Inherent Protector
An inherent protector is another safety device used to protect an electric-driven compressor motor from overload damage. It accomplishes this purpose by disconnecting the motor from the power supply when the motor temperature or current becomes excessive. Its function is similar to that of the overload relay.
An inherent protector is essentially a thermostat operated by the snap action of a bimetal disc. As shown in Figure 6-63, it consists of a heater, thermostatic disc, and contacts.
In operation, the motor current flows through both the motor winding and the heater. When an overload condition occurs, the critical temperature level is reached in the motor winding at exactly the same time that the protector reaches its tripping point. When the temperature rises to the rating of the bimetal disc, the disc snaps open, reversing its curvature, and cuts off the flow of current to the motor. When the temperature drops, the action is reversed.
Inherent protectors are available for all sizes of single-phase compressor motors. For three-phase motors, they are available for motor sizes up to 11⁄2 hp.