Type of Torque in the Synchronous Motor

In this article, we will learn about the different torques in synchronous motor.

The torque developed by a synchronous motor under running conditions is known as running torque. The running torque is determined by the power rating and the speed of the motor.

These torques are always kept in mind whenever a synchronous motor is selected for any particular application. Different Torques are:

1. Locked Rotor Torque.
2. Running Torque.
3. Pull-In Torque.
4. Pull-Out Torque.

LOCKED ROTOR TORQUE:

As the name suggests, rotor of the synchronous motor is locked (i.e, kept stationary) and the motor is supplied with rated voltage and frequency. So, the minimum torque which is developed in the rotor is called Locked Rotor Torque.

RUNNING TORQUE:

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It is the torque developed by the motor under running conditions. It is determined by the power rating and speed of the driven machine.

PULL IN TORQUE:

First of all, synchronous motor is started as induction motor. Its speed is kept 2%-5% below the synchronous speed. Then DC excitation is applied and the rotor starts to step with the synchronously rotating stator field.
So, pull in torque is the maximum constant torque at rated voltage and rated frequency under which a synchronous motor will pull a connected load into synchronism when the DC excitation is applied.

PULL OUT TORQUE:

It is the maximum value of the torque which a synchronous motor can develop at rated voltage and rated frequency without losing its synchronism.