Motor protection

There are basically two ways to protect an electric motor from overload during operation:

  • Monitoring of its power consumption e.g. by Motor protection switch or Motor Protection Relays
  • Direct monitoring of the temperature of the motor winding e.g. by thermistors, which is also known as full motor protection.

Motor protection switch

The motor protection switch basically serves as overload protection, not-self starting (which means a very high current for induction motors) and failure of an external conductor of a 3-phase motor.
Two releases are installed: a thermal release using a bi-metal, which must be adjusted to the rated motor current, and an electromagnetic release, which provides short-circuit protection.

Motor protection switch - Symbol, function

Motor protection switch - Symbol, function


Motor protection relay or overload relay

With the motor protection relay or overcurrent relay (overload relay) the switch-off is only thermal and via control contacts, that means a contactor must be used as switching stage.

Motor protection relay

Motor protection relay


PTC Thermistors

PTC Thermistor

PTC Thermistor

Problem: The motor winding can overheat even if the rated current is not exceeded. Reasons for this can be: Ventilation slits are dirty, ambient temperature too high. The motor protection switch would not respond here.

Solution: The temperature of the motor winding is measured directly by a thermistor (PTC thermistor).

Note: Since the temperature of the motor winding is detected directly, the full motor protection is suitable for all types of electric motors, independent of the current and connection type.