Electric Motor Overload Relay

A typical overload event is that of a motor bearing failure in which a bearing begins to seize the motor shaft but yet still allows the shaft to turn. This type of bearing failure causes a continuous drag on the motor rotor, causing a high motor current. Overload relays are normally used in conjunction with an electromechanical contactor, which may be used to disconnect power from equipment. Electric motors are one type of electrical load which can be started and stopped using a contactor.

The overload relay senses an overload condition by monitoring the current in each of the three-phases received by the motor windings. The overload relay includes current sensing elements that are wired in series with the three phases passing through the contactor. In this way, the overload relay can monitor current flowing in the three phases through the contactor, and based on current magnitude and duration, may interrupt the current flow through the contactor armature circuit to open the contactor contacts when an overload occurs.