Pulsing Torque

The pulsating torque generated in the starting of synchronous machines happens because of the protrusions of the poles, Gates B93 since the torque during the start hesitates around an average value with frequency equal to twice the slip and its magnitude is indicated by the degree of asymmetry of the circuit of the rotor. If the field is excited prematurely at start, it will generate a large component of pulsating torque at the slip frequency, making it impossible to arrive at synchronism. This asymmetry also causes a decrease in the average torque when the rotor reaches speed greater than half the synchronous speed.

This fact is the negative sequence component of the air gap flux interacting with the stator, generating resistive torque. The field windings involving only the direct axis of the rotor, is also responsible for generating the pulsating torque. For the synchronous motor with the excited field, the electromagnetic geometry of the rotor repeats once every two poles, plus a degree of asymmetry. For these reasons motor startup is normally done with the field windings short-circuited or a resistor is connected to its terminals.