Many single-phase motors used with refrigeration-duty, semi-hermetic and hermetic compressors require an external relay to assist in starting. How much do you know about these necessary products?

An external relay is required because the motor is enclosed in an oil-vapor-rich environment. If an internal relay were used, any arcing that might occur as the contacts open or close could cause the oil vapor to ignite - not good, and potentially very dangerous.

The main purpose of this starting relay is to bring in an additional electrical circuit, giving the motor the extra torque it requires to start. This additional electrical circuit can be the addition of a motor's start winding, the addition of a start capacitor, or the addition of both the start winding and a start capacitor.

Several types of starting relays are used to assist in the starting of single-phase compressors. These include the current relay, potential relay, and PTC (positive temperature coefficient) relay. Each performs the same function but in a different manner.

Three Types Of Relays

A current relayuses the high-inrush current drawn by a motor when starting to energize the coil of a relay and bring in the additional starting circuit. The coil of this relay is wired in series with the run winding to sense that initial inrush of current.

The contacts of this relay are normally open and are wired in series with the starting circuit. As the motor attempts to start, the contact point closes and brings in the starting circuit. Once the motor starts, the contact's points open, removing the starting circuit from the motor.

A potential relay uses the induced voltage created across the start winding to energize the coil of the relay. The coil of this relay is wired in parallel with the motor's start winding. The coil is energized when the induced voltage across the start winding reaches its designed value, commonly referred to as its pickup voltage.

When the coil is energized, it causes a normally closed set of contacts to open, taking the starting components out of the circuit. This set of contacts remains open as the motor operates; they close again when the motor shuts down.

A PTC relay uses a positive temperature coefficient thermistor to remove the starting components from the circuit. The circuit is wired so that the PTC thermistor is in series with the starting components. When the motor starts, current flows through both the thermistor and the starting components. As current flows through the thermistor, the thermistor quickly heats up and its resistance quickly increases to a point where it is so high, it will essentially re-move the starting components from the circuit. By this time, the motor will have started.

Once used, a PTC relay must be allowed to cool down before it can be used again. There must be at least a several-minute delay between stopping and restarting a motor, in order to give the PTC relay time to cool down.

Joe Marchese is owner of Coldtronics, Pittsburgh. He can be reached at 412-734-4433,, or

Publication date: 04/05/2004