1.The duration of sudden short circuit test on a synchronous motor is usally about
2. If one-phase of a 3 phase synchronous motor is short circuit,motor
3.If the excitation of an Alternator operating in parallel with other Alternator decreased,its:
4.A synchronous motor is connected to an infinite bus at 1.0pu voltage and draw 1.0pu current at unity power factor.The synchronous reactance is 1pu and resistance is negligible.What is the load angle?
5.BY reversing______the direction of rotation of a synchronous motor can be reversed
6.An alternator operating principle is quite similar to that of
7.The following system is used in connection of various lamps and appliances in parallel.Identify the system
8.On which of the following parameters does the power factor of an alternator depend?
9. What is purpose of dampers in a large generator?
10.Stability of a synchronous machine
41.. If the load of a synchronous motor is increases while keeping the field excitation constant, then
42.A single-phase alternator has a synchronous Reactance of 2 ohms and negligible resistance. If it supplies 10 A to a purely capacitive load at 200V, then the generated emf will be
43.Suppose a synchronous generator connected to an infinite bus is supplying electrical power at unity PF to the bus. If its field current is now increased.
44. The field winding of a 3-phase synchronous motor is short-circuited directly. If a 3-phase balanced voltage is impressed across the stator terminals, then the rotor will
45.. In a synchronous machine, the electric circuits are armature circuit, field circuit and damper circuit and Xd is d-axis synchronous reactance; X'd is d-axis transient synchronous reactance and X"d is d-axis sub-transient synchronous reactance. If there is no damper winding, then
46. 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 are correct:
47. Which of the following will change in a three-phase synchronous motor, as a consequence of excitation variations?
1. Pull-out torque
2. Torque angle
3. Magnitude and power factor of stator current
4. Output power
Select the correct answer using the codes given below:
48. 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 15. Neglecting the effect of harmonics, the angle between the axis of the main field mmf and the axis of the armature mmf will be
49. A 3-phase alternator delivers power to a balanced 3-phase load of power factor 0.707 leading.It is observed that the open-circuit emf phasor leads the corresponding terminal voltage phasor by 15 Neglecting the effect of harmonics, the angle between the axis of the main field mmf and the axis of the armature mmf will be
50. On which of the following does the back emf set up in the stator of a synchronous motor depend
1.Rotor speed
2.Rotor excitation
3.Coupling angle
choose the correct answer from the options given below:
51. Let us assume that two alternators running exactly in synchronism. Find the synchronizing power of the system.
52. What will be the effect on a salient pole synchronous motor if its field current is switched off (provided the motor runs at no load)?
Explain:- Reluctance Torque:
Reluctance torque is the torque generated because the motor is moving to a position where the reluctance seen by the armature flux is declining.
Reluctance exists only when there is an unsymmetrical air gap.
Therefore it exists only for Salient pole synchronous machines and does not exist for the cylindrical machine.
Reluctance torque comes into play when any winding is disconnected i.e. filed winding or armature winding in the running conditions
Reluctance torque also produces reluctance power which makes the machine more stable
The machine will continue to run at synchronous speed due to reluctance torque (i.e. even when field winding or armature windings are disconnected)
53. On which of the following parameters does the power factor of an alternator depend?
54. For very low speed and high power applications, the best suited ac motor is
55. When a synchronous motor is started, the field winding is initially
56.A 10 pole, 25 Hz alternator is directly coupled to and is driven by a 60 Hz synchronous motor. Ihe number of poles of a synchronous motor are
57.Which one of the following methods gives voltage regulation higher than the actual value in an alternator ?
58. Two mechanically coupled alternators deliver at 50 Hz and 60 Hz respectively The highest speed of alternators is
59.To reduce the peripheral speed of an alternator, diameter of the rotor is:
60.For modern alternator, what be the specific value of SCR?
61. A rotary converter generally:
62. A Rotary Converter in General Construction and Design is more or less like
63. Which of the following methods may be used to convert AC system to DC?
64. In a single phase Rotary Converter the number of slip rings will be:
65. A rotary converter is a single machine with
66. A Rotary Converter operate at a
67. In a synchronous machine, in case the axis of field flux is in line with the armature flux then
68. A synchronous motor is found more economical over a load of :
69.By reversing the direction of rotation of a synchronous motor can be reversed
70. It will be the effect on a salient pole synchronous motor if its field current is switched off (provided the motor runs at no load)?
71.The angle between rotating stator flux and rotor poles is called:
72.Synchronous motors are to be used in situations where
73. n a synchrounous machine, if the field flux is ahead of the armature field axis, in the direction of rotation, the machine works as-
74. In a synchrounous machine, if the armature field axis is ahead of the field flux , in the direction of rotation, the machine works as-
75. Which type of torque in synchronous motor is also called as break away torques?
Explain:-
1.The pull-up torque is the minimum torque developed by an electric motor when it runs from zero to full-load speed (before it reaches the break-down torque point).
2.Pull-out torque is defined as the maximum value of the torque which a synchronous motor can develop at rated voltage and frequency without losing synchronism.
3.Starting torque is the torque at the time of starting of the motor. It is also known as breakaway torque.