The torque required to operate the synchronous motor at every moment from its initial state to final shutdown is important in understanding its characteristics. Also to use the synchronous motor for different applications, the types of torques associated with the motor should be known.
The various types of torque associated with the synchronous motor are pull-in torque, pull-out torque, starting torque, and running torque. Let us discuss them in detail.
Starting torque is also known as the locked rotor torque. It is the minimum value of the torque developed by a synchronous motor with the rotor locked (i.e. stationary rotor) at any angular position of the rotor on the application of rated voltage and frequency. The locked rotor torque is provided by the armature windings of the motor.
It is sometimes called as breakaway torque. It is the torque required to start the rotation of the motor from its stationary position. It can be as low as 10% for centrifugal pumps and as high as “200 or 250% of full load torque” for loaded reciprocating two-cylinder compressors.
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. Peak output power determines the maximum running torque that would be required by the driven machine.
Since the synchronous motor is not a self-starting motor, it is started as an induction motor and runs at a speed of 2% to 5% below the synchronous speed. The DC excitation voltage is then applied, and the rotor pulls into phase with the stator’s rotating magnetic field, which is revolving at synchronous speed.
Pull-in torque is the maximum torque at rated voltage and frequency under which a synchronous motor will pull a connected load into synchronism when DC excitation is applied to the motor.
There is a limit to the mechanical load that can be applied to a synchronous motor. With the increase in the load, the torque angle or the load angle (δ) also increases so that a stage is reached. At this stage, the motor torque will be less than the load torque hence the rotor gets pulled out of synchronism and the motor comes to the rest.
Therefore, Pull-out torque or breakdown torque is the maximum value of load torque that a synchronous motor can develop at rated voltage and frequency without losing synchronism.