Electrical Machines (Synchronous Machines)

1. In a synchronous motor, damper winding is provided in order to
(a) stabilize rotor motion
(b) suppress rotor oscillations
(c) develop necessary starting torque
(d) both (b) and (c)

2. In a synchronous motor, the magnitude of stator back e.m.f. Eb depends on
(a) speed of the motor
(b) load on the motor
(c) both the speed and rotor flux
(d) d.c. excitation only

3. An electric motor in which both the rotor and stator fields rotates with the same speed is called a/an ........motor.
(a) d.c.
(b) charge
(c) synchronous
(d) universal

4. While running, a synchronous motor is compelled to run at synchronous speed because of
(a) damper winding in its pole faces
(b) magnetic locking between stator and rotor poles
(c) induced e.m.f. in rotor field winding by stator flux
(d) compulsion due to Lenz’s law

5. The direction of rotation of a synchronous motor can be reversed by reversing
(a) current to the field winding
(b) supply phase sequence
(c) polarity of rotor poles
(d) none of the above

6. When running under no-load condition and with normal excitation, armature current Ia drawn by a synchronous motor
(a) leads the back e.m.f. Eb by a small angle
(b) is large
(c) lags the applied voltage V by a small angle
(d) lags the resultant voltage ER by 90º.

7. The angle between the synchronously-rotating stator flux and rotor poles of a synchronous
motor is called........ angle.
(a) synchronizing
(b) torque
(c) power factor
(d) slip

8. If load angle of a 4-pole synchronous motor is 8º (elect), its value in mechanical degrees is
........
(a) 4
(b) 2
(c) 0.5
(d) 0.25

9. The maximum value of torque angle a in a synchronous motor is ........degrees electrical.
(a) 45
(b) 90
(c) between 45 and 90
(d) below 60

10. A synchronous motor running with normal excitation adjusts to load increases essentially
by increase in its
(a) power factor
(b) torque angle
(c) back e.m.f.
(d) armature current.

11. When load on a synchronous motor running with normal excitation is increased, armature
current drawn by it increases because
(a) back e.m.f. Eb becomes less than applied voltage V
(b) power factor is decreased
(c) net resultant voltage ER in armature is increased
(d) motor speed is reduced

12. When load on a normally-excited synchronous motor is increased, its power factor tends to
(a) approach unity
(b) become increasingly lagging
(c) become increasingly leading
(d) remain unchanged.

13. The effect of increasing load on a synchronous motor running with normal excitation is to
(a) increase both its Ia and p.f.
(b) decrease Ia but increase p.f.
(c) increase Ia but decrease p.f.
(d) decrease both Ia and p.f.

14. Ignoring the effects of armature reaction, if excitation of a synchronous motor running with constant load is increased, its torque angle must necessarily
(a) decrease
(b) increase
(c) remain constant
(d) become twice the no-load value.

15. If the field of a synchronous motor is underexcited, the power factor will be
(a) lagging
(b) leading
(c) unity
(d) more than unity

16. Ignoring the effects of armature reaction, if excitation of a synchronous motor running with constant load is decreased from its normal value, it leads to
(a) increase in but decrease in Eb
(b) increase in Eb but decrease in Ia
(c) increase in both Ia and p.f. which is lagging
(d) increase in both Ia and φ

17. A synchronous motor connected to infinite busbars has at constant full-load, 100% excitation and unity p.f. On changing the excitation only, the armature current will have
(a) leading p.f. with under-excitation
(b) leading p.f. with over-excitation
(c) lagging p.f. with over-excitation
(d) no change of p.f.

18. The V-curves of a synchronous motor show relationship between
(a) excitation current and back e.m.f.
(b) field current and p.f.
(c) d.c. field current and a.c. armature current
(d) armature current and supply voltage.

19. When load on a synchronous motor is increased, its armature currents is increas- ed provided it is
(a) normally-excited
(b) over-excited
(c) under-excited
(d) all of the above

20. If main field current of a salient-pole synchronous motor fed from an infinite bus and
running at no-load is reduced to zero, it would
(a) come to a stop
(b) continue running at synchronous speed
(c) run at sub-synchronous speed
(d) run at super-synchronous speed

21. In a synchronous machine when the rotor speed becomes more than the synchronous speed during hunting, the damping bars develop
(a) synchronous motor torque
(b) d.c. motor torque
(c) induction motor torque
(d) induction generator torque

Answers
1. d 2. d 3. c 4. b 5. b 6. c 7. b 8. a 9. b 10. d 11. c 12. b 13. c 14. a 15. a 16. d 17. b 18. c 19. d 20. b 21. d 22. d 23. c 24. c

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