Potential Relays - What Happened to Terminal 3?

Potential relays also known as voltage relays are used in capacitor-start capacitor-run (CSR) motors. CSR motors have very high starting torque but run efficiently once started. The potential relay’s only function is to assist in starting the motor. Once the motor has started, the potential relay is no longer in the circuit.

We often see potential relays installed on residential, single phase compressors. Understanding how the voltage relay’s connections and sequence of operation is important when diagnosing compressors that aren’t starting.

Potential Relay Connections

• Potential relays have 5 terminals: 1, 2, 4, 5, and 6.
• Only 3 terminals are used: 1, 2, and 5
• Terminals 2 to 5 are the coil terminals.
• Terminals 1 to 2 are the normally closed contact terminals
• Terminal 1 is always connected to the start capacitor.
• Terminal 2 is is always connected to the start winding of the compressor.
• Terminal 5 is always connected to the common terminal of the compressor.
• Terminals 4 and 6 are connection or dummy terminals and have no bearing on the operation of the potential relay.

Legend SC - Start Capacitor

RC - Run Capacitor

SR - Start Relay

R - Compressor Run

C - Compressor Common

S - Compressor Start

The highlighted region in the illustration above show the potential relay’s wiring path.

Potential Relay Connections

• Potential relays have 5 terminals: 1, 2, 4, 5, and 6.
• Only 3 terminals are used: 1, 2, and 5
• Terminals 2 to 5 are the coil terminals.
• Terminal 1 is always connected to the start capacitor.
• Terminal 2 is is always connected to the start winding of the compressor.
• Terminal 5 is always connected to the common terminal of the compressor.
• Terminals 4 and 6 are connection or dummy terminals and have no bearing on the operation of the potential relay. Terminals 1 to 2 are normally closed contacts.

Potential Relay Operation

1. The SC and RC are wired in parallel, increasing the capacitance at start-up and providing the torque when power is first applied.
2. The high resistance coil of the potential relay operates on counter electromotive force (CEMF) generated in the start winding.
3. During the start-up period the CEMF is insufficient to operate the relay.
4. The CEMF generated across the start winding causes a small current to flow in the start winding and through the potential relay coil.
5. Once the compressor motor reaches 75-80% of its full load speed the CEMF (back voltage) is high enough to energize the coil of the potential relay, opening up the contact between terminals 1 and 2.
6. When terminals 1 and 2 are open, the start capacitor is no longer in the circuit and the compressor is now operating as a permanent split capacitance (PSC) motor.
7. When the power is removed and the compressor rotation begins to decrease, so does the CEMF.
8. Once the potential relay no longer has enough voltage to hold the contacts open and the contacts between terminal 1 and 2 return to their normally closed position by spring power.

The CEMF which opposes line voltage and can be across the coil of the potential relay at terminals 2 and 5. The CEMF is usually a higher voltage than the line voltage and can be as high as 400-450 volts.