Current start relay

Current start relay DEFAULT

by Embraco 3 minutes Read

The refrigerator does not works the whole time. It goes on and off during the cooling cycle, maintaining the ideal temperature inside the cabinet. For the compressor to be connected, the start relay is required, it is responsible for supplying energy to the unit. The relay is in the on/off operating command. But when a refrigerator needs repairing, how does one know if the start relay needs to be replaced?

To know more about compressor check, click here.

How to test the start relay: One of the symptoms that the relay is damaged is when the refrigerator heats up and certain clicks can be heard. Before replacing it by a new start relay, do the test to discover if it really is the part which has a problem. Learn to test the start relay of the compressor: 1. Disconnect the refrigerator As you are going to work in the rear part, for your protection, the appliance should not be energized. This is a safety recommendation which must be followed when you are working with parts or equipment driven by electricity. 2. Remove the connectors of the relay Remove the protective cover of the compressor and remove the relay. With pliers, preferably needle-nose pliers, take the connector of the wire and remove the relay. Pay attention to not take the wire, but the connector. Note if there are signs of corrosion in the relay – if this is the case, opt to replace it. 3. Identify the model of the start relay There are different types of relays. The electrical unit of a refrigeration system is determined based on the voltage, frequency and operating envelope of the compressor. 4. Do the test with a multimeter  

Electromechanical Relay EM

Step 1: With the relay in any position, check if there is continuity between terminals 1 and 2 of the relay. If there is not, change the relay.

Step 2: With the relay in the vertical position, coil of the relay upwards, check if there is continuity between terminals 1 and 3 of the relay. If there is not, change the relay and repeat step 1.

Step 3: With the relay in the vertical position, coil downwards, check if there is continuity between terminals 1 and 3. If there is, change the relay

Electromechanical Relay F, EG and PW

Step 1: With the relay in the vertical position, coil downwards, check if there is continuity between terminals 10 and 11 of the relay. If there is not, change the relay.

Step 2: With the relay in the vertical position, coil upwards, check if there is continuity between terminals 10 and 11 of the relay. If there is, change the relay and repeat step 1.       

Relay PTC

Step 1: With the aid of an ohmmeter, measure the ohmic resistance between terminals 2 and 3. At ambient temperature, the values should be near those presented in the list below:

Relay PTC– Relay PTC 8EA 1B1X – 2.8 to 5.2 Ω; – Relay PTC 7M4R7XXX / 8M4R7XXX / 8EA14CX– 3.8 to 5.6 Ω; – Relay PTC 8EA4BX / 8EA3BX / 8EA21CX – 3.5 to 6.5 Ω; – Relay PTC 8EA5BX – 14 to 26 Ω; – Relay PTC 7M220XXX / 8M220XXX / 8EA17CX – 17.6 to 26.4 Ω. 

How to change the start relay As already mentioned, there are different types of relays. When it is necessary to make a change, do not try to improvise or make use of not recommended solutions. This can cause problems, such as the burn-out of the compressor. Always check out the technical sheet of the compressor (click here and check out the electronic catalog of Embraco). See the step-by-step to change the start relay: 1 – Remove the start relay After having located the relay, remove it paying great attention to the color and where the wire is connected. 2 – Fit the new relay With the new relay positioned, connect the correct wire in the output of the relay. The relay will always be in the two pins above and the thermal protector in the pin below. Fit the relay and press to fasten it in the compressor. 3 – Place the protective box again Fit the protective box and place the screws you have removed, to fasten it. You can now connect the refrigerator again!  

In addition to finding more technical information here on the Cooling Club website, you can also check our channel on YouTube. Click here to access.


The relay utilizes a steel armature centered in solenoid coil field and bridging type contact arm which closes by armature movement. All working parts are enclosed in rugged thermoplastic  casing and air.

core construction is used to avoid residual magnetism.  A double pair of contacts give two breaks in series to assure longer contact and relay life.

The relay is normally open with its solenoid coil connected to main winding of the motor and starting winding connected in series with contacts of the relay.  When voltage is applied to the motor and high current flows through main winding and in series relay coil, which creates the magnetic  field and picks up the armature to close the contact to bring start winding in circuit.

Start winding in the circuit increases the speed of the motor and reduces the current in main winding and relay solenoid coil. As current reduces magnetic field developed by coil reduces and drops out armature to open the contacts to disconnect the start winding. 

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Current Relay Operation

From Fig. 1b notice that when voltage is applied to the compressor, the run winding will pull locked-rotor amperage (LRA) of up to 40 A. Since the coil of the current relay is in series with the run winding, it will also see this large current, which will be strong enough to pull the current relay’s contacts closed. When the contacts close, it will provide a path for voltage to reach the start winding. This voltage will also pass through the start capacitor, which causes a phase shift in the magnetic field in the start winding and the torque will be strong enough to start the compressor motor even though it's under 70-80 psi of pressure.

Above: Fig 1.

The current relay has a return spring connected to its contacts that tries to pull them open. After the motor starts and picks up speed, the rotor will produce enough counter EMF to allow the full-load current ( FLA) to drop substantially. When the motor’s current drops to the FLA level of 3 to 4 A, the spring will pull the contacts open. This drop in current from LRA to FLA occurs in 1 to 2 seconds as the motor reaches full speed. When the contacts open, the start capacitor and start winding are removed from the circuit and their current drops to zero. Since the run winding continues to be energized, it will draw FLA and the motor will continue to run. A graph of the locked rotor current and full-load current is also shown in Fig. 2b. Note that LRA only exists for several seconds while the rotor is coming up to speed. This current is strong enough to pull the current relay’s contacts closed for the few seconds the motor requires to start.

Above: Fig 2.

Since the current relay is mounted on the outside of the compressor, it can easily be replaced if it becomes faulty. It can also be tested easily by checking voltage from L2 to the S terminal of the motor during the time the motor is starting. If no voltage is present at terminal S during start, the start capacitor should be removed from the circuit and an attempt made to restart the motor. Allow the motor to try to start for only a few seconds during this test because continued LRA current will damage the motor.

If terminal S on the motor receives voltage when the capacitor is removed, one can assume that the capacitor is open. If the terminal still does not receive voltage, the current relay is faulty and should be replaced.

How The Compressor Current Starting Relay Works \u0026 Troubleshooting

Current Starting Relays

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


Start relay current

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How The Compressor Current Starting Relay Works \u0026 Troubleshooting

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