Potential relay

The potential relay is used in all capacitor start/capacitor run motors. They’re also used in capacitor start/induction run motors. However, it’s rare to see that style of a motor in residential and lite commercial in our industry.

The potential relay may also be called a start relay. Without it, the start capacitor could explode or destroy the windings in the motor within seconds. Using a start capacitor with a potential relay is known as a hard start kit.

Schematic layout of the relay above

With the metal rotor turning and surrounded by motor windings, this generates a significant voltage called back emf. As back emf is generated across the start winding, a small current will flow through the start winding and across the coil of the potential relay. With this happening, the potential relay takes the start capacitor out of the circuit using this back emf when the motor reaches about 3/4 of its speed.

With power removed from the motor, the back emf generating across the start winding of the motor will become too low and the contacts that were opened will again close.

The potential relay has a coil wired between terminals 2 and 5 and NC (normally closed) contacts between terminals 1, and 2. 4 and 6 are dummy/ convenience terminals.

Testing the relay

Checking a potential relay is quite simple as there’s not much to them.

Coil – Between terminals 2 and 5

The coil of the relay is between terminals 2 and 5. It’s very high resistance, so use a high setting on your meter if it doesn’t self-adjust. You should have a reading in the thousands of ohms. I’ve typically seen them around 10kΩ – 12kΩ. OL or infinity would indicate no continuity through the coil meaning a broken wire in the coil, and this potential relay would need replacement.

NC Contacts – Between terminals 1 and 2

With power off to the appliance, disconnect the wires on terminals 1 and 2. Using an Ohm meter, check resistance across the two terminals. You should read near 0Ω or very very close. While set to continuity with the beep, you should hear the beep sound indicating the contacts are closed.

If you read OL or infinity, that means the NC contacts are not open. They would be stuck open. You would need to replace this potential relay.

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