The refrigerator has stopped starting, is humming but not cooling, or the circuit breaker in the electrical panel is tripping - all these symptoms often indicate the failure of one small but critical component. This element is start-up protection relay, which is responsible for starting the compressor electric motor and protecting it from overloads. Understanding the principle of its operation and the ability to diagnose a breakdown allows you to save significant money on calling a technician, since even an inexperienced home craftsman can often replace this part.
Inside this compact device is hidden a complex mechanism that combines the functions of a starter and a thermal fuse. Start-up protection relay provides increased voltage to the starting winding of the motor at the moment of start, and then turns it off when the engine picks up operating speed. At the same time, the built-in thermal release monitors the current strength, breaking the circuit in case of dangerous overheating, which prevents the burning of an expensive compressor.
Diagnostics and replacement of this unit require care and compliance with safety precautions, because you are dealing with high voltage electric current. In this article, we will analyze in detail the design of various types of relays, consider the troubleshooting algorithm and provide step-by-step instructions for installing a new part. The exact marking of the relay must match the original, since the starting current parameters may differ for different compressor models.
The operating principle and design of the starting protection relay
The fundamental task of this component is to briefly connect the starting winding of an asynchronous motor to the network. When the refrigerator is turned on, the compressor rotor is stationary, and to create the initial torque, additional force is required, which is provided by the starting winding. Starting relay automatically supplies voltage to this winding, and after the rotor spins to a certain speed, it opens the circuit, putting the engine into operating mode on one working winding.
The second, no less important function is protection. The built-in bimetallic element reacts to the strength of the current passing through the windings. If the compressor is jammed or another malfunction occurs, the current increases sharply, the bimetallic plate heats up, bends and opens the contacts, de-energizing the unit. This prevents the insulation from melting and causing a fire. There are two main types of such devices: posistor and induction, and their internal structure differs significantly.
Induction relays, often called "cat-tongue" relays, use an electromagnetic coil and a movable core. 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. When the engine starts, the current drops, the magnetic field weakens, and the core, under the influence of gravity (which is why the correct position of the refrigerator is important), drops, opening the circuit. PTC models do not have moving mechanical parts and work by changing the resistance of the semiconductor element when heated.
⚠️ Attention: When installing the relay, strictly monitor the orientation of the device. Induction relays work only in a certain position (the “UP” arrow should point up), otherwise the core will not drop and the starting winding will not turn off, which will lead to the compressor burning out.
Differences between posistor and induction types
Modern household appliances are increasingly equipped positor relays, which are considered more reliable and durable. Their work is based on the property of a posistor - a ceramic element whose resistance increases sharply when heated. In a cold state, the resistance of the posistor is low, and current flows freely through the starting winding. The passing current heats the element, its resistance increases thousands of times, and the current through the starting winding practically stops.
Induction (electromagnetic) devices are found in older models of refrigerators, such as Atlant, Biryusa or Soviet ZIL. Their design includes a coil wound on a copper frame and a movable core with a contact group. The mechanical nature of the switching makes them sensitive to vibrations and the position of the housing, but they are easier to diagnose visually - often a malfunction can be identified by characteristic ringing or sticking of contacts.
Visually distinguish these two types quite simple. A posistor relay is usually a plastic cube with one or two wires going to the posistor and has no moving parts inside when shaken. An induction relay often has a more complex shape; inside it, when shaken, a knocking sound of a moving core can be heard, and on the body there may be a marking with an arrow pointing up. Replacing one type with another is not always possible and requires careful study of the electrical circuit of your refrigerator.
Is it possible to replace an induction relay with a posistor relay?
In most cases, direct replacement is impossible without changing the connection diagram, since the operating principle is fundamentally different. An inductive relay opens the circuit mechanically, while a posistor relay opens the circuit by changing the resistance. Installing the wrong type may result in the refrigerator not starting or the starting winding burning out.
The cost of posistor analogues is usually lower and their service life is longer, which makes them the preferred choice for repairs if the compressor design allows such an upgrade. However, to preserve the originality of the repair and guarantee compatibility, it is better to select a part with an identical serial number.
Typical signs of a relay malfunction
Diagnostics begins with an analysis of the behavior of the refrigerator. If you notice that the unit hums for 3-5 seconds, then a click is heard, the light in the chamber goes out, and there is silence for several minutes, after which the cycle repeats - this is a classic symptom of a malfunction trigger mechanism or the compressor itself. In such a situation, the relay tries to start the motor, but due to lack of contact or mechanical sticking, it cannot perform its function.
Another alarming sign is a burning smell or melted plastic in the compressor area. This may indicate that the relay does not open the starting circuit, and the motor winding operates in short circuit mode, overheating. Also, a malfunction may be indicated by a situation when the refrigerator turns on, works for a while, and then suddenly turns off with a characteristic click, even if it is still warm inside.
Often users confuse a relay failure with a compressor failure. To differentiate these problems, pay attention to the following nuances:
- 🔌 Clicks and humming: If you hear a click, there is a hum, but the engine does not start - most likely the problem is in the relay or a jammed compressor.
- 🌡️ Case heating: If the relay or compressor housing becomes extremely hot in a short time operation, this is a sign of overload.
- 💡 Light in the chamber: When the protection is triggered, the light in the refrigerator goes out along with the motor stopping, as the general power circuit is broken.
It is also important to check the condition contacts. If the relay is inductive, the contacts could simply burn out and stop conducting current. In posistor models, the ceramic element itself is often destroyed or the contact is broken at the place where the wires are soldered.
Dismantling and visual inspection of the part
Before you start checking with a multimeter, you need to remove the relay from the refrigerator. This process does not require complex tools, but requires compliance with electrical safety measures. First of all, de-energize the refrigeratorby removing the plug from the socket. Any manipulations with live electrical components are strictly prohibited.
Get to the compressor, which is usually located at the lower rear of the unit. Remove the protective plastic or metal cover covering the motor-compressor. The relay is placed directly on the compressor pins. Carefully, without jerking, remove the retaining bracket or frame holding the relay and pull it away from the contacts. Often it is simply inserted into the grooves and is held in place due to the tight fit.
Visual inspection can immediately answer the question of serviceability:
- 👀 Fusing: A blackened or melted case indicates overheating and breakdown of the insulation.
- 🔊 Sound: Shake the relay near your ear. If nothing is knocking inside the induction device (the core is jammed) or, on the contrary, it rattles too much (the core has collapsed) - the part is faulty. The posistor relay must be “deaf.”
- 🔥 Smell: The sharp smell of burnt plastic or chemicals is a sure sign that the insides of the relay are burnt.
If no external defects are found, we proceed to instrumental diagnostics. Do not rush to throw away the old part, even if it looks burnt - you will need it to select an analogue in the store according to the markings.
Checking a relay with a multimeter: step-by-step instructions
For accurate diagnostics, you will need a multimeter (tester) switched to resistance measurement mode (Ohm). If you have an induction relay, it must be oriented correctly when testing, since the core must be in the lower position (simulating a running engine) for the contacts to be open, or in the upper position (start) for them to be closed. In posistor relays, the test is for the integrity of the circuit and the resistance of the posistor.
The test process is as follows:
- Connect the multimeter probes to the input and output contacts of the relay.
- In normal condition (cold posistor or open contacts of the induction relay) the device should show either infinite resistance or a certain value (for a posistor about 20-50 Ohms).
- If the device shows zero (short circuit) or infinity where there should be a circuit, the relay is faulty.
To check the posistor, you can apply 220V voltage to it (being careful!) and measure the current. In the first seconds there should be current, and then, as it heats up, it should practically disappear. If the current flows constantly, the posistor is “broken,” if there is no current at all, it is broken.
☑️ Algorithm for checking with a multimeter
Do not forget that the multimeter only checks electrical integrity. Mechanical sticking of contacts or loss of magnetic properties of the coil may not be visible on the device, but will interfere with normal operation. Therefore, if the relay is old and in doubt, it is easier and more reliable to replace it than to repair it.
Compatibility and markings table
When purchasing a new part, it is extremely important to pay attention to the markings. Manufacturers use different codes, but basic parameters such as engine power and refrigerant type must be the same. Below is a table of common types of relays and their compatibility with compressors.
| Relay type | Marking (example) | Compatible compressors | Features |
|---|---|---|---|
| Induction | RT-21, R-3 | Atlant, Biryusa (old) | Requires vertical installation |
| Posistor | 100N, 102N | Danfoss, Secop, Atlant (new) | Works in any position |
| Posistor | PTC 12-36 | LG, Samsung, Bosch | Universal parameters |
| Combined | RTK-1, RTK-3 | Soviet refrigerators | Combined with a thermal relay |
Always check the rated power (in Watts or horsepower HP) on the relay and compressor housing. Installing a relay with a lower response threshold will lead to constant shutdowns, and with a larger one - to a lack of engine protection.
Replacing the relay and first start
The process of installing a new part usually does not cause difficulties. Make sure the new relay has the same seat configuration as the old one. Slide the relay onto the three pins on the compressor (two for motor power and one for ground or thermal protection). It should fit tightly and all the way. Secure the device with the metal bracket or frame that was removed earlier.
Connect the wires if they were disconnected. Typically a plug-in connection is used, but on some models the wires may come with the relay and require twisting or soldering. If soldering is required, use high-quality solder and insulate the joints with heat shrink.
⚠️ Attention: After installation, check whether the relay vibrates during operation. Sometimes it is necessary to place a small piece of foam or rubber between the relay and the compressor housing to reduce noise, but not at the expense of cooling.
What to do if the new relay also does not start the compressor?
If replacing the relay did not help, the problem may be deeper. Perhaps the compressor itself is faulty (an interturn short circuit or wedge), the wiring is burned out, or the thermal relay (thermostat) is faulty. In this case, professional diagnostics will be required by measuring the resistance of the motor windings.
Plug in the refrigerator. In the first seconds, a characteristic hum may be heard, turning into a smooth hum of a running engine. If after 10-15 seconds there is no shutdown click and the compressor runs smoothly and the chamber begins to get colder, the repair was successful. Observe the operation of the unit for the first hour to make sure that the new part does not overheat.
Questions and answers (FAQ)
Is it possible to temporarily start the refrigerator without a starting relay?
Theoretically, you can start the motor by applying voltage to the starting winding manually, but operate the refrigerator without relay strictly prohibited. Without a protective function, any jamming or overload will lead to instant combustion of the compressor, the cost of which is not comparable with the price of the relay.
Why does a new relay burn out after a short time?
A common reason is incorrect selection of power or type of relay. Also, the cause may be a malfunction of the compressor itself (difficult starting, bearing wear), due to which starting currents exceed the norm, or problems with the voltage in the network.
Where can I find the markings on the relay?
The markings are usually placed on the side or top of the plastic case. Look for designations like PTC, R600a (type of refrigerant), as well as numbers indicating power (for example, 1/4 HP or 180W).
Is it possible to clean the relay contacts instead of replacing them?
If the relay is inductive and the contacts are simply burnt, they can be carefully cleaned with fine sandpaper. However, there is nothing to clean in posistor relays - there are no mechanical contacts in the usual sense, and in the event of a malfunction, only complete replacement of the element helps.