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Electrical Engineering Objective Questions { DC Motors }

15.  v xB=E is the equation of the electrical machines which
A. is a dc machines
B. has magnetic poles
C. works as a motor
D. converts mechanical energy into electrical energy

16.  The function of commutator in a dc machine is
A. to improve commutation
B. to change dc voltage into dc voltage
C. to vchange ac voltage into dc voltage
D. to provide easy speed control

17. Under commutation in a dc machine gives rise to
A. sparking at the leading edge of the brush
B. sparking at the trailing edge of the brush no sparking at all
C. sparking at the middle of the brush

18.  A conductor is rotating within a magnetic field. At which of the following positions do the zero voltages occur?
A. along the axis of the magnetic field
B. at right angles to the axis of the magnetic field
C. at 45? with the axis of the magnetic field
D. none of these

19.  Which of the following forms an energy converter?
A. piezo-electric effect
B. magneto-striction effect
C. Hall effect
D. all of these

20.  Which of the following parts helps the commutation process?
A. interpoles
B. compensating winding
C. pole shoes
D. all of these

21. Commutation in a dc machine may be hnprovedby
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

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