A modern refrigerator is a complex unit, but its heart - the compressor - often causes a breakdown that can be diagnosed independently. When the refrigerator stops cooling, and the motor hums but does not start, or the circuit breaker trips, the first thing the technician checks is the integrity of the electric motor windings. Understanding how to find the start and run windings is a fundamental skill for anyone who repairs household appliances.
A malfunction in the motor electrical circuit can be caused by a broken wire, an inter-turn short circuit, or a breakdown to the frame. To carry out an accurate diagnosis, you will need a minimum set of tools, the main one of which is a multimeter. In this article we will examine in detail the physical principles of operation of a single-phase motor, methods for visually identifying contacts and an algorithm for checking resistance, which will allow you to accurately identify each terminal.
Safety when working with electrical appliances comes first, since the compressor operates on a 220 volt network. Before starting any manipulations, you must completely turn off the power to the refrigerator by removing the plug from the socket. Do not neglect this rule, even if it seems to you that you are just “looking” at the contacts - residual voltage or accidental switching on can lead to serious consequences.
The design of the electric motor and the principle of operation of the windings
The electric motor of a refrigeration compressor is an asynchronous single-phase unit. Unlike three-phase industrial motors, here, to create a rotating magnetic field, two separate windings located in the stator are needed. Working winding (the main one) occupies most of the stator slots and is wound with a thicker wire, which provides it with less electrical resistance. It is responsible for creating the main torque when the engine is running.
The second key element is starting winding. It is wound with a wire of smaller diameter and has greater resistance. Its task is to create the initial phase shift necessary to start the rotor. In modern refrigerators, this process is controlled by a starting relay, which supplies voltage to the starting winding only at the moment of startup, and then turns it off. If these terminals are mixed up when connecting, the compressor may hum, overheat, or not start at all.
⚠️ Attention: An attempt to start the compressor by applying 220V voltage directly to the starting winding without a starting relay or capacitor will lead to instantaneous burnout of the turns and failure of the unit.
Structurally, the terminals of all windings are assembled in a sealed casing and brought out through three insulated terminals arranged in a triangle. Depending on the compressor model (for example Atlant, Secop or LG), the location of these contacts may vary, but the physical principle of their operation remains unchanged. Understanding which winding is responsible for what helps not only in diagnostics, but also in the correct selection of a starting relay.
Visual diagnostics and marking of contacts
The first stage of searching for windings is an external inspection of the contact group. On most modern compressors, the terminals are marked in Latin letters, which greatly simplifies the task. The standard marking is as follows: C (Common) - common wire, R (Run) - working winding, S (Start) - starting winding. Sometimes you can find the designations M (Main) instead of R, which means the same thing.
However, you should not blindly rely on the markings, especially if we are talking about old Soviet refrigerators or units after unqualified repairs. The paint on the terminals may have worn off, or the previous technician may have used a non-standard relay. In such cases, a visual analysis of the thickness of the wires comes to the rescue, if it is possible to look inside the case (which is rare) or an analysis of the location of the contacts relative to each other.
Usually three contacts form an equilateral triangle. In different compressor models, the upper corner can be either a common wire or one of the phase wires. There is a common, but not universal rule: if you look at the contacts in a triangle, where the apex is directed upward, then the lower left corner is often the working winding, the lower right is the starting winding, and the upper is the common winding. But this is only an empirical observation, which requires mandatory testing with the device.
If the markings on your compressor are erased, do not try to guess the purpose of the contacts at random. A wrong pinout could cost you a new compressor. Always use a multimeter for verification, even if the letters seem obvious to you. This will only take a couple of minutes, but will save the equipment.
Method of testing with a multimeter
The most reliable way to determine where the starting winding is and where the working winding is is to measure the resistance. To do this, you will need a digital multimeter switched to the resistance (Ohms) measurement mode at the 2 kOhm or 200 Ohms limit. Before starting measurements, make sure that the probes of the device are in good condition, and the contacts on the compressor are cleaned of oxides and dirt.
The measurement process consists of alternately ringing pairs of contacts. Since we have three pins, we will get three resistance values. The algorithm of actions is as follows:
- 🔌 Measure the resistance between the first and second contacts, write down the result.
- 🔌 Measure the resistance between the second and third contacts, write down the result.
- 🔌 Measure the resistance between the first and third contacts, write down the result.
After receiving three numbers, you need to analyze them. The sum of the two smaller resistances should be equal to the largest resistance (taking into account the instrument error). The largest value is the resistance between the ends of the working and starting windings (total resistance). The two smaller values are the resistances between the common wire and each of the windings.
For example, if you got the values 20 Ohms, 12 Ohms and 32 Ohms, then 32 Ohms is the sum of 20 and 12. Therefore, the contact that participated in the 20 Ohms and 12 Ohms measurements (but did not participate in the 32 Ohms measurement directly as a sum) is common wire. The contact that is paired with a common 12 ohms is the working winding (lower resistance). The contact that gave 20 Ohms is the starting winding (higher resistance).
Table of typical resistance values
For quick orientation, it is useful to know the approximate resistance ranges for compressors of different powers. Low-power refrigeration motors (for small chambers) will have higher resistance than powerful compressors for two-chamber refrigerators. Below is a reference table to help you understand whether your measurements are within the normal range.
| Compressor type | Working winding resistance (Ohm) | Starting winding resistance (Ohm) | Total resistance (Ohm) |
|---|---|---|---|
| Low power (up to 100 W) | 18 – 25 | 35 – 45 | 55 – 70 |
| Medium power (100-150 W) | 12 – 18 | 25 – 35 | 40 – 50 |
| High power (>150 W) | 8 – 12 | 15 – 25 | 25 – 35 |
| Inverter (BLDC) | Not applicable | Not applicable | Phase check |
It is important to note that the values in the table are averaged. Actual performance depends on the temperature of the windings (a cold motor will show less resistance than a hot one) and the specific series of the manufacturer. For example, compressors Secop (formerly Danfoss) often have slightly different characteristics compared to their Chinese counterparts QD or Jiaxipera.
If your measurements differ radically from the table ones (for example, they show one or infinity), this indicates a malfunction. A one on the multimeter display usually means an open circuit (infinite resistance), and a zero or close to zero value means a short circuit. In such cases, resuscitation of the windings is impossible; replacement of the unit is required.
Why does the resistance change when heated?
The resistance of the copper from which the windings are made depends on temperature. As the engine heats up, the resistance increases. Therefore, measurements should always be carried out on a cooled compressor to obtain correct data.
Checking for insulation breakdown (short circuit to housing)
Determining the windings is only half the task. It is critical to ensure that none of the windings “pierce” into the compressor housing. This phenomenon, called insulation breakdown, is fatal to humans and can lead to fire. Even if the windings are intact, the presence of contact with the metal body makes operation of the refrigerator impossible.
To check, set the multimeter to the continuity mode (with a sound signal) or resistance measurement to the maximum limit (2 MOhm and higher). Press one probe to the cleaned area on the compressor housing (where there is no paint or rust), and with the second touch all three contacts in turn.
⚠️ Attention: If the device shows at least some resistance (even several Megaohms) or makes a squeak when touching the housing, the compressor is faulty. It is strictly prohibited to operate such a unit!
Sometimes breakdown can only occur when heated. If there is no breakdown “in the cold”, but the RCD or circuit breaker knocks out during operation, perhaps the thermal expansion of the windings leads to contact of the turns with the housing. In such a situation, diagnostics with a multimeter may be ineffective, and the use of a professional megohmmeter that supplies high voltage is required to check the insulation.
It is also worth checking the condition of the starting relay. Often the problem lies not in the compressor itself, but in sticky relay contacts that do not turn off the starting winding. This leads to the fact that the starting winding, designed for short-term operation, overheats and burns out. Visually, a burnt winding may have a blackened varnish or a characteristic burning smell emanating from the casing.
Connection diagram and starting the compressor
After you have managed to find the starting and operating winding, you need to correctly assemble the circuit for testing or temporary starting. The classic circuit includes a start relay, which is placed directly on the contacts of the compressor. The relay body usually has an arrow indicating the "UP" direction, which is important for the correct operation of the mechanical switch.
In a circuit with a start relay, current first passes through the operating winding, creating a magnetic field. At the same time, current flows through the relay coil and the starting winding. When the rotor picks up speed, the current in the working winding drops, the magnet in the relay weakens and opens the contacts of the starting circuit. If you mix up the leads, this process will not happen correctly.
☑️ Check before connecting to the network
For a temporary start without a native relay (for example, for pumping out freon or diagnostics), craftsmen often use a button. The circuit is assembled like this: the phase goes from the network to the common wire, and also through the button to the starting winding. The working winding is constantly connected to the network. By pressing the button, you apply power to the starting winding, the motor starts, release the button - the starting winding is turned off.
Maintaining polarity in the alternating current (AC) circuit does not matter for the motor itself, but it is important for the correct operation of the starting relay if it has a capacitor or a specific coil. Always try to reproduce the standard connection diagram indicated on the refrigerator sticker or in the technical documentation.
Frequent diagnostic errors and their consequences
One of the most common mistakes is ignoring the state of contacts. Oxidized or burnt terminals may produce a false high resistance, which will lead to an erroneous diagnosis of “burnt out winding.” Always clean the contacts to a metallic shine before taking measurements.
Another mistake is using a faulty or discharged multimeter. If the battery in the device is dead, the resistance readings will be greatly distorted. Test the device with a known resistor or simply by connecting the probes to each other - the resistance should be close to zero (0.3-0.5 Ohm, taking into account the probe wires).
Also, inexperienced craftsmen often confuse a thermal relay with a starting one. The thermal relay protects against overcurrent and opens the circuit of the working winding when overheated. It is connected in series with the working winding. If it is installed incorrectly or replaced with an unsuitable one, the compressor will constantly turn off or not start at all.
Is it possible to start the compressor without a start relay?
You can start it temporarily using the button, as described above, but long-term operation without a relay is unacceptable. The starting winding will burn out in a few seconds or minutes of continuous operation, since it is not designed for direct current.
What to do if the resistance of the windings is normal, but the compressor does not work?
This may indicate a mechanical wedge (jamming of the piston group) or a malfunction of the starting relay/capacitor. Sometimes the “wedging” method by applying increased voltage helps, but this is a risky procedure, accessible only to professionals.
Why does it hum, but does not start?
The hum indicates that current is flowing into the windings and a magnetic field is created, but the torque is not enough to break the rotor from its place. Most often, the culprit is a faulty starting relay (does not supply a pulse to the starting winding) or an interturn short circuit that reduces the engine power.
How to distinguish an inverter compressor from a conventional one?
Inverter compressors are often labeled BLDC or DC Inverter. They have three working terminals of the stator windings (without division into starting and working in the classical sense) and the resistance between them is approximately the same. They need to be checked differently, and you cannot start them directly from 220V.
Is it dangerous to change the compressor yourself?
Yes, this is associated with the risk of electric shock and working with refrigerants under pressure. In addition, a vacuum pump and a pressure gauge station are required to charge the system. If you do not have these tools, it is better to limit yourself to diagnosing the electrical part.