Sudden stop of the compressor and lack of cold in the chambers is a signal that requires the immediate attention of the owner of household appliances. Often the cause of a breakdown is the failure of the start-up relay, in particular the widespread model P3. This small but critical component is responsible for starting the motor and protecting its windings from overloads, ensuring stable operation of the entire refrigeration system. Without a working starting mechanism, the motor simply will not be able to overcome the initial resistance and start.
Replacing this element does not always require calling an expensive technician, since the design of old and many modern refrigerators allows you to perform this procedure yourself with minimal tools. It is important to understand that a relay P3 is an electromechanical device where physical contacts can burn or stick over time, which leads to a characteristic clicking sound without starting the motor. Correct connection requires attention to the circuit and compliance with safety precautions, since the work is carried out with high voltage.
In this article we will analyze in detail the operating principle of the device, its diagnostic algorithm and the step-by-step replacement process. You will learn how to correctly determine the polarity of the windings, clean the contacts and install a new component so that it will last for many years. Accurately follow the instructions will avoid repeated breakdowns and ensure safe operation of your refrigerator.
The principle of operation and design of the P3 relay
Start-protective relay type P3 is a combined device that combines two main functions: starting an asynchronous motor and protecting it from current overloads. Inside the housing there is a coil connected in series to the operating winding circuit of the compressor, and a movable core with a contact group. When voltage is applied, current passes through the coil, creating a magnetic field that draws in the core and closes the contacts of the starting winding, allowing the rotor to gain the necessary speed.
As soon as the engine reaches operating mode, the current in the circuit drops, the magnetic field weakens, and under the influence of gravity (or a spring, depending on the modification) the contacts open. The starting winding is turned off, and the motor continues to operate on the working winding. In parallel with this process, thermal protection operates: the bimetallic plate heats up when current passes and opens the circuit when critical values are reached, preventing burnout of the winding insulation. Winding insulation is the most vulnerable point that protects this mechanism.
⚠️ Attention: The relay P3 is a polarized device. This means that it will only work correctly in a certain position relative to the horizon. Installation upside down or on its side will lead to incorrect opening of the contacts and rapid combustion of the compressor starting winding.
Structurally, the device is made in a dielectric housing that protects internal components from moisture and dust. On the rear panel there are terminals for connecting to the compressor contacts and supplying power wires. The quality of the contact materials directly affects the service life: cheap analogues use metal that is prone to rapid oxidation and carbon formation, which increases the contact resistance.
Why is the P3 relay called a “tablet”?
In the professional environment of repairmen, the P3 relay is often called a “tablet” because of the characteristic round shape of the body and flat profile. This nickname has been attached to the model since the days of Soviet refrigerators, where such compact starters were used. The shape is due to the need to place the device in a narrow space between the compressor housing and the rear wall of the refrigerator.
Diagnostics of faults before replacement
Before you start purchasing a new component and installing it, you need to make sure that the problem lies in the relay, and not in the compressor itself or the wiring. The first sign of a malfunction is often a characteristic sound: the refrigerator hums, then there is a loud click, followed by silence, and after a while the cycle repeats. This indicates that the trigger mechanism is trying to start the motor, but the contacts do not close or stick, preventing it from entering the operating mode.
For accurate diagnostics, remove the relay from its seat. It is usually attached to the three contact pins of the compressor and secured with a metal bracket or simply held in place by a tight fit. After removing the case, visually inspect the insides: the presence of burning, melted plastic or blackened contacts indicates a breakdown. Blackened contacts is a sure sign that the device has worked.
Next, you need to test the circuit using a multimeter. In the normal state (horizontal position), the contacts between the input and output to the starting winding should be open. If you turn the device over, you hear a click and the circuit should close. Checking the thermal protection is also mandatory: in a cold state it should ring short, and when heated (you can gently warm it up with a hairdryer) it should open the circuit. If this does not happen, the protective function is broken.
- 🔍 Visual inspection for melting, cracks in the case and burning smell.
- 🔊 Listening to the operation: absence of a start click or its repetition too often.
- ⚡ Testing with a multimeter: checking the integrity of the coil and mobility contacts.
- 🌡️ Thermal protection test: reaction of a bimetallic plate to heating.
In some cases, the cause of failure may not be the relay itself, but an interturn short circuit in the compressor windings, due to which the starting current becomes too high and knocks out the protection. Therefore, if a new relay burns out immediately after installation, a more in-depth engine diagnosis is required. Interturn short circuit is often accompanied by a change in the color of the oil in the system or the appearance of a sour odor.
Preparation for work and the necessary tools
The quality of repair work directly depends on the preparation of the workplace and the availability of suitable tools. Replacing a relay P3 is not a complicated operation, but requires care. The first step is to ensure access to the compressor, which is usually located at the lower rear of the unit. To do this, the refrigerator must be moved away from the wall and, if necessary, placed on its side, having previously secured the doors so that they do not swing open.
The most important safety rule is to completely turn off the power supply. The cord must be unplugged from the socket, and at least 10-15 minutes must pass from the moment of switching off before starting work, so that the capacitors are discharged and the refrigerant in the system is stabilized. Working under voltage is strictly prohibited, as the risk of electric shock in conditions of humidity and a metal case is very high. Safety should be priority number one.
To perform dismantling and installation, you will need a minimum set of tools. Usually a flathead and Phillips screwdriver, needle-nose pliers and a multimeter are enough to check. It is also recommended to prepare isopropyl alcohol or a special contact cleaner to treat the terminals if there are oxides on them. Clean contacts ensure a reliable connection and reduce the risk of sparking.
☑️ Tools for replacing relays
If you plan to clean the contacts of the old relay (which is a temporary solution), you may need fine sandpaper or a needle file, but this is not required for a new part. When working with wires, make sure that they do not have insulation damage. If the insulation is broken, use heat-shrink tubing or electrical tape to restore the dielectric layer before connecting.
Step-by-step instructions for connecting relay P3
The process of installing a new start-up protection device requires careful attention to the connection diagram, since incorrect switching can lead to instant failure of both the new relay and the compressor windings. There are three contacts on the compressor body, covered with a plastic cover or located under the terminal block. They are designated by letters or symbols: C (Common) - common, R (Run) - working winding, S (Start) - starting winding. In domestic markings they can be designated as O, P i P respectively.
Remove the old one relay and carefully study its design. The connection diagram is often printed on the inside of the cover or on the body itself. The relay P3 is placed directly on the compressor pins. The common power wire is usually connected to a terminal that goes through a relay to the working winding, and the starting circuit is switched by a moving contact. It is important not to confuse the top and bottom of the device: the arrow UP or the inscription TOP should look straight up.
When installing a new component, first push the relay onto the contact pins of the compressor until it stops. Make sure the fit is snug and the contacts are not loose. Then connect the lead wires to the appropriate terminals on the relay body. If the wires have male terminals, they should fit tightly into the relay sockets. After connecting all the wires, secure the relay with a metal bracket, if provided by the design, or make sure that it is securely fixed in place.
| Designation on the diagram | Contact function | Wire color (standard) | Winding resistance |
|---|---|---|---|
C (Common) |
Common input | Black / Red | Sum of R and S |
R (Run) |
Working winding | White / Blue | Low (20-40 Ohm) |
S (Start) |
Start winding | Green / Yellow | High (40-100 Ohm) |
PE |
Grounding | Yellow-green | To the body |
After visually checking all connections, you can begin a test run. Without removing the back cover yet (if it is removable) and ensuring access for observation, turn on the refrigerator. The engine should start the first time, without a long hum. If the start-up was successful and the compressor runs smoothly, without vibrations or unusual sounds, the installation can be considered complete.
Installation features and typical errors
When installing a relay P3 masters often make mistakes that nullify all efforts and can lead to repeated failure. One of the most common mistakes is ignoring the orientation of the device. As already mentioned, the relay must be positioned strictly vertically. Even a slight tilt can disrupt the gravitational opening of the contacts, causing the starting winding to remain on for too long and burn out. Device orientation is critical for its operation.
Another common mistake is bad contact at the junction of the wires. If the terminals are oxidized or poorly crimped, heat will begin to be generated at the contact point, which will lead to melting of the plastic and a fire hazard. Always check the tightness of the terminals. If you notice that the old terminals are loose, it is better to replace them or carefully tighten them with pliers, having first de-energized the device.
⚠️ Attention: Never use a relay from a different type of compressor or with a different rated current. The relay P3 is designed for a certain range of starting currents. Installing an unsuitable analogue (for example, from a more powerful refrigerator) will result in the contacts not opening in time, which is fatal for the engine.
It is also important to pay attention to the condition of the wires going to the relay. Often, due to the vibration of the compressor, the cores break at the point where the wire exits the insulation. Visually, the insulation may be intact, but there is a conductor inside. Before installing a new relay, carefully feel the wires along their entire length and, if necessary, replace the damaged section. To connect, use wires with heat-resistant insulation that can withstand heat from a running compressor.
Some try to “reanimate” the old relay by sanding the contacts. This measure is temporary and ineffective. The surface of the contacts has a special coating (often silver), which is removed by cleaning with abrasive. The remaining base metal quickly oxidizes, and after a few days or weeks the problem returns, but with more serious consequences for the compressor.
Checking performance after installation
After the relay is installed and the refrigerator is turned on, it is necessary to carry out a final check. During the first minutes of operation, carefully monitor the start of the compressor. It should happen quickly, without a lingering buzz. If the motor hums for 3-5 seconds and clicks, turn off the device immediately - this is a sign that the starting phase is not being processed correctly and the engine does not return to operating mode. Prolonged start destructive for the windings.
Check the temperature of the relay housing and compressor through 15-20 minutes of work. The relay may get hot, but it should not be scalding hot or smell like burning plastic. Compress