Potential relay how does it work

An electric circuit with voltage and current will be generated in the start winding and relay coil, causing the relay coil to energize and open the contacts between contacts 1 and 2.

This action occurs because the relay coil is wrapped around an iron core, which will magnetize once the coil is energized. This opening of contacts causes the start capacitor to be taken out of the circuit, and the motor will continue to run as a permanent split capacitance PSC motor.

Once the operating control opens and power is taken away from the motor, the motor speed will gradually decrease along with the BEMF generated. The relay coil will now de-energize, and the contacts between terminals 1 and 2 will return to their normally closed position as the motor comes to a stop.

Pick-up voltage — An example of a potential relay rating specification is listed in Figure 4. The pick-up voltage for a specific potential relay will be listed as a minimum and a maximum. The actual pick-up voltage must stay within its range for proper operation. If the pick-up voltage generated by BEMF 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 draws and can open motor-protective devices.

However, if the pick-up voltage generated by BEMF 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 draws if the relay coil opens circuit. Continuous coil voltage — Potential relays also have a continuous voltage rating. Drop-out voltage — Potential relays also have a drop-out voltage rating.

Notice in Figure 4, it takes more BEMF pick-up voltage to pick up and open the contacts than it does to hold them open. Once the cycling control opens the circuit, the rotor will decrease in speed, thus generating less BEMF across the start winding and relay coil. As the BEMF drops below the dropout voltage, the contacts between 1 and 2 will return to their normally closed position and be ready for the next starting cycle. Because of these three voltage rating specifications, service technicians must realize that potential relays must be sized to each individual compressor.

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 convenient tables via the internet.

Whenever possible, the model number on the old relay should be used when ordering a new relay. A simple ohmmeter is all that is needed to troubleshoot a potential relay.

After taking all connecting wires off the relay, measure the resistance across the 1 and 2 terminals. The resistance should read close to zero, since they are normally closed contacts. It creates a direct path from L2 run side through the start winding, and as soon as it heats up, the higher resistance essentially removes it from the circuit.

That is NOT the same technology as a start capacitor in any way, and in my experience, they don't work well and are prone to failure, at least in air conditioning systems. However, the most traditional and time-tested method of start assisting a compressor in HVAC is the good old start capacitor and the potential relay. Photo Courtesy of Rectorseal. A run capacitor is sized to create an optimal phase shift for a compressor running at full speed and full design load because the run capacitor never comes out of the circuit.

While a run capacitor has heat dissipation capability for constant duty, a start capacitor MUST be taken out of the circuit VERY quickly to avoid melting down and causing compressor damage. When you see that increased voltage across the capacitor, that is back EMF being generated by the motor, just like in a generator pretty cool, huh? Some hard start manufacturers wire the coil on the potential between start and common, and some wire it between start and run.

You will find that most OEMs wire between start and common, but this does not mean that wiring between start and run is bad; it just needs to be designed correctly for that purpose Kickstart does it this way. A properly sized start capacitor and potential relay are not BAD for a compressor, they just must be sized and installed correctly, and there are some cases where they are more likely to be useful than others. When in doubt, a factory start capacitor with a potential relay is the best and safest bet.

Notify me of follow-up comments by email. Notify me of new posts by email. What does compressor relay do? The AC compressor relay is responsible for supplying power to the AC compressor and clutch. The AC compressor relay is no different then other electrical relays - its electrical contacts will eventually wear or burn out, and the relay must be replaced.

How do you test a motor starter relay? You can also check for resistance by placing the red probe of your multimeter to the ignition circuit terminal and the other to the ground terminal. If the voltage you read is not 12V when the ignition switch is turned on, the starter relay is faulty.

Another way to test for resistance by use of a wire jumper. What does a compressor start relay do? What causes a relay to fail? In fact, the life of a relay is essentially determined by the life of its contacts. Degradation of contacts is caused from high in-rush currents, high- sustained currents, and from high voltage spikes.

Relays can also fail due to poor contact alignment and open coils. How do you know if a relay is broken? With the relay removed from the fuse box, the multimeter set to measure DC voltage and the switch in the cab activated, first check to see if there are 12 volts at the 85 position in the fuse box where the relay plugs in or wherever the relay is located. If there isn't, check to see if the appropriate fuse has blown. What religions are in Iraq?