METHODS OF STARTING OF SYNCHRONOUS MOTOR

(1) By using a starting motor. This motor is directly coupled to the motor. It may be an induction motor which can run on a synchronous speed closer to the synchronous speed of the main motor.

(2) Starting as an induction motor. This is the most usual method in which the motor is provided with a special damper winding on rotor poles. The stator is switched on to supply either directly or by star delta/reduced voltage starting. When the rotor reaches more than 95% of the synchronous speed, the dc circuit breaker for field excitation is switched on and the field current is gradually increased. The rotor pulls into synchronism

(A) Pull-in torque. It is the maximum constant load torque under which the motor will pull into synchronism at the rated rotor supply voltage and rated frequency, when the rated field current is applied

(B) Nominal pull in torque. It is the value of pull in torque at 95 percent of, the synchronous speed with the rated voltage and frequency applied to the stator when the motor is running with the winding current.

(C) Pull out torque. It is the maximum sustained torque which the motor will develop at synchronous speed for I minute with rated frequency and with rated field current.

(D) Pull up torque. It is the minimum torque developed between standstill and .pull in point. This torque must exceed the load torque by sufficient margin to ensure satisfactory acceleration of the load during starting.

(E) Reluctance torque. It is fraction of the total torque with the motor operating synchronously. It results from saliency of the poles. It is approximately 30% of the pull-out torque.

(F) Locked rotor torque. It is the maximum torque which a synchronous motor will-develop at rest, for any angular positions of the rotor at the rated voltage and frequency.

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