Phase monitoring relay
Also known as: phase failure relay, phase sequence relay
Short definitionAn electronic relay that watches the three-phase supply. When a phase is lost, the sequence is reversed, the voltages become unbalanced or go out of limits, it opens the control circuit and the compressor stops.
A motor that loses one phase does not stop by itself; it keeps turning by drawing excessive current from the other two, and the winding burns out minutes later. The relay measures the three voltages continuously, releases its contact on a fault, and the contactor drops out. Once the supply recovers, the set delay runs out and it allows a restart. The delay also prevents needless stops on momentary dips. Its price is tiny next to that of the compressor, so it is not left out of the panel.
In quotes with scroll and screw compressors, phase sequence protection is a must, because these machines compress in one direction only. If two phases are swapped during a changeover to the generator or a repair on the network, a scroll runs backwards, builds no pressure and fails quickly. For unbalance, motor manufacturers mostly ask for a 2% limit. The relay is fitted on the supply side of the contactor; it cannot see a burnt contact or a loose terminal on the motor side, which is left to the thermal overload relay.