Compressors may have one of three voltage rating specifications. Service technicians need to know that potential relays must be sized to each individual compressor.

All potential relays have voltage ratings or voltage specifications. An example of a potential relay specification is shown in Figure 1.

Pickup voltage: The pickup voltage is the back EMF (electromagnetic field) voltage generated across the start winding by the motor’s rotor when it is up to about three-quarters speed.

The pickup voltage for a specified potential relay is listed as a minimum and a maximum. The actual pickup voltage must stay within this range for proper operation. If the pickup voltage generated by back EMF is under the minimum, the contacts between terminals 1 and 2 will never open. The start capacitor will then stay in the circuit. This will cause high amp draw and the motor’s protective devices will open.

If the pickup voltage generated by back EMF is above the maximum, the relay coil stands a good chance to overheat and open circuit. Again, the contacts between 1 and 2 would stay closed, causing high amp draw if the relay coil opens circuit.

Continuous coil voltage: Potential relays also have a continuous coil voltage rating. This is the maximum back EMF that the relay’s coil can tolerate continuously without overheating and opening circuit.

Dropout voltage: Potential relays also have a dropout voltage rating. This is the back EMF voltage that must be generated across the relay coil to “hold” the contacts between 1 and 2 open once they have been picked up (opened).

Note: It takes more back EMF to pick up and open the contacts (more pickup voltage) than it does to hold them open (dropout voltage). Once the cycling control opens the circuit, the rotor will decrease in speed, thus generating less back EMF across the start winding and relay coil. As the back EMF drops below the dropout voltage, the contacts between 1 and 2 return to their normally closed position and are ready for the next starting cycle.

If you’re not sure, consult with a service manual, the compressor manufacturer, or a supply house for information on selecting the correct potential relay.

Replacement relays can be cross-referenced for different manufacturers using conventional tables. For example, Steveco has cross-reference material that list two other relay manufacturers (RBM and GE), in which a cross-reference can be made for a replacement potential relay with the same specifications. (See Figure 2.) Whenever possible, the model number of the old relay should be used when ordering a new relay.

Next month’s column will cover troubleshooting the potential starting relay.

Tomczyk is a professor of HVAC at Ferris State University, Big Rapids, MI, and the author of Troubleshooting and Servicing Modern Air Conditioning & Refrigeration Systems, published by ESCO Press. To order, call 800-726-9696. Tomczyk can be reached at tomczykj@tucker-usa.com (e-mail).

Publication date: 10/07/2002