Diagnostics of the compressor is the fundamental stage from which any serious repair of household refrigeration equipment begins. It is this unit that is the heart of the system, creating the necessary pressure for the circulation of the refrigerant. If your refrigerator stops freezing, hums but does not start, or knocks out the circuit breaker, the first thing you need to do is to rule out an electrical breakdown or a break in the motor.
The test procedure winding resistance is relatively simple and does not require complex professional equipment, just a basic set of tools is enough. Understanding how to properly measure and interpret these values can help you save time and money by avoiding calling a repairman in the event of an obvious breakdown. In this article we will analyze in detail the algorithm of actions, resistance standards for various types of motors and typical errors in diagnostics.
Before starting measurements, it is important to understand the physical meaning of what is happening. A compressor motor is an electromechanical device where electrical energy is converted into mechanical energy. Any deviation in the integrity of the copper wire from which the coils are wound is immediately reflected in the electrical parameters. Multimeter in this case, it acts not just as a measuring device, but as a window into the internal state of the engine, allowing you to look there without opening the sealed casing.
Necessary tools and measures safety
Safety should be your absolute priority when working with electrical appliances. The refrigerator compressor runs on 220 volts, which poses a real threat to life if handled carelessly. In addition, incorrect connection of the probes or selection of the measurement mode can lead to failure of the measuring device itself. Therefore, preparing the workplace and tools takes no less time than the diagnosis itself.
⚠️ Attention: All work on checking the electrical circuits of the compressor should be carried out exclusively with the refrigerator disconnected from the network. Remove the plug from the socket before touching any internal components.
To conduct a quality diagnosis, you will need the following kit: a digital multimeter with the ability to measure resistance (Ohms) and check continuity (continuity), a set of screwdrivers for removing the protective casing, pliers for removing terminals and, possibly, electrical tape for marking wires Digital instruments are preferable to analog dial instruments, as they give more accurate readings and have overload protection.
Pay special attention to the appearance of the compressor. If you notice traces of oil, soot, or deformation of the housing, this may indicate that the problem is not only electrical, but also mechanical in nature. In such cases, a simple test may not provide a complete picture, and deeper intervention will be required.
It is also important to check the condition of the wires going to the start-up relay. Often the problem lies not in the motor itself, but in oxidized contacts or a broken wire, which is easy to replace. Make sure that the insulation of the wires is not melted and does not have cracks through which current may leak to the housing.
☑️ Preparation for diagnostics
Access to the electrical contacts of the compressor
To get to the winding terminals, you need to remove the protective plastic or metal casing located at the bottom of the refrigerator at the rear. It is usually secured with a few screws or snaps. After removing the casing, you will see the compressor itself, to the bottom of which is attached a black plastic barrel - this is start-protection relay along with the starting capacitor (if your model has one).
Removing the relay is a critical step. It simply slides onto three pins coming out of the compressor housing. Carefully, without excessive force, remove the relay down. If it is stuck or rusty, you can slightly rock it from side to side, but try not to damage the plastic elements. On the inside of the relay you will see a connection diagram, which will be useful during assembly.
After dismantling the relay, three contacts arranged in a triangle will open in front of you. These are the winding terminals. Depending on the compressor model, they may be marked with letters or numbers, or have color-coded wires coming from the relay. The standard marking looks like this: C (Common) - common wire, S (Start) - starting winding, R (Run) - working winding. Sometimes the numbers 1, 2, 3 are used instead of letters.
Some modern models use inverter-type compressors, where control occurs through an electronic unit. In such cases, the classic three-contact circuit may differ, and the presence of a start relay may not be present as a class. For such units, a simple test with a multimeter is often insufficient, since additional electronics may be built inside.
How to identify contacts without markings?
If there are no letters on the terminals and the relay is removed, you can determine the common contact by measuring the resistance between the pairs. Between the working and starting windings the resistance will be maximum. The common wire (C) will give average resistance values relative to the other two contacts. The sum of the resistances C-R and C-S must be equal to the resistance between R and S.
Theory of operation: starting and operating windings
To understand the measurement results, you need to know how the asynchronous compressor motor works. It has two main windings, wound with copper wire of different diameters. The working winding made with thicker wire and has lower electrical resistance. It is designed for constant operation of the engine and the creation of the main magnetic flux.
Start winding is wound with a thinner wire, so its resistance is always higher than that of the working one. Its task is to create an initial torque to start the rotor. As soon as the engine picks up speed, the starting relay opens the starting winding circuit, and the motor continues to operate only on the working winding. It is the difference in the thickness of the wire and the number of turns that creates the difference in resistance, which we will measure.
The ratio of the resistances of these windings is not a random value. Typically, the resistance of the starting winding is 1.5–2 times higher than the resistance of the working winding. If you see that the values are equal or differ by a factor of 10, this is a sure sign of a malfunction. Also
The third element in this bundle is the compressor housing. In good condition, there should be no electrical connection between any windings and the metal housing. The resistance here should tend to infinity. The appearance of conductivity on the body means a breakdown of the insulation, which makes the operation of the refrigerator deadly.
Step-by-step instructions: measuring resistance with a multimeter
Let's move on to the practical part. Set the multimeter to resistance measurement mode. This is usually a sector designated by the symbol Ω (Ohm). Select the measurement limit of 200 ohms or 2 kohms (2000 ohms). If you do not know the approximate values, start with a higher limit so as not to overload the device, and then reduce it for greater accuracy.
Before starting measurements, check the multimeter itself by shorting the probes together. The device should show zero or a value close to zero (0.1–0.5 Ohm), which confirms the serviceability of the probes and the device. Now you can begin diagnosing the compressor terminals.
- Find the common contact (C). To do this, measure the resistance between all three pairs of contacts. The pair that gives the highest resistance (for example, 25 ohms) is the ends of the working and starting windings. The third, remaining contact is the common one (C).
- Measure the resistance between the common contact (C) and each of the other two. Between C and the working winding (R) the resistance will be lower (for example, 10 Ohms), and between C and the starting winding (S) it will be higher (for example, 15 Ohms).
- Add the resulting two values (10 + 15 = 25 Ohms). This sum should match the resistance measured between the two outer terminals (R and S). If the math converges, then the windings are intact.
Write down all the values obtained. You will need them for comparison with tabular data or for consultation with a specialist if the results are questionable. Also, be sure to check that there is no short circuit to the housing: touch one probe to any contact, and the other to a stripped place on the copper tube or compressor housing.
Interpretation of readings and table of normal values
Interpretation of results is the most important stage of diagnosis. The numbers on the multimeter screen will tell you more about the condition of the engine than a visual inspection. Normal resistance values may vary depending on the manufacturer (Atlant, Danfoss, Secop, LG) and the power of the model, but the general patterns remain the same for all asynchronous motors.
Below is a table with approximate resistance values for household compressors. Remember that deviations within 10-15% can be considered acceptable due to instrument error or engine temperature.
| Measurement type | Normal value | What does deviation mean |
|---|---|---|
| Working winding (C-R) | 10 – 25 Ohm | Zero - short circuit, Infinity - open |
| Start winding (C-S) | 20 - 50 Ohm | Significantly above the norm - poor contact |
| Between the windings (R-S) | Sum of C-R and C-S | Mismatch of the sum - turn short circuit |
| To the body (Any-Case) | Infinity (1) | Any value is an insulation breakdown |
If the multimeter shows infinity (one on the left of the screen or no reading) between the contacts, this means an open circuit. The break can be internal (the winding has burned out) or external (the internal thermal relay has tripped if it is built into the housing and has open contacts, although in older models the thermal relay is often sealed inside the casing, but modern ones often have external contacts).
Indication 0 Ohm or close to it between any two contacts that should not be directly connected indicates a short circuit. This is the most severe malfunction requiring compressor replacement. The resistance to the housing is also dangerous: even if it is several Megaohms, this indicates a violation of the dielectric properties of the insulation, and such a compressor can break through to the ground when heated.
Typical faults and their symptoms
Based on measurements, several main types of breakdowns can be diagnosed. Interturn short circuit - an insidious defect in which the winding resistance may be slightly below normal, but not zero. At the same time, the engine may hum, overheat and knock out the automatic transmission, but formally it will “ring”. It is difficult to identify such a short circuit without a special stand, but a strong discrepancy with the passport data should alert you.
Winding break often happens in the starting winding, since the wire there is thinner. At the same time, the compressor hums, tries to start, clicks the relay, but does not rotate. After a few seconds the thermal protection is triggered. If the continuity test shows infinity on the starting winding, the motor is faulty.
Another problem - breakdown to the housing. This often occurs due to aging insulation, overheating, or moisture entering the casing. The danger of such a malfunction is that the refrigerator may work, but its metal parts will be energized. Touching such a refrigerator may result in an electric shock.
⚠️ Attention: If you find a breakdown to the housing, further operation of the refrigerator is prohibited until the malfunction is eliminated. Even if it freezes, the risk of electric shock is too great.
Sometimes the cause of “incorrect” resistance is oxidation of the contacts at the compressor terminals. If the pins are coated with black deposits or oxide, the multimeter may show unstable readings. In this case, you can carefully clean the contacts with fine sandpaper or a blade and repeat the measurements.
Is it possible to start a compressor without a relay?
Theoretically, by applying voltage directly to the working and starting windings (simulating the operation of a relay), you can start the motor. However, doing this without experience and special starting devices is dangerous: you can burn the starting winding in seconds or get an electric shock.
Frequently asked questions (FAQ)
What resistance is considered normal for an Atlant compressor?
For most compressor models Atlant The resistance of the working winding is about 15-20 Ohms, and the starting winding is 25-35 Ohms. The exact data depends on the specific series and year of manufacture, but the principle of “starting is greater than working” is always preserved.
Why does the multimeter show 1 (infinity) between all contacts?
This means an open circuit. Either one of the windings inside the motor burned out, or, as happens more often in modern models, the internal thermal overheating relay tripped. Let the compressor cool completely (2-3 hours) and try again.
Is it possible to check the compressor without removing the relay?
No, you can only qualitatively check the winding resistance directly at the compressor terminals. Through the relay, you measure the resistance of the entire circuit, including the contacts of the relay itself and the starting capacitor, which will not give accurate information about the condition of the engine.
What to do if the resistance is normal, but the refrigerator does not work?
If the electrical part is working, the problem may be mechanical (jamming of the piston group) or in the refrigerant system (freon leak, clogged capillary tube). It is also worth checking the start relay - it could burn out, even if the motor is intact.