How Does Thermal Protection in a Motor Work?


Electric motors convert electrical energy into mechanical energy using conductors and magnetic fields. Motors require thermal protection to prevent damage due to overheating when this conversion occurs.

Amplifier Overload Capability

  • Amplifiers, or devices used to increase the power supplied by a signal, prevent excess current by providing a specific amount of overload capacity. Amplifiers with a current rating equal to, or less than, the rating of the motor offer thermal protection by not allowing signals carrying excess power to flow through the motor.


  • Fuses act as a sacrificial strip or wire that melts and interrupts excessive motor current. Fuses offer a simple and inexpensive means of thermal protection, but do not permit a controlled shutdown of the motor servo. When an overload occurs, a fuse will fail and interrupt the circuit, immediately shutting down the connection between the electric source and the motor.

Overload Relay

  • Overload protection relays work via a bimetallic strip, which is heated by the motor current and, once a certain level of heat is reached, releases a spring. The spring operates the overload protection relay contacts, which divert the circuit. The operation time of the overload protection relay is fixed, or constant, no matter the current rating of the motor.

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  • "Electric Motors and Drives: Fundamentals, Types and Applications"; Austin Hughes; 2005
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  • "Essentials of Electric Motors and Controls"; Charles M. Trout; 2009

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