Contactor
Also known as: power contactor, motor contactor
Short definitionAn electromagnetic switch that closes its main contacts on a small control current through its coil and so powers heavy loads such as compressors, fans and heaters.
The output of a thermostat or controller cannot carry more than a few amperes, while a compressor needs tens of amperes. The contactor stands in between. When the coil is energised the magnet pulls in the armature and three contacts close together; when power is removed a spring separates them all. Coil voltage is usually 24 V or 230 V and must match the control circuit. The auxiliary contacts on top are used for interlocking and signalling.
In a panel quote the type and current of the load decide the choice: the AC-3 rating applies for motors, AC-1 for heaters. The compressor contactor is taken slightly above the maximum operating current; the AC-3 class itself covers the starting current of 5–8 times that. On a compressor that starts often, one size larger lasts longer. A worn contact either fails to pass one phase or welds shut. This is why a defrost heater gets a separate safety thermostat.