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Electrical Engineering Objective Questions { Synchronous Motors }



36. Statement :I. A synchronous motor has no starting torque but when started it always runs at a fixed speed II. Neglecting residual magnetism, both cylindrical rotor and salient pole synchronous motors produce some mechanical power even though the field is unexcited III. A single phase reluctance motor is not self starting even if paths for eddy currents are provided in the rotor IV. A single phase hysteresis motor is self-starting Combinations
A. I, II, III and IV
B. I, III and IV
C. I and III
D. I, II and IV

37.  Statements :I. The constant speed of a synchronous motor can be changed to a new fixed value by changing the supply frequency II. In a synchronous motor, copper losses do not vary with load III. Speed regulation of a 3-phase synchronous motor is zero IV. The size of a synchronous motor decreases with the decrease of flux density Combinations
A. I, II and HI
B. II, III and IV
C. I and 11
D. I and III

38. A synchronous motor, connected to an infinite bus, is working at a leading p.f. Its excitation e.m.f. Ef and alternator terminal voltage Vt are related as
A. Ef > Vt and Ef lags Vt
B. Ef< Vt and Et. lags Vt
C. Ef > Vt and Et. leads Vt
D. Ef< Vt and Et. lags Vt

39. An alternator and a synchronous motor of the salient-pole type are operating at lagging pfs. Their excitation emf Ef is respectively given by
A. Vt + Xd Id, Vt ? Id Xd
B. Vt + ra la + Id Xd, Vt ? la ra ? Id Xd
C. Vt + ra Iq + Id X Vt ? ra lq ? Id Xd
D. Vt + ra Id + Id Xd, Vt ? ra Id ? Id Xd

40. Under steady-state working conditions of a synchronous motor and an alternator, the relative speed between the magnetic fields of stator and rotor should respectively be
A. zero, synchronous speed ns
B. ns, ns
C. zero, zero
D. ns, zero

41. Two 550 kVA alternators operate in parallel to supply the following loads.(i) 250 kW at 0.95 p.f., lagging (ii) 100 kW at 0.85 p.f. leading One machine is suplying 200 kW at 0.9 p.f. lagging. The p.f. of other machine must be
A. 0.89 leading
B. 0.95 leading
C. 0.95 lagging
D. 0.89 lagging

42. Synchronous motors are generally not self-starting because
A. the direction of rotation is not fixed
B. the direction of instantaneous torque reverses after half cycle
C. starters cannot be used on these machines
D. starting windings is not provided on the machines

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