# ONLINE MCQ EE-Synchronous Motors 3

31. The constant speed of a
synchronous motor can be changed to new fixed value by
(A) changing the applied voltage
(B) interchanging any two phases
(D) changing the frequency of supply.

32. A 3 phase, 400 V, 50 Hz
synchronous motor is operating at zero power factor lagging with respect to the
excitation voltage. The armature reaction mmf. produced by the armature current
will be
(A) demagnetizing
(B) magnetizing
(C) cross-magnetizing
(D) none of the above.

33. In a synchronous motor, the
torque angle is
(A) the angle between the rotating stator flux and rotor poles
(B) the angle between magnetizing current and back emf
(C) the angle between
the supply voltage and the back emf
(D) none of the above.

34. A 3 phase, 400 V, 50
Hz, 4 pole synchronous motor has a load angle of 10° electrical. The
equivalent mechanical degrees will be .
(A) 10°
(B)5√2 degrees
(C) 5 degrees
(D)
1 degree.

35. A 3 phase, 400 V, 50 Hz synchronous motor
has fixed excitation. The load on the motor is doubled. The torque angle, φi will become nearly
(A) φr /2
(B) φr
(C)2 φr
(D) √ 2 φr

36. The hunting in a synchronous motor takes
place when
(A) friction
in bearings is more
(B) air gap
is less
is variable
is constant.

37. V curves for a synchronous motor represent
relation between
(A) field
current and speed
(B) field
current and power factor
(C) power
factor and speed
(D) armature
current and field current.

38. The breakdown. torque of a synchronous motor
varies as
(A)1
/(applied voltage )
(B) 1/(applied voltage )2
(C)
applied voltage
(D)
(applied voltage)2.

39. Hunting in a synchronous motor cannot be due
to
(A) variable
frequency
(B) variable
(C) variable
supply voltage
(D) windage
friction.

40. When the excitation of an unloaded salient
pole synchronous motor suddenly gets disconnected
(A) the
motor stops
(B) it
runs as a reluctance motor at the same speed
(C) it
runs at a reluctance motor at a lower speed.

41. Which synchronous motor will be smallest in
size ?
(A) 5
HP, 500 rpm
(B) 5
HP, 375 rpm
(C) 10
HP, 500 rpm
(D) 10
HP, 375 rpm.

42. A synchronous machine has its field winding
on the stator and armature winding on the rotor. Under steady running
conditions, the air-gap field
(A) rotates
at synchronous speed with respect to stator
(B) rotates
at synchronous speed with direction of rotation of the rotor
(C)
remains stationary with respect to stator
(D) remains
stationary with respect to rotor.

43. If the field of a synchronous motor is
under-excited, the power factor will be
(A) unity
(B)
lagging
(D) more than unity.

44. The name plate of an induction motor reads 3
phase. 400 V, 50 Hz. 0.8 of lagging, 1440 rpm. On similar lines the name plate
of a synchronous motor should read
(A) 3
phase, 400 V, 50 Hz, 0.8 pf lagging, 1500 rpm
(B) 3
phase, 400 V. 50 Hz, 0.8 pf leading, 1500 rpm
(C) 3
phase, 400 V, 50/60 Hz, 0.8 pf lagging, 1500 rpm
(D) 3
phase. 400 V, 50/60 Hz, 0.8 pf leading, 1500 rpm.