Different Torques of a Synchronous Motor
Different Torques of a Synchronous Motor:
Various torques associated with a synchronous motor are as follows:
1. starting torque
2. running torque
3. pull-in torque and
4. pull-out torque
(a) Starting Torque
It is the torque (or turning effort) developed by the motor when full voltage is applied to its stator (armature) 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.
(b) Running Torque
As its name indicates, 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 required by the driven machine. The motor must have a breakdown or a maximum running torque greater than this value in order to avoid stalling.
(c) 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.
(d) Pull-out Torque
The maximum torque which the motor can develop without pulling out of step or synchronism is called the pull-out torque. Normally, when load on the motor is increased, its rotor progressively tends to fall back in phase by some angle (called load angle) behind the synchronously-revolving stator magnetic field though it keeps running synchronously. Motor develops maximum torque when its rotor is retarded by an angle of 90º (or in other words, it has shifted backward by a distance equal to half the distance between adjacent poles). Any further increase in load will cause the motor to pull out of step (or synchronism) and stop.