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In case of synchronous motor, which of the following are true? a. Synchronous motor is not self-starting b. If the rotor poles are excited by field current and the stator terminals are connected to AC supply, motor vibrates. c. Variable frequency supply is used to start synchronous motor. d. By using frequency converter, a synchronous motor can be brought from standstill to its desired speed.

In case of synchronous motor, which of the following are true? a. Synchronous motor is not self-starting b. If the rotor poles are excited by field current and the stator terminals are connected to AC supply, motor vibrates. c. Variable frequency supply is used to start synchronous motor. d. By using frequency converter, a synchronous motor can be brought from standstill to its desired speed. Correct Answer All are true

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

 

Excitation:

If the rotor poles are excited by field current and the stator terminals are connected to the AC supply, the motor vibrates.

Starting of synchronous motor:

A variable frequency supply is used to start the synchronous motor. A variable frequency drive unit is included in a motor-control circuit to achieve speed control, then starting the synchronous motor is very easy-simply adjust the frequency to a very low value for starting, and then raise it up to the desired operating frequency for normal running.

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