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

  • 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.

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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Products & Services
Motor Controllers
Motor controllers receive supply voltages and provide signals to motor drives that are interfaced to motors. They include a power supply, amplifier, user interface, and position control circuitry.
Ground Fault Relays
Ground fault relays protect electrical equipment from ground faults. A ground fault is an unintentional current path between a current-carrying conductor and a grounded surface.
Stepper Motors (rotary)
Stepper motors use a magnetic field to move a rotor in small angular steps or fractions of steps. They provide precise positioning and ease of use, especially in low acceleration or static load applications.

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