A sudden stop in the operation of the refrigeration chamber often leaves the owner with a choice: call a technician or try to find the problem on his own. The most common and critical component, the failure of which leads to loss of cold, is compressor. It is he who is responsible for the circulation of the refrigerant, and if this element stops starting or hums, but does not pump, the first step is to diagnose the electrical part.
Checking the performance of the motor is impossible without understanding the basic principles of operation of the electric motor and having a simple measuring instrument. In most cases, the failure lies in a winding break, an interturn short circuit, or a breakdown to the housing. In order not to spend money on buying a new unit with a working motor or not to install a unit that is obviously not working, you need to know how to ring a refrigerator with a multimeter and correctly interpret the readings.
This procedure requires a minimum set of tools and compliance with safety precautions, since the work is carried out with electric current. Before proceeding with disassembly and measurements, it is necessary to completely de-energize the device and prepare the workplace. Competent diagnostics will save significant money and time, eliminating the need to call a specialist for a basic check.
⚠️ Attention: Before starting any diagnostic work, be sure to remove the refrigerator plug from the socket. The voltage of 220 volts is dangerous to life, and the capacitors in the starting relay can retain charge.
Preparation of tools and safety precautions
To conduct high-quality diagnostics, you will need a digital or dial multimeter capable of measuring resistance in the range from 0 to 20 kOhm and checking the integrity of the circuit. You also cannot do without a set of screwdrivers to remove the protective casing at the rear of the unit, where the motor-compressor is located. It is advisable to have pliers and electrical tape on hand in case you need to insulate the wires after checking.
Particular attention should be paid to external inspection. If the compressor housing shows signs of oil, soot, or melted insulation, this may indicate a serious internal problem. In such cases diagnosis may show a short circuit, but the visual signs often speak for themselves. Make sure that there is good lighting and no dampness in the work area, since water is a conductor and can distort the readings of the device.
☑️ Preparation for diagnostics
Some refrigerator models, for example, from brands Indesit or Atlant, have a specific terminal location boxes. Before removing the casing, it is useful to take a photograph of the wire connection diagram so as not to mix up the contacts during assembly. This is especially important if you plan to change the starting relay or the compressor itself after checking.
Removing the starting relay and accessing the contacts
To get to the winding contacts, you need to remove the starting and protective relay, which are usually attached to the side of the compressor. They are a black or white plastic block placed directly on the motor terminals. Carefully remove the fixing brackets or screws holding the box and disconnect the wires, first remembering their location.
At the end of the compressor you will see three terminals forming a triangle. These are the entry points for electricity into the motor windings. Depending on the manufacturer, the marking may differ, but the standard circuit assumes the presence of a common point, a starting winding output and a working winding output. Often contacts are designated by letters or colors: C (Common) - general, S (Start) - starting, R (Run) - working.
If the relay has a complex structure or is connected through a connector, do not use excessive force. The plastic of older refrigerators can be brittle. Models Samsung and LG are characterized by the use of specific fasteners that require careful handling. After removing the relay, inspect its contacts for carbon deposits - this is a common reason that the motor does not start, although it itself is in good working order.
Searching for a breakdown on the housing (short circuit)
The first and most important stage of the check is to exclude a breakdown of the winding insulation on the metal body of the compressor. This is a situation when a phase wire inside the motor touches the stator, which makes the operation of the device deadly and leads to the constant knocking out of plugs or RCDs in the panel. To check, switch the multimeter to resistance measurement mode (20 kOhm limit or continuity mode).
Press one probe of the device to any of the three contacts on the compressor, and the second to a cleaned place on the metal body of the engine (you can use the place under the fastening screw or a cleaned section of the tube). If the device shows one (infinity) or a value close to infinity, then there is no breakdown. If you see specific resistance numbers (for example, several Ohms or kOhms) or hear a squeak, it means that the insulation is broken.
⚠️ Attention: The presence of resistance between the contacts and the housing means that the compressor is faulty and must be replaced. It is prohibited to operate such a refrigerator!
This procedure must be repeated for each of the three contacts in turn. Only after making sure that there is no breakdown to the housing at all terminals can you proceed to the next stage. Sometimes the breakdown can be “floating” and appear only when heated, so even good readings on a cold engine do not give a 100% guarantee, but they allow you to weed out obvious defects.
Why does breakdown occur on the housing?
Most often, insulation breakdown occurs due to aging of the varnish coating of the windings, overheating of the engine during prolonged operation or moisture getting inside electrical part. In rare cases, this is a factory defect.
Checking the integrity of the windings and searching for a break
After eliminating a ground fault, it is necessary to check the windings themselves for integrity. The refrigerator motor has two main windings: working and starting. The working winding is wound with a thinner wire and has a larger number of turns, so its resistance is always higher. The starting winding is made of a thicker wire and has less resistance.
To check, switch the multimeter to resistance measurement mode (limit 200 Ohms). Measure the resistance between each pair of contacts. You need to get three values: between pins 1 and 2, between pins 2 and 3, and between 1 and 3. The sum of the two smaller values should equal the largest. If the device shows one (infinity) when measuring between some contacts, it means there is a break in the circuit open.
Typical resistance values for a working compressor of a domestic refrigerator vary within the following limits:
| Winding type | Resistance range (Ohm) | Characteristics |
|---|---|---|
| Working (Run) | 10 - 35 Ohm | High resistance, thin wire |
| Start | 15 - 50 Ohm | Resistance is higher than working, depends on the model |
| Total (C-R + C-S) | Sum of values | Checking circuit integrity |
| Breakage to the housing | 0 or a specific number | Unacceptable (must be infinity) |
It is worth noting that for inverter compressors, which are found in modern models Bosch or Electrolux, the resistance of the windings can be significantly lower, sometimes less than 10 Ohms. In such cases, it is important not to confuse low resistance with a short circuit. It is always better to check the exact data in the technical documentation for the specific engine model.
Diagnostics of interturn short circuit
The most difficult defect to identify is the interturn short circuit. In this case, the insulation between the turns of the wire inside the winding is broken, and some of the turns fall out of operation. In this case, the resistance drops, but not to zero, and the multimeter can show quite “working” values, although the engine will not work correctly.
The main sign of an interturn short circuit is the strong heating of the compressor and the activation of the thermal protection relay a short time after starting. If you have tested the windings and they show integrity, but the engine hums, does not start or turns off immediately, the likelihood of this defect is high. Accurate identification is possible only with the help of special instruments that measure inductance, or by the method of exclusion.
An indirect sign may be a burning smell or a change in the color of the oil in the system (if there is access). Also pay attention to the current consumption: with an interturn short circuit current in the circuit it will significantly exceed the nominal value indicated on the nameplate. If you have a current clamp, start the compressor (for a short time) and measure the starting and operating current.
⚠️ Attention: Technical characteristics and current limits may vary depending on the specific series of the refrigerator. Always check the data with the sticker on the body of your device.
Interpretation of results and conclusions
Summarizing the diagnostic results, you can get a clear picture of the condition of the unit. If all three contacts communicate with each other, have a resistance within acceptable limits and do not ring into the housing, then the electrical part of the engine is working. In this case, the problem most likely lies in the start relay, thermostat or freon leak, and the compressor itself does not need to be changed.
If a break is detected, a breakdown to the housing or an obvious inter-turn short circuit (confirmed by heating and current), the compressor must be replaced. Repairing the windings at home is impossible due to the need to rewind the motor in a vacuum and use a special varnish. Attempts to revive such a motor are usually temporary.
Replacing a compressor is a labor-intensive process that requires a vacuum pump and skills in working with the refrigeration circuit. If you do not have the appropriate equipment, after self-diagnosis it is better to turn to professionals, already knowing the exact cause of the breakdown. This will avoid imposing unnecessary services and unnecessary costs.
Is it possible to start the compressor directly without a relay?
Theoretically, it is possible by applying voltage to the operating and starting windings, but this is dangerous and requires precise knowledge of the circuit. Without a starting relay, the engine can burn out in a few seconds.
Frequently asked questions (FAQ)
What resistance is considered normal for a compressor?
The normal resistance of the working winding is in the range of 10–35 Ohms, and the starting winding is 15–50 Ohms. The main rule: the sum of the resistances of the two smaller sections must be equal to the resistance of the largest section. It is also important that there is no resistance (breakdown) to the housing.
Why does the compressor hum but does not start?
Most often this indicates a faulty start relay or a jammed compressor piston group. Less commonly, the reason lies in the interturn short circuit of the windings or insufficient voltage in the network.
Is it possible to check a compressor without a multimeter?
It is impossible to accurately determine the condition of the windings without a device. However, you can indirectly evaluate the work: if, when connected to the network, the machine immediately knocks out or a strong hum is heard followed by a click of the relay, these are signs of a serious malfunction.
What to do if the resistance of the windings is very different from the norm?
If the resistance is significantly lower than the norm, an interturn short circuit is likely. If higher, poor contact or breakage is possible. In both cases, the compressor is considered faulty and requires replacement.