Overload Relay Heater
As shown in Figure 6-62, an overload relay consists of a bimetal contact in series with the motor coil, and a heater in series with the compressor motor. An overload relay heater is a small electric heating device designed to work in conjunction with the bimetal con- tact. When the compressor motor becomes overloaded or stalled, the heavy continuous current through the heater causes the bimetal contact to bend until it opens the motor-starter coil circuit. This action stops the flow of current through the motor starter and results, in turn, in the opening of the load contacts to stop the flow of current to the compressor motor.
An overload relay heater must be accurately sized for the installation. Generally the manufacturer of the cooling equipment will provide instructions for sizing overload relay heaters.