10.4 Motor protection

Motor protection is provided by the set of devices ensuring that deterioration due to abnormal supply, motor or load operating conditions is avoided.

Main faults affecting an asynchronous motor

• overload;

• internal or external phase short-circuits;

• phase cutting, inversion of two phases and phase unbalance;

• stator frame fault;

• excessive start-up time and locked rotor;

• drop in supply voltage;

• closing of the switching device while motors still hold a considerable remanent voltage;

• too many successive start-ups;

• drop in current;

• rotor frame fault (for slipring rotor motors).

Main faults affecting a synchronous motor

To the faults listed for asynchronous motors may be added: loss of synchronism;

• loss of excitation;

• rotor frame fault;

• prolonged asynchronous operation upon start-up;

• overload and short-circuit in excitation windings;

• generator operation.