The output of a thermostat, pressure switch or controller can carry only a few amperes; a compressor, however, draws tens of amperes. The contactor is the bridge between the two. When the control voltage (mostly 230 V or 24 V) reaches its coil, the electromagnet pulls in the moving armature, the three main contacts close together and the motor is energised. When the voltage is cut the spring opens the contacts. The auxiliary contacts are used for interlocking and signalling. On small single-phase compressors the same job is done by a relay.
Two values are decisive in selection: the type of load and its current. For motors one looks at the rated current in utilisation category AC-3, for heaters in AC-1; the same contactor carries a higher current in AC-1. The compressor contactor is chosen a little above the compressor’s maximum operating current; the starting current is 5–8 times this, and the AC-3 category already takes this start into account. The coil voltage must be the same as the control circuit. In applications with frequent starts choosing one size larger extends the life.
The contacts make a small arc and wear at every opening and closing. Electrical life is around a few hundred thousand to one million operations at full load; in a short-cycling compressor that makes more than six starts an hour this life is used up in a few years. Worn contacts heat up, one phase fails to make contact and the motor is left on two phases, or the contacts weld and the load never switches off again. Both situations are dangerous for the compressor.
A welded contactor means the risk of fire at the defrost heater and, at the compressor, of not stopping even when the pressure switch opens; that is why an independent safety thermostat is fitted in heater circuits. In maintenance the colour of the contacts, any humming noise and the tightness of the terminals are checked; a loose terminal blackens and burns. At low voltage the coil cannot pull the armature in fully, it chatters and finishes off the contacts in a short time. Contacts are not sanded; a worn contactor is replaced. For replacement a contactor of the same category, current and coil voltage is used; the auxiliary contact arrangement is also kept.