pole slipping

Pole slipping typically occurs under severe fault conditions which cause a transient torque on the generator which exceeds the ability of the field to hold the generator rotor synchronised to the stator. A generator is most susceptible to this problem when it has a low excitation, as this produces a weak magnetic field. For this reason, capability diagrams show the stability limit for the machine when in an under-excited state. Outside of this line, pole slipping becomes a real possibility in the event of a system fault.

The 'slip' occurs when the rotor experiences a sudden physical and electrical shift in position relative to the stator, after which the field recovers enough strength to lock the rotor back in synch with the stator. When his occurs, the violent acceleration and deceleration associated with pole slip causes enormous stress on the generator and prime mover, and may result in anything from winding movement to shaft fracture or worse. It is a very serious fault.