1. 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. starter cannot be used on these machines
D. starting winding is not provided on the machines
Answer: B. the direction of instantaneous torque reverses after half cycle
Explanation:
- A synchronous motor is not self-starting because the direction of instantaneous torque reverses after half a cycle. When the motor is at rest, the direction of the instantaneous torque is determined by the phase angle between the magnetic field and the stator current.
- However, after half a cycle, the direction of the instantaneous torque reverses, causing the motor to stop rather than start.
- This is because the synchronous motor relies on a constant rotational speed, which is maintained by the synchronous speed of the magnetic field.
- If the motor were to start, it would have to accelerate to the synchronous speed, which would require a torque in the opposite direction to the instantaneous torque. Therefore, a separate starting device, such as a starter or external torque, is needed to overcome this issue and get the motor started.
2. In case one phase of a three-phase synchronous motor has short-circuited the motor will
A. not start
B. run at 2/3 of synchronous speed
C. run with excessive vibrations
D. take less than the rated load
Answer: A. not start
Explanation:
- If one phase of a three-phase synchronous motor is short-circuited, the motor will not start. When a three-phase synchronous motor is supplied with three-phase power, the stator windings generate a rotating magnetic field that drives the rotor to rotate at the same speed as the field.
- If one of the phases is short-circuited, the balance of the stator magnetic field will be disrupted, causing the motor to become unstable and unable to rotate.
- As a result, the motor will not start. It is important to ensure that all phases of a three-phase synchronous motor are functioning properly in order for the motor to operate correctly.
3. A pony motor is basically a
A. small induction motor
B. D.C. series motor
C. D.C. shunt motor
D. double winding A.C./D.C. motor
Answer: A. small induction motor
Explanation:
- A pony motor is a small auxiliary motor that is used to start a larger motor. It is typically an induction motor that is used to provide the initial torque needed to get the larger motor started.
- Once the larger motor is up to speed, the pony motor is disconnected and the larger motor continues to operate on its own.
- The term "pony motor" is often used to refer specifically to the small motor that is used to start a large synchronous motor, but it can also be used to refer to any small auxiliary motor that is used to start a larger motor.
4. A synchronous motor can develop synchronous torque
A. when under loaded
B. while over-excited
C. only at synchronous speed
D. below or above synchronous speed
Answer: C. only at synchronous speed
Explanation:
- A synchronous motor can develop synchronous torque only at synchronous speed. Synchronous torque is the torque developed by a synchronous motor when it is operating at its synchronous speed, which is the speed at which the rotor of the motor rotates at the same speed as the stator magnetic field.
- At this speed, the rotor is locked in step with the stator field and is able to follow it as it rotates.
- As a result, the rotor experiences a torque that is in the same direction as the stator field, causing it to rotate.
- Synchronous motors are able to develop synchronous torque only at synchronous speed because they rely on a constant rotational speed to produce torque. If the motor is operating at a speed that is different from the synchronous speed, it will not be able to follow the stator field and will not experience a synchronous torque.
- As a result, synchronous motors are not self-starting and require a separate starting device, such as a starter or external torque, to get them up to speed. Once the motor is operating at synchronous speed, it can develop synchronous torque regardless of whether it is underloaded or over-excited.
5. A synchronous motor can be started by
A. pony motor
B. D.C. compound motor
C. providing damper winding
D. any of the above
Answer: D. any of the above
Explanation:
- A synchronous motor can be started by a pony motor, a DC compound motor, or by providing a damper winding.
- A pony motor is a small auxiliary motor that is used to provide the initial torque needed to start a larger motor. It is typically an induction motor that is used to get the synchronous motor up to speed, after which the pony motor is disconnected and the synchronous motor continues to operate on its own.
- A DC compound motor is a type of motor that combines the characteristics of both a DC series motor and a DC shunt motor. It can be used to start a synchronous motor by providing the necessary starting torque and then adjusting the field current to maintain the desired speed.
- A damper winding is a type of winding that is added to the stator of a synchronous motor to provide additional starting torque. It is typically used in conjunction with a starter to help the motor accelerate to its synchronous speed. Once the motor is up to speed, the starter and damper winding are disconnected and the motor continues to operate on its own.
6. A three-phase synchronous motor will have
A. no slip-rings
B. one slip-ring
C. two slip-rings
D. three slip-rings
Answer: C. two slip-rings
Explanation:
- In a three-phase synchronous motor, the stator windings are connected to a three-phase power supply and the rotor is driven by the resulting magnetic field.
- However, the rotor of a synchronous motor is typically an electromagnet, and the field on the rotor is produced by passing direct current (DC) through the rotor windings.
- In order to supply the DC power to the rotor windings, the synchronous motor uses two slip-rings, which are mounted on the shaft of the motor and are connected to the rotor windings.
- The slip-rings allow the DC power to be transferred to the rotor windings while the rotor is rotating.
7. Under which of the following conditions hunting of synchronous motor is likely to occur ?
A. Periodic variation of load
B. Over-excitation
C. Over-loading for long periods
D. Small and constant load
Answer: A. Periodic variation of load
Explanation:
- Hunting of a synchronous motor is likely to occur under conditions of periodic variation of load. Hunting refers to the oscillation of the rotor speed around the synchronous speed of the motor, which can occur when the load on the motor varies periodically.
- When the load on a synchronous motor varies, the motor must adjust its speed to maintain the synchronous speed. If the load variation is small and constant, the motor will be able to maintain a steady speed without any problems. However, if the load variation is large or occurs at a high frequency, the motor may have difficulty adjusting its speed, which can cause the rotor to oscillate around the synchronous speed. This oscillation is known as hunting.
- Over-excitation and overloading for long periods can also cause instability in a synchronous motor, but they are less likely to result in hunting compared to periodic variation of load. Small and constant load conditions are unlikely to cause hunting in a synchronous motor.
8. 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 as a reluctance motor at a lower speed
D. none of the above
Answer: A. the motor stops
Explanation:
- When the excitation of an unloaded salient pole synchronous motor suddenly gets disconnected, the motor stops. A salient pole synchronous motor is a type of synchronous motor that has salient (projecting) poles on the rotor, which are magnetized by a direct current (DC) excitation.
- The excitation is supplied to the rotor through a set of slip-rings, which are mounted on the shaft of the motor and are connected to the rotor windings.
- If the excitation is suddenly disconnected, the rotor will lose its magnetization and will no longer be able to generate a torque. As a result, the motor will stop.
- It will not run as a reluctance motor at the same speed or at a lower speed, as the rotor will not have the necessary magnetization to interact with the stator magnetic field.
9. When V is the applied voltage, then the breakdown torque of a synchronous motor varies as
A. V
B. V312
C. V2
D. 1/V
Answer: A. V
Explanation:
- The breakdown torque of a synchronous motor varies as V, rather than V^2. The breakdown torque is the maximum torque that a synchronous motor can produce before it becomes unstable and unable to operate properly.
- It is typically reached at or near the synchronous speed of the motor.
- The breakdown torque of a synchronous motor is directly proportional to the applied voltage, V.
- This means that if the applied voltage is doubled, the breakdown torque will be doubled. Similarly, if the applied voltage is halved, the breakdown torque will be halved.
10. The power developed by a synchronous motor will be maximum when the load angle is
A. zero
B. 45°
C. 90°
D. 120°
Answer: C. 90°
Explanation:
- The power developed by a synchronous motor will be maximum when the load angle is 90 degrees. The power developed by a synchronous motor is determined by the product of the torque developed by the motor and the rotational speed of the motor.
- The load angle, also known as the power factor angle, is the phase angle between the applied voltage and the current in the motor.
- At a load angle of 90 degrees, the voltage and current in the motor are in quadrature, which means that the motor is operating at its maximum power capability.
- As the load angle decreases, the power developed by the motor decreases, as the voltage and current are no longer in quadrature and the motor is unable to develop its full torque.
- At a load angle of zero, the power developed by the motor is zero, as the voltage and current are completely out of phase.