When the refrigerator stops cooling, and only a quiet hum or periodic clicks of the relay are heard from the back, most often the culprit is compressor. In such a situation, self-diagnosis can save significant money, since calling a technician to replace the unit is expensive. The first step in checking the serviceability of the motor is to determine the integrity of its electrical windings.
The electric motor of the refrigeration unit has a special design that differs from conventional motors. Inside the case there are two main coils, which must be correctly identified for further measurements. Working winding provides the main rotation of the shaft, and the starting one helps overcome inertia at the moment of start. If you confuse their purpose when connecting or diagnosing, you can instantly disable an expensive unit.
In this article we will analyze in detail the method of searching for contacts, the principle of operation starting relay and the algorithm for checking resistance with a multimeter. You will learn to distinguish a working motor from a burnt one and understand why it is important to follow the sequence of actions when working with high-voltage components of household appliances.
Electric motor design and output contacts
A hermetically sealed compressor installed at the bottom of the refrigerator is a complex mechanism where the electric motor and pump group are located in a single metal housing. On the top of the unit there is a contact block from which three terminals extend. These terminals are the only point of access to the internal windings without opening the case, which is strictly forbidden to do at home.
The contacts on the block can be arranged in a triangle or in a row, and their markings often depend on the manufacturer. On some models, such as Atlant or Indesit, the terminals may be designated by letters, for example, C, S, R, which corresponds to the English names of the windings. However, on old Soviet or Chinese compressors there may be no markings at all, and then you will have to rely only on the color marking of the wires or resistance measurements.
The main task of the master at this stage is to understand which of the three pins belongs to what. An error in identification will lead to the fact that when voltage is applied, the motor will not start, but will only hum, heat up and eventually burn out due to lack of torque. Therefore, visual inspection and understanding of the physics of the process are critical here.
⚠️ Attention: Before any manipulations with the electrical part of the compressor, be sure to disconnect the refrigerator from the 220V network. Residual voltage or accidental contact with live parts under load can lead to electric shock.
Why are the windings not symmetrical?
Unlike three-phase motors, a single-phase refrigerator motor requires a phase shift to start. Therefore, the starting winding is made of thinner wire and has a higher resistance to create the necessary magnetic distortion when turned on.
Differences between the working and starting windings
The fundamental difference between the two coils inside the motor is their electrical resistance and the thickness of the wire used. Working winding (main) is wound with a thicker wire, since the main current flows through it during operation of the compressor. Accordingly, its resistance is always lower than that of the starting winding.
The starting winding is intended only for short-term operation when the engine starts. It is wound with a thinner wire, which causes its high resistance. As soon as the motor picks up speed, starting relay or the electronics turn off this circuit, and further rotation occurs only due to the working coil.
When taking measurements with a multimeter, you will see a significant difference in the readings. If the resistance of one of the pairs of contacts is significantly higher than that of the other, then this is a sign of a starting circuit. In some circuits, especially in capacitor-start compressors, the difference may not be so obvious, but the law “lower resistance - working winding” remains the basic rule of diagnosis.
Necessary tools for diagnostics
To properly check the compressor, it is not enough just to look at the wires. You will need a set of tools that will allow you to obtain accurate digital values for the insulation resistance and the coils themselves. The main device here is a multimeter, preferably digital, with a mode for measuring resistance up to 2 kOhm.
In addition to the measuring device, you will need tools to dismantle the protective casing and remove the starting relay. Often a flat-head screwdriver, pliers and electrical tape are required to mark the wires after they have been disconnected. It will not be superfluous to have on hand a technical passport for a specific refrigerator model, if you can find it.
- 🔌 Multimeter —the main device for measuring winding resistance and checking for breakdown.
- 🔧 Set screwdrivers —for removing the rear cover of the compressor and the terminal box.
- 🧤 Dielectric gloves —to ensure safety when working with electrical circuits.
It is important that the multimeter probes are sharp and clean, since the contacts on the compressor often oxidize. Poor contact of the probe with the terminal will give incorrect readings, which can lead to an erroneous diagnosis. Before starting work, check the device itself by connecting the probes to each other - the screen should show zeros or a value close to them.
☑️ Preparation for diagnostics
Algorithm for determining windings with a multimeter
The determination process begins with a visual inspection of the contact group. If there are no letters on the terminals C, S, R, proceed to measurements. Take a multimeter, turn on the resistance measurement mode (limit 2000 Ohms or 2 kOhms) and alternately apply the probes to pairs of contacts.
You will get three measurements between three contacts. Record the resulting values. The sum of the resistances of the two smaller values should be approximately equal to the largest value (resistance between the extreme contacts). The largest value is the sum of the resistances of the working and starting windings, measured in series.
The contact that is common to the two smaller resistances is called general (Common). Wires go from it to both the working and starting windings. The remaining two contacts are the conclusions of the windings themselves. The one with less resistance between it and the common contact is the output of the working winding. The one where the resistance is greater is the trigger terminal.
Consider an example: you have three contacts A, B, C.
Measurements: A-B = 20 Ohm, B-C = 50 Ohm, A-C = 70 Ohm.
Here A-C (70 Ohms) is the sum. The common contact is B, since it is involved in measurements 20 and 50.
Between A and B - 20 Ohms (less), which means A is the working winding.
Between B and C - 50 Ohms (more), which means C is the starting winding.
Table of typical resistance values
Although the exact values depend on the motor power and manufacturer, there are typical ranges that are typical for household refrigerators. Knowing them, you can quickly cut off clearly faulty units. Below are indicative data for compressors with power from 100 to 200 W.
| Winding type | Resistance range (Ohm) | Characteristic features |
|---|---|---|
| Working (Main) | 10 - 25 Ohm | Thick wire, low resistance |
| Start | 30 - 80 Ohm | Thin wire, high resistance |
| Total | 40 - 100 Ohm | Measurement between winding terminals |
| Insulation (housing) | Infinity (∞) | Any value indicates a breakdown |
If your measurements are radically different from the table ones, for example, they show 2-3 Ohms or, conversely, go to infinity (one on the screen of a digital multimeter), this indicates a malfunction. In the first case, an interturn short circuit likely occurred, in the second - an open circuit.
It is worth considering that compressors with a high energy efficiency class A+ or inverter models may have lower resistance due to the use of more advanced materials and a different stator design. However, the proportion “starting is always greater than operating” remains almost always.
Fault diagnosis: breakage and breakdown
The most common problem faced by owners of old refrigerators is winding break. If the multimeter shows infinite resistance when testing any of the windings, then the circuit is broken. It is impossible to restore such a compressor at home; the unit must be replaced.
The second dangerous defect is a breakdown in the housing. To check it, one multimeter probe is applied to any contact of the block (after having previously cleaned the contact area from varnish or oxides), and the second one is applied to a copper tube or an unpainted part of the compressor housing. The device should show infinity.
⚠️ Attention: If the device shows at least some resistance between the winding and the housing (even a few megaohms), operating such a compressor is deadly. Leakage current can lead to an electric shock when touching the body of the refrigerator.
An inter-turn short circuit also occurs, which is more difficult to diagnose with a simple ohmmeter. In this case, the winding resistance will be lower than the nominal one, and when you try to start the compressor, it will hum strongly, get hot and knock out the circuit breaker. This can be determined by the smell of burning or thermal heating of the case after a short-term switching on.
Frequent errors when connecting and testing
One of the common mistakes is an attempt to start the compressor directly, bypassing the starting relay, applying voltage directly to the working and starting windings. Without a properly selected capacitor or relay, such a start can lead to the burnout of the starting winding in a split second, since it is not designed for long-term operation.
Another mistake is ignoring the state of the contacts. Rust, carbon deposits or oxidation on the terminal block introduce additional resistance, distorting the measurement results. Before diagnostics, be sure to clean the contacts with fine sandpaper or a file until they have a metallic shine.
Do not also forget about the temperature coefficient of resistance. Measurements taken on a cold and hot compressor will differ. Copper changes its resistance when heated, so to accurately compare with tabular data, it is better to carry out diagnostics on a cooled unit.
⚠️ Attention: Technical characteristics of compressors may vary slightly depending on the year of manufacture and the specific batch. If you doubt the readings, check the data with the manufacturer's official datasheet or contact a service center.
FAQ: Frequently Asked Questions
Is it possible to start a compressor without a start relay?
Theoretically It is possible if you manually briefly apply voltage to the starting winding at the moment the working winding is turned on, but this requires high qualifications and is dangerous. It is easier and safer to buy a universal starting relay, which is suitable for many models.
What to do if the resistance of both windings is the same?
In a working single-phase compressor, the resistances should be different. If they are equal, this may indicate a serious inter-turn short circuit in one of the windings, which has changed its parameters, or a specific design of the motor (which is extremely rare in household appliances).
Why does the compressor hum, but does not start?
Most often the culprit is a stuck starting relay or faulty starting winding Also, the reason may be mechanical jamming of the piston group due to lack of oil or moisture entering the system.
Is it possible to replace a compressor from one refrigerator to another?
Yes, if the power, refrigerant (freon), type of oil and overall dimensions of the seats are the same. However, it is better to choose a model recommended by the manufacturer to avoid problems with performance and service life.