The thermal overload relay is fitted directly below the contactor, and the current of the three phases going to the motor passes through the bimetal strips inside it. As the current rises the strips heat up and bend; at a certain point the mechanism trips, the auxiliary contact opens and the contactor coil is de-energised. Its response resembles the heating of the motor: with a small excess it opens after minutes, with a large excess within seconds. Because the starting current lasts only briefly it does not trip the relay. A tripped relay can be reset only a few minutes later, once the bimetals have cooled.
The setting dial is set to the rated current on the motor nameplate; the relay does not open up to about 1.05 times this value and opens within two hours at 1.2 times. The trip class is chosen according to the starting time: class 10 opens within 10 seconds at the latest at 7.2 times the set current and is suitable for most compressors and fans. The relay does not protect against short circuit; there must be a fuse or a motor protection circuit breaker ahead of it. A relay is chosen whose setting range has the motor current near its middle.
Most of today’s thermal overload relays are sensitive to phase loss: if one of the phases is lost the bimetals bend differently and the relay opens earlier than normal. Even so, a separate phase monitoring relay provides faster and more definite protection. Hermetic and semi-hermetic compressors also have temperature sensors (thermistors) embedded in the winding, together with their module; the thermal overload relay monitors the current, the thermistor the winding temperature directly. Electronic overload relays do the same job more precisely and are not affected by ambient temperature.
If the overload has tripped, one does not simply press the button and move on without finding the cause. The usual causes are these: high condensing pressure (dirty condenser, stopped fan), low or unbalanced voltage, loss of a phase, mechanical seizure, a weak contactor contact and a loose terminal. Raising the setting does not solve the problem; it leaves the motor unprotected. The automatic reset position is not recommended for refrigeration compressors; the relay is left in the manual reset position so that the fault is noticed. When the relay trips, the current and voltage of the three phases are measured and recorded.