Why synchronous motor is not self starting:

When three phase currents are flowing in the 3- phase winding on the stator of a synchronous motor, a synchronously rotating magnetic field is set up. If the rotor is stationary then relative speed between standstill rotor field and rotating stator field is equal to the synchronous speed. Consequently the torque angle between rotating stator poles and stationary rotor poles is a function of time. Therefore the electromagnetic torque varies sinusoidally with time. It reverses during each half cycle and therefore, the average value of electromagnetic torque over a complete cycle is zero. Hence a synchronous motor, on is own, has no net starting torque.

                               Schematic diagram of Synchronous machines

For another way of describing that a synchronous motor has no net starting torque, refer to two pole synchronous motor. With three currents in the three phase armature winding, stator N,S poles rotate at synchronous speed. At the instant, stator N,S poles produce a clockwise torque on the rotor as shown in Fig. (1). After a half cycle  i.e., after 0.01 sec [= 1/2f] for a 50 Hz supply system.

stator poles occupy the position as shown in Fig.(2). Now the stator N. S poles repel stationary rotor N. poles, thus producing a counter-clockwise torque. The rotor is urged to rotate clockwise, but soon after, i.e. , after 0.01 sec, the rotor is urged to rotate anti-clockwise. The rotor due to its inertia can’t respond to such quick reversals electromagnetic torque. Consequently the rotor remains at standstill showing thereby that in a synchronous motor, the net starting torque is zero. In other words the relative speed between stator field and rotor field is equal to Ns, but for the production of unidirectional torque two fields must be stationary w.r.t each other.

  • In brief, when the supply is given to the stator, the stator poles rotate at synchronous speed but the rotor is not able to catch the quick reversal of the stator poles due to mechanical inertia of the rotor. Then the stator is subjected to clockwise torque and counter clockwise torque. Therefore average torque is equal to zero during starting condition. Hence synchronous motor is not self starting.


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