One type of starting relay used on fractional-horsepower compressors is the current relay. This type of starting relay uses the initial high inrush current draw of a compressor’s run winding to energize its coil and bring in its additional starting circuit. The coil of this relay is wired in series with the run winding so as the compressor starts it draws the same initial inrush current as the run winding. The contacts of this relay are normally open and are wired in series with the starting circuit. As the compressor attempts to start, the higher current draw causes the relay’s coil to be sufficiently energized, closing its contacts and bringing in the starting circuit. This gives the compressor the initial torque required to start. As the compressor’s rotor approaches its normal operating speed, the current draw drops causing the relay’s contacts to open and remove the starting circuit.
A defective current relay can cause a compressor not to start. If the relay has an open coil or its contacts are stuck open, the starting circuit will not be energized during the starting of the compressor. This will cause the compressor not to have the required starting torque. If its contacts are stuck closed, the starting components of the motor will not be removed from the circuit and the compressor will cycle off on its overload.