From Protection of Electrical Networks
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
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;
10.4.1 Protection of medium voltage motors
Protection against overloads
These are revealed by the occurrence of overcurrents which, if prolonged, cause excessive heating leading to accelerated ageing of the motor.
Protection is ensured by the following methods:
either by a thermal overload protection (see section 7.7);
or by a temperature monitoring device (see section 7.23).
Protection against internal or external phase short-circuits
External faults may occur at the terminals or in the power cable between the switchboard and motor. Internal faults may occur between two turns of the same phase or different phases following the deterioration of the insulators. They produce considerable phase-to-phase overcurrents.
Protection may be ensured by the following methods:
By independent time phase overcurrent protection devices for equipment with circuit-breaker.
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