Thermal overload relay
Also known as: overload relay, thermal relay
Short definitionIf the motor draws more than the set current for some time, the bimetal strips inside heat up, break the control circuit and the contactor drops out. It protects the winding against overload and burn-out.
It sits directly below the contactor, and the three phase currents of the motor flow through its bimetals. As current rises the strips bend, and at a threshold the mechanism trips. Its behaviour mimics the heating of the motor: on a small excess it waits for minutes, on a large one seconds are enough, and it ignores the brief starting current. After a trip it cannot be reset until it has cooled for a few minutes. It does not cover short circuits; a fuse or motor protection circuit breaker is needed upstream.
In the panel the dial is turned to the current on the motor nameplate, and that current should fall in the middle of the relay’s range. Trip class 10 suits most compressors and fans. If the relay has tripped, look for the cause first: a fouled condenser, a fan that is not turning, unbalanced or low mains voltage, a lost phase, a loosened terminal. Raising the setting does nothing except leave the motor unprotected. Manual reset is preferred on refrigeration compressors, otherwise the fault goes unnoticed.