A three-phase motor does not stop when one of the phases is lost; it tries to keep turning by drawing a much higher current from the remaining two phases, and its windings burn out within minutes. The phase monitoring relay measures the voltage of the three phases continuously; when it sees a problem it opens its output contact, the contactor drops out and the motor stops. When the supply recovers it closes the contact again after the set delay. It is a small, cheap device; its price is insignificant beside the compressor it protects. It is found as standard in most refrigeration panels.
Models that combine four protections are common. Phase loss: one phase is missing. Phase sequence: the phases are connected in reverse and the motor would turn backwards. Imbalance: the voltage difference between the phases has exceeded the set percentage (usually adjustable between 5 and 15%). Undervoltage and overvoltage: the supply has gone outside a certain proportion (for example ±10–15%) of the rated value. The trip and reclose delays are set between a few seconds and a few minutes. The delay prevents unnecessary stoppages during momentary voltage dips.
Phase sequence protection is mandatory for scroll and screw compressors. These compressors compress in one direction only; a scroll compressor running backwards operates noisily, builds no pressure and is damaged in a short time. All it takes is for two phases to be swapped during maintenance on the supply or on a changeover to a generator. A reciprocating compressor compresses in both directions, but the oil pump of some models delivers in one direction only. At first start-up the direction of rotation is also verified by watching the suction and discharge pressures.
Voltage imbalance may look small but its effect is large: a 2% voltage difference between the phases creates a current imbalance several times greater in the windings and noticeable extra heating; motor manufacturers usually require that 2% is not exceeded. The relay is connected on the supply side of the contactor. It does not see a break on the motor side (a burnt contact, a loose terminal); that is the job of the thermal overload relay and the thermistor. If the relay trips frequently the problem is not in the relay but in the supply; the voltages are monitored with a recorder and, if necessary, the distribution company is contacted.