 10, 2020

Chapter 6 : DC Machines-Page 2

Chapter 6 : DC Machines

11. All rotating electrical machines have

A. rotating armature and fixed field
B. rotating field and fixed armature
C. either (a) or (b) above
D. none of the above

... Answer is (A)



12. All rotating electric machines are basically

A. D.C. machines
B. A.C. machines
C. Electro-Mechanical convertors
D. Machines using electromagnetic induction

... Answer is (C)


13. The sole purpose of commutator in a dc generator is to

A. Increase output voltage
B. Reduce sparking at brushes
C. Provide smoother output
D. Convert the induced alternating voltage into unidirectional pulses

... Answer is (D)



14. If residual magnetism of a shunt generator is destroyed accidentally, it may be restored by connecting its shunt field

A. to earth
B. to an alternator
C. in reverse
D. to a battery

  ... Answer is (D)


15. Commutation in a dc machine may be improved by
A. reducing to number of turns in the armature and segments of commutator
B. increasing the resistances of brushes
C. neutralizing the reactance voltage by producing a reverse emf in the coil undergoing commutation
D. all of these
Answer is (D)


16. Which of the following parts helps the commutation process?

A. Interpoles
B. Compensating winding
C. Pole shoes
D. All of these

  ... Answer is (B)



17. Which of the following forms an energy converter?

A. piezo-electric effect
B. magneto-striction effect
C. Hall effect
D. all of these

... Answer is (D)


18. A conductor is rotating within a magnetic field. At which of the following positions do the zero voltages occur?

A. at 45° with the axis of the magnetic field
along the axis of the magnetic field
C. at right angles to the axis of the magnetic field

D. None of these

... Answer is (A)


19. Under commutation in a dc machine gives rise to

A. sparking at the trailing edge of the brush no sparking at all
B. sparking at the middle of the brush
C. sparking at the leading edge of the brush
D. None of the above
  ... Answer is (A)


20. The function of commutator in a dc machine is

To change ac voltage into dc voltage
B. To provide easy speed control

To improve commutation
D. To change dc voltage into dc voltage

  ... Answer is (A)



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