Current starting relays are used on single-phase, fractional horsepower motors requiring low starting torque. Their main function is to assist in starting the motor. Start and run capacitors can be used in conjunction with current relays to boost both starting and running torque.

Current relays are often seen when capillary tubes or fixed orifices are used as metering devices; the reason being that systems employing capillary tubes and fixed orifices as metering devices will equalize pressures during their off cycle. This will cause a lower starting torque than systems that do not equalize their pressure during the off cycle, as with a conventional thermostatic expansion or automatic expansion metering device.

Some typical applications for current relays on compressors might include domestic refrigerators, drinking fountains, small window air conditioners, smaller ice machines, and smaller supermarket display cases.

Current starting relays consist of a low resistance coil and a set of normally open contacts. The coil is wired between terminals L and M. The contacts are wired between terminals L and 2 when a start capacitor is used. If the start capacitor is not employed, the contacts may be wired differently between terminals L and S. Terminals L, S, and M are typical identifier terminals for current relays. L is for Line, S is for Start winding, and M is for Main winding. This should help technicians wire a current relay to a compressor's motor.

Remember, when a start capacitor is being used, terminals 2 and 3 may come into play. Also, when wiring capacitors to a motor using a current relay, always wire the starting capacitor in series with the start winding. The run capacitor will always be wired between the run and start winding terminals. Also, remember that different manufacturers may vary their terminal designations somewhat.


When power is applied through the cycling control, both the run (main) winding and the relay coil see locked rotor current because they are in series with one another. The start winding cannot experience any current flow because of the normally opened contacts of the relay being wired in series with it.

Because both run winding and relay coil experience locked rotor current, the relay coil will form a strong electromagnetic field around it from the high locked rotor current draw of the run winding. Because the relay coil has a very low resistance (usually under 1 ohm), it will not be a large power consumer to interfere with the run winding's power consumption needs. The strong electromagnetic field formed around the relay's coil will make it an electromagnet. This is caused from the iron core that the relay coil is wrapped around. This magnetism is the force that will close the normally open contacts in series with the start winding and start capacitor. The motor's rotor now starts to turn.

Once the start winding is closed, the motor will quickly accelerate in speed. Once the motor has reached about three-fourths of its rated speed, the current draw of the run winding will decrease from a counter electromagnetic force (CEMF) on it. It is this reduced flow that will decrease the electromagnetic force in the iron core the relay coil is wrapped around. Now, spring pressure or gravity forces the contacts between (L and 2) or (L and S) back to their normally opened position.

On capacitor start motors, this action takes both the start capacitor and start winding out of the circuit. On capacitor start-capacitor run motors, the action only takes the start capacitor out of the circuit. The start winding will be left in the circuit by the run capacitor's wiring. But line power will not be directly applied to the start winding.

The run capacitor will help with the running torque and also limit the current draw through the start winding while the motor is in the running mode. This configuration makes the motor a permanent split capacitance (PSC) motor only while running.


A simple ohmmeter is all that is needed to troubleshoot a current starting relay. After taking the connecting wires off of the relay and disconnecting it from the motor, measure the resistance across the relay coil between L and M. Since this relay coil wire is a very short and fat wire, its resistance will be very low (usually less than 1 ohm). If the resistance is close to this, the coil is good.

However, if the resistance reads infinity, the relay coil is opened and the run winding will never be energized because of the open circuit in the relay coil. The motor will not hum or try to start because of the opened circuit. If the coil is found to be opened, discard the relay and install a new one.

Use the model and serial number on the old relay when ordering a replacement. Often cross-reference charts are handy when replacing relays made by different manufacturers. Never wire just any current relay to a compressor. The amperages and counter electromotive forces are different for each motor. If in doubt, contact the compressor manufacturer.

The contacts of a current relay can also be measured with an ohmmeter. First, take the wires off of the relay and disconnect it from the motor. Before ohming the contacts, it is critical to place the relay in the same position that it would be in when installed on the motor. If the relay is held in the wrong position (upside down), the normally open contacts will become normally closed from gravity forces overcoming spring forces.

Once in the proper position, simply ohm between (L and S) or (L and 2), depending if a start capacitor is used or not. The contacts should be open and the ohmmeter should read infinity. If the ohmmeter measures resistance, the contacts would be stuck or arced closed. This condition would cause the start winding and capacitor to be energized continuously, causing very high amp readings when the motor is running. The amperage reading would be somewhere between Locked Rotor Amps (LRA) and Running Load Amp (RLA). Motor protector devices will soon protect the motor windings and open the circuit.

If the relay is then turned upside down, the contacts should close. An ohmmeter reading should now read 0 ohms. If this doesn't occur, the contacts probably will never close, and the motor will never leave its locked rotor position. High amp draws (locked rotor amp ‘LRA') will be experienced and a humming sound will occur until the motor protection devices open the circuit.

Publication date: 09/04/2006