SOLUTION

The maximum value of torque that a synchronous motor, can develop without losing its synchronism, is known as Pull out torque.

Torque Characteristics of Synchronous Motor

The different types of torques developed by a synchronous motor are as follows:

Pullout torque: Maximum steady-state torque developed by the motor for 1 minute before it pulls out of step due to overload or the maximum value of torque that a synchronous motor can develop without losing its synchronism.

Starting or breakaway torque: Torque developed at the instant of starting at zero speed with rated voltage applied at rated frequency.

Pull-up torque: Minimum torque developed by the motor during acceleration from rest to the pull-in speed.

Pull-in torque: It is defined as the maximum load torque under which the motor will pull its connected load inertia into synchronism at rated voltage and frequency when excitation is applied. At the pull-in point, the motor torque just equals the load torque. The torque developed by the armature and field windings becomes zero at synchronous speed and cannot pull the motor into step. The torque developed by a synchronous motor (when operating as an induction motor) at 95% speed (5% slip) is defined as “nominal pull-in torque” and is used as a characteristic value of the motor.

Synchronous torque: Steady-state (load dependent) torque developed during operation.

Maximum torque: The peak of the torque-slip curve at rated voltage. The stable operating range of the motor is between the slip corresponding to the maximum torque and the synchronous speed. If during transient changes in load, the slip increases beyond this point, the motor may stall.

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