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Consider the following statements: 1. A synchronous motor has no starting torque but when started it always runs at a fixed speed. 2. A single-phase reluctance motor is not self-starting even if path for eddy currents are provided in the rotor 3. A single-phase hysteresis motor is self-starting Which of these statement(s) is/are correct?

Consider the following statements: 1. A synchronous motor has no starting torque but when started it always runs at a fixed speed. 2. A single-phase reluctance motor is not self-starting even if path for eddy currents are provided in the rotor 3. A single-phase hysteresis motor is self-starting Which of these statement(s) is/are correct? Correct Answer 1, 2 and 3

Synchronous motors:

  • Synchronous motors are constant speed motors. They run at the synchronous speed of the supply.
  • They are generally used for constant speed operation under no-load conditions such as to improve the power factor.
  • Synchronous motors have fewer losses than induction motors at a given rating.

 

Self-Starting Property:

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  • At the start the rotor is stationary and take a positive cycle of AC supply, N pole of the stator (Ns) is opposite to N pole of the rotor (Nr)
  • The repulsion on the rotor will move clockwise or counterclockwise
  • Now Take negative half cycle N pole of the stator will become S pole
  • Now S pole of the stator (Ss) will attract N pole of the rotor (Nr)
  • Because of attraction, during the negative cycle and repulsion during the positive cycle, the resultant rotation is zero
  • So resultant torque is zero
  • So synchronous motor is not self-starting

 

Single-phase synchronous motors:

  • Very small single-phase motors have been developed which run at true synchronous speed.
  • They do not require d.c. excitation for the rotor.
  • Because of these characteristics, they are called unexcited single-phase synchronous motors.

 

Reluctance motors: All reluctance torque motors require induction motor starting torque to come up fairly close to synchronous speed, where pull in can occur as a result of reluctance torque and the motor operates as a reluctance motor at a constant synchronous speed therefore no reluctance motor is self-starting.

Hysteresis motors: Hysteresis motors and sub-synchronous motors are self-starting and come up to full synchronous speed, developing high rotor resistance because of hysteresis loss and consequently, fairly high starting torque but poorer running torque than reluctance torque motors.

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