Electrical Machines - Synchronous Machines

Synchronous Machines - IAS previous year questions.....

1. A synchronous machine working in the motor mode, is fed from an infinite bus and is delivering half full-load. If an increase in field current causes an increase in the armature current,then the motor will [IAS-1994]
a. Absorbs active power and reactive power from the bus.
b. Delivers active power and reactive power to the bus
c. Absorbs active power from the bus and delivers reactive power to the bus
d. Absorbs reactive power from the bus and delivers active power to the bus

2. In a synchronous generator operating at zero pf lagging, the effect of armature reaction is
[IAS-1995]
a. Magnetising
b. Demagnetising
c. Cross magnetising
d. Both magnetising and demagnetising

3. Consider the following statements about a three phase synchronous generator synchronized to an infinite bus when its mechanical input is increased gradually with field current held constant:
1. The power factor of the current supplied becomes more lagging.
2. The power factor of the current supplied improves.
3. The power factor remains unity. 4. The load angle is increased Of these statements [IAS-1996]
a. 1 alone is correct
b. 2 alone is correct
c. 2 and 4 is correct
d. 3 and 4 is correct

4. A 3-phase alternator delivers power to a balanced 3-phase load of power factor 0.707 lagging. It is observed that the open-circuit emf phasor leads the corresponding terminal voltage phasor by Neglecting the effect of harmonics, the angle between the axis of the main field mmf and the axis of the armature mmf will be [IAS-1997]
a. 30 degrees electrical
b. 60 degrees electrical
c. 90 degrees electrical
d. 150 degrees electrical

5. A 3-phase alternator delivers power to a balanced 3-phase load of power factor 0.707 lagging. It is observed that the open-circuit emf phasor leads the corresponding terminal voltage phasor by Neglecting the effect of harmonics, the angle between the axis of the main field mmf and the axis of the armature mmf will be  [IAS-1998]
a. 240 V  b. 220 V  c. 200 V  d. 180 V

6. In a synchronous machine, the electric circuits are armature circuit, field circuit and damper circuit and X is d-axis synchronous reactance; X' is d-axis transient synchronous reactance and X" is d-axis sub-transient synchronous reactance. If there is no damper winding, then [IAS-1999]
a. X = X'
b. X' = X"
c. X = X"
d. X = X' = X"

7. A 3-phase 20-pole,synchronous generator has 180 stator slots with single-layer full pitch coils.There are 6 conductors per slot and all coils per phase are committed in have series. The rotor is driven at 300 rpm and the flux per pole (sinusoidally distributed) is 25 mWb. The voltage induced per phase is nearest to
[ IAS-2000]
a. 1000   b. 500   c. 1500   d. 2000

8. An alternator is operating on infinite bus. It will develop maximum synchronizing power when it is operating at [ IAS-2001]
a. No load
b. Full load
c. Steady state stability limit
d. Any load

9. Which of the following information is obtained, by Potier's method using open circuit characteristic and zero power factor characteristics ? [ IAS-2003]
a.field current equivalent to armature reactance.
b. Synchronous reactance
c. Leakage reactance
d. Both a and b

10. A 3-phase cylindrical rotor alternator is delivering rated power to a grid at its rated power factor On account of some fault, steam supply to the turbine of the alternator is cut-off. The alternator would [ IAS-2004]
a. Stop
b. continue running as an alternator at synchronous speed Ns and deliver active power to the grid
c. continue running as a synchronous motor at Ns and deliver reactive power to the grid.
d. continue running as a synchronous motor at and absorbs reactive power from the grid.

11. Consider the following statements for a single -phase hysteresis motor :
1.Torque is constant from standstill up to synchronous speed .
2.Rotor is provided with narrow slots embedded with bars.
3. Torque at starting is somewhat more than that at synchronous speed
4. As the name suggests. only hysteresis torque is produced from zero speed right up to
5. Rotor material possesses very wide hysteresis loop
which of the statements given above [ IAS-2005]
a. 1,2 and 5 correct.
b. 3 and 5 correct.
c. 1 and 4 correct.
d. 2,3 and 4 correct.

12. An under-excited synchronous motor is operating at no load on an infinite bus of voltage 7.o p.u and drawing a current of 0.8 p.i. Its synchronous reactance is 0.5 p.u. What is the p.u. value of its excitation emf ? [ IAS-2006]
a. 1.4       b. 1.0      c. 0.6      d 0.4