Different Torque of a Synchronous Motor

DIFFERENT TORQUE OF A SYNCHRONOUS MOTOR

Various torques associated with a synchronous motor are as follows:

1) Starting torque

2) Running torque

3) Pull in torque

4) Pull out torque.

Starting Torque:

It is the torque developed by the motor when full voltage is applied to its stator winding. It is also sometimes called breakaway torque. Its value may be as low as 10% as in the case of centrifugal pumps and as high as 200 to 250 % of full load torque as in the case of loaded reciprocating two-cylinder compressors.

Running Torque:

It is the torque developed by the motor under running conditions. It is determined by the horse-power and speed of the driven machine. The peak horsepower determines the maximum torque that would be by the driven machine. The motor must have a break down or a maximum running torque greater than this value in order to avoid stalling.

Pull in Torque:

A synchronous motor is started as induction motor till it runs 2 to 5% below the synchronous speed. Afterwards excitation is switched on and the rotor pulls into step with the synchronously rotating stator field. The amount of torque at which the motor will pull into step is called the pull in torque.

Pull out torque:

The maximum torque which the motor can develop without pulling out of step or synchronism is called pull-out torque.

Salient Pole Synchronous Motor

Cylindrical-rotor synchronous motors are much easier to analyze than those having salient-pole rotors. It is due to the fact that cylindrical-rotor motors have a uniform air-gap whereas in salient pole motors, air gap is much greater between the poles than along the poles. Cylindrical-rotor theory is reasonably accurate in predicting the steady state performance of salient pole motors. Hence salient pole theory is required only when very high degree of accuracy is needed or when problems concerning transients or power system stability are to be handled.

The d-q currents and reactance for a salient pole synchronous motor are exactly the same as discussed for salient pole-synchronous generator. The motor has d-axis reactance X_d and q-axis reactance X_q. Similarly motor armature current I_a has two components = I_d and I_q.

With the help of Figure (b) 3.64,

The magnitude of the excitation or the back emf.

E_b = V cos α – I_a R_a – I_d X_d

Similarly for synchronous motor with R_a = 0

If R_a is not negligible. It can be proved that: