HVAC: Potential Relay Explained (Potential Relay Wiring Diagram) 521 Relay Sequence Of Operation

HVACR (Heating, Ventilation, Air Conditioning / Refrigeration) Potential Relay Explained (Potential Relay Wiring Diagram) 521 Relay Sequence Of Operation / Potential Relay Wiring / Potential Relay VS Current Relay / Potential Relay Troubleshooting / Potential Relay Refrigeration / Potential Relay Connection / Potential Relay Start Capacitor Wiring Diagram

HVACR POTENTIAL RELAY:
Potential or “voltage” relays are used with single-phase capacitor-start/capacitor-run motors, which need relatively high starting torque. Their main function is to assist in starting the motor.
Knowing the sequence of operation for this type of starting relay can help you diagnose, confirm, or rule out certain service problems.

Potential starting relays consist of a high resistance coil and a set of normally closed contacts. The coil is wired between terminals 2 and 5, with the contacts between terminals 1 and 2.

Terminals 4 and 6 are used for capacitors and/or condenser fan connections and have no electrical significance to the starting relay itself, as shown in Figure 1. In fact, terminals 4 and 6 are sometimes referred to as “dummy” terminals and are simply used for wire connections.

OPERATION:
When power is applied through the cycling control, both the run and start windings are energized. The run and start capacitors provide the phase shift for starting torque because their capacitances add up when wired in parallel. In fact, both capacitors are wired in series with the start winding and in parallel with the run winding.

The run capacitor limits the current that will pass through the start winding when the motor is running, because they are wired in series. The run capacitor also provides running torque when the motor is up and running.

The operation of the potential relay is based on the increase in back-electro-motive force (back-EMF) or a bucking voltage that is generated across the start winding as the motor increases in speed. The large metal mass of the motor’s rotor turning at high speeds with motor windings in close proximity has a voltage-generating effect.

The generated back-EMF opposes line voltage and can be measured across the start winding or across the coil of the potential relay at terminals 2 and 5. The back-EMF is usually a higher voltage than the line voltage and can be in the 400-V area.
All motors have different magnitudes of back-EMF, thus requiring different potential relay designs.

The back-EMF voltage generated across the start winding causes a small current to flow in the start winding and potential relay coil, because they are in the same circuit. When the back-EMF has built up to a high enough value (referred to as pick-up voltage), the contacts between terminals 1 and 2 will be picked up (“opened”). This will take the start capacitor out of the circuit.

Pick-up voltage usually occurs when the motor has reached about three-fourths of its speed. The start winding is still in the back-EMF circuit, keeping the relay’s coil energized while the motor is running at full speed.

The contacts open because of the electromagnetic force (magnetism) generated in an iron core around which the relay’s coil is wrapped.

When the cycling control opens, line voltage is taken away from the motor. The motor’s rotor decreases in speed. Thus, the back-EMF generated across the start winding decreases in magnitude. The relay’s coil now sees a lower back-EMF and no longer can generate enough magnetism in its iron core to keep contacts 1 and 2 open.

The contacts therefore return to their normally closed position by spring power as the motor coasts to a stop.

With the start capacitor out of the circuit, the motor becomes a permanent split capacitance (PSC) motor in the running mode only.

[zerotrace]

Sir, you are one of the best HVAC teachers on the Internet. I tip my hat to you. God has truly blessed you with the ability to communicate complex ideas in a simple way. We cannot thank you enough for all you have done for us.

[walterfernandes]

Pretty cool my guy, you have a gift on how to teach anything. May God bless you

[billyvandervoet]

1, 2, 5 relays are one of the most overlooked parts in diagnosing a so called bad compressor. Many unseasoned Techs don’t understand them, they are very simple but extremely critical. You explained them perfectly, I hope less experienced techs will take a little time to watch this video.

[paulwel]

Best explanation of what that device does. I couldn’t figure out why it can only be oriented one way. Now I know! Keep up the great videos!

[Salt_stain]

Thank you from a pipe fitter apprentice. Making it seem easy!

[robertdavenport]

Great video once again. Excellent explanation as well keep up the great work

[jamesallen]

Tyvm! Very succinct. I will forward this to our Service Department Technicians.

[keitharnett]

Great video, thanks for putting this together. I'm familiar with these in well pump controllers, where they serve the exact same purpose -- put the start cap in the circuit to get the pump motor running, and then remove the start cap from the circuit. Although, I never knew exactly how they worked -- and now I do!. You have to love the simplicity! Thanks again.

[izzybell]

Interesting and helpful video. Great job JMT as always.

[austindoeshvac]

Great explanation as always! Could you possibly make a video of wiring a potential relay, start cap, run cap and the compressor coming from the contactor? Thanks and great job as always JMT!

[bonganiedwardmthethwa]

Thank you once again for the easy to follow explanation on a rather confusing piece of item the potential relay. I know understand it better. Thanks to you and your depth knowledge of this field.
I wonder if you can help me and others like me with the pictorial sketch and explanation of the wiring on freezer box with the power connection, dual pressure switch, thermostat, solenoid valve, defrost timer and heater connection starting from the very same box with normally house the potential relay, start and run capacitor.

[jaseduar]

Great explanation, how can I know if this relay is not working properly or damaged

[geneborne]

As a 5 yr old.... I now understand this perfectly. Our brains are configured to think coil voltage will close/make the circuit (N.O.).... this concept (N.C.) is opposite and I believe the reason for difficulty understanding. Thanks and following.

[ebenezerservices]

Excellent explanation; just a question is it a good thing to use a hard start kit in case an emergency?

[Anointingify]

Thanks for the precise explanation. I will like to know if both potential relay and the current relay can be used together on a compressor.

[joehead]

Back EMF is generated in the motor itself. Pick-up voltage, generally speaking, 0.75 percent of total back emf of motor start winding at operational speed. Relay only sensing back emf of start winding.

[briancarlisi]

Never understood nor took the time to see how those worked. Thanks for the excellent explanation and keep these types of videos coming. BTW… You sound a lot like my cousins from Long Island, lol.

[syedsadiqll]

Helpful video, thank you Sir for sharing this video

[musakavalci]

Another banging vid. Troubleshooting tips and tricks?

[mohdahmed]

Thank you for sharing this valuable information