Synchronous Motor

A Synchronous Motor is a machine that converts AC electric power to mechanical power at a constant speed. A synchronous motor is a doubly excited machine. Its rotor poles are excited by direct current (DC), and the stator windings are connected to the AC supply. The resultant of the fluxes due to both rotor current and stator current is the air gap flux. An important feature of a synchronous motor is that it can draw either lagging or leading reactive current from the AC supply system.

If two alternators A and B are operating in parallel and the power input to one of the alternators say A is increased, it starts delivering more power. Since the demand of the load is unaltered, the alternator B will thus be relieved off its load by an equal amount. If this process is continued to a point where the output power of alternator A becomes more than the total load supplied. Hence, the alternator B instead of supplying power starts receiving power from the bus bar. Thus, the machine starts working as a motor.

When both the alternator A and B or any one of them is operating in parallel and the driving force of any of them is removed, it continues to run as a motor receiving power from the other alternators or bus bar. The machine receiving power from the bus bar is called as Synchronous Motor.

Main Features of Synchronous Motor

Some of the main features of synchronous motor are as follows:-

  • It runs either at synchronous speed or not at all. That is while running it maintains a constant speed. The speed is independent of load.
  • The synchronous motor is not inherently self-starting. It has to run up to synchronous speed by any means before it can be synchronized to the supply.
  • The Synchronous Motor operates under both lagging and leading power factor.
  • If the counter torque is increased beyond the maximum torque that the machine can develop while running, the motor will stop.


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