In the process of fault diagnosis In a refrigeration unit, a situation often arises when it is necessary to replace a failed motor-compressor or restore its functionality after a winding burnout. Self-replacement or rewinding is impossible without an accurate understanding of the electrical circuit of the motor. Identification of conclusions is the first and critical step in this process, since an erroneous connection can instantly disable the equipment.
Modern and old models of refrigerators, be it Indesit, Atlant or LG, use a similar principle for constructing single-phase asynchronous motors. However, the markings on the terminal block may be missing, worn off, or not consistent with standard markings due to previous repairs. In such cases, the only reliable tool for the master is multimeter knowledge of the physics of the operation of an electric motor.
This manual will help you understand the intricacies of winding continuity, understand the difference between the working and starting circuits, and also correctly interpret the received readings from the device. We will consider identification methods without markings, typical resistance values for different powers and an algorithm of actions when detecting anomalies in the circuit.
The principle of operation and the design of the windings
A single-phase refrigerator compressor is structurally different from three-phase industrial analogues in the presence of two distinct stator windings. The first, known as working (main) winding, is designed to create the main magnetic flux, ensuring rotation of the rotor in normal mode. It occupies most of the stator slots and is made of thicker wire, which causes its low electrical resistance.
The second winding is called starting. Its task is to create the initial torque and phase shift necessary to start the rotor from a stationary state. Since this winding is only turned on for a few seconds at startup (via a starting relay or capacitor), it is wired with a smaller diameter and has a significantly higher number of turns. It is this design feature that allows them to be distinguished using measuring instruments.
It is important to understand that at the moment of starting both windings operate in parallel, creating a resulting magnetic field. As soon as the engine reaches about 75% of rated speed, the centrifugal switch or thermal relay opens the starting winding circuit. If this mechanism is broken, or if the windings are mixed up when connecting, the engine will either not start, or will hum and overheat, which will quickly lead to thermal breakdown insulation.
⚠️ Attention: Never attempt to apply voltage to the compressor until you are sure that the insulation of the windings is intact relative to the housing. Applying voltage to the motor with a breakdown on the housing can lead to electric shock and failure of the wiring in the apartment.
Necessary tools and safety measures
Before starting any work with the electrical equipment of the refrigerator, you must strictly follow the electrical safety rules. First of all, the device must be completely de-energized. Simply turning off the refrigerator with a button is not enough - it requires physical disconnection power cord from the outlet. Only after this can you begin to dismantle the compressor casing and remove the protective cover of the terminal box.
To carry out high-quality diagnostics, you will need a set of tools, which every master or home craftsman should have. The main device is a digital multimeter capable of measuring resistance in the range from several ohms to tens of megaohms. You may also need pliers, screwdrivers for removing covers and, possibly, electrical tape for marking the wires after testing.
- 🔧 Digital multimeter —the main device for measuring resistance and checking for breakdown.
- 🧤 Dielectric gloves —additional protection when working with internal components.
- 📝 Marker or stickers —for marking the found contacts to avoid confusion.
- 🔦 Flashlight —for illuminating the darkened compressor niche in the back of the refrigerator.
Particular attention should be paid to visual inspection. If traces of burning, insulation melting or black spots are visible on the windings, further diagnostics may be useless - such a compressor requires replacement or professional rewinding. Also check the condition of the starting relay: often it is its malfunction (sticking contacts) that leads to burnout of the starting winding.
Method of testing with a multimeter
The process of determining the pins begins with access to the contact group of the compressor. Typically these are three pins arranged in a triangle. On a working compressor, resistance should be determined between any two contacts. If the multimeter shows infinity (OL or 1 on the screen) between any pair of contacts, this indicates an open circuit, which makes the motor inoperable.
The search algorithm consists of sequentially measuring the resistance between each pair of contacts. We have three contacts in total, therefore we will get three resistance values. Let's designate the contacts conventionally as A, B and C. We measure the resistance between A-B, B-C and A-C. The sum of the two smaller resistances should always be equal to the third, largest value. This is an axiom for a healthy engine.
The key point is to understand the resistance ratio. As mentioned earlier, the working winding has less resistance, and the starting winding has more resistance. Therefore, once you have three values, you can immediately identify the pairs. The largest value is the sum of the resistances of the working and starting windings (series connection). The two smaller values are the resistance of each winding individually.
☑️ Algorithm for naming the compressor
For ease of data interpretation, you can use the following logic: if you received values, for example, 20 Ohms, 50 Ohms and 70 Ohms, then 70 Ohms is the total resistance of the circuit (Operating + Launcher). Therefore, 20 Ohms is working winding, and 50 Ohms is starting. The contact that is common for measurements of 20 Ohms and 70 Ohms will be one of the ends of the working winding, and the contact common to 50 Ohms and 70 Ohms will be the end of the starting winding.
Table of typical resistance values
Resistance values may vary depending on the compressor power, manufacturer and model year. Low-power compressors for minibars will have a higher resistance, while powerful units for large two-chamber refrigerators will have a lower resistance. Below is a reference table to help you navigate the normal ranges.
| Compressor power | Working winding resistance (Ohm) | Starting winding resistance (Ohm) | Examples models |
|---|---|---|---|
| Small (up to 100 W) | 25 - 45 Ohm | 40 - 70 Ohm | Compressors for mini-refrigerators |
| Medium (100-150 W) | 15 - 25 Ohm | 25 - 40 Ohm | Atlant, Biryusa, old Indesit |
| High (150-200 W) | 10 - 18 Ohm | 18 - 30 Ohm | LG, Samsung, Whirlpool |
| Inverter models | 1 - 5 Ohm (phase-phase) | Not applicable (3 phases) | Modern Bosch, Siemens |
It is worth noting that for inverter compressors, the method of searching for starting and operating windings is not applicable in the classical form, since they are three-phase and controlled by a complex electronic unit. In such cases, the resistance between any three contacts should be the same. An attempt to start an inverter motor directly from a 220V network will lead to its immediate combustion.
Why can the resistance “float”?
The resistance of the windings depends on the temperature of the copper. A cold engine will show some values, a warm one - others. In addition, cheap multimeters can have an error of up to 10-15% at low ranges. Always let the engine cool down before taking measurements.
Diagnostics of breakdown to the frame and interturn short circuit
In addition to identifying the pins, it is critical to check the condition of the insulation. A housing breakdown is a situation where electrical contact occurs between the winding and the metal housing of the compressor. To check, one multimeter probe is applied to any contact of the terminal block (preliminarily cleaned of oxides), and the second is applied to a copper tube on the case or a specially cleaned place on the metal.
The device, set in resistance measurement mode to the maximum limit (Megaohms), should show infinity. Any deviation of the arrow or change in numbers on the screen indicates a violation of the insulation. The operation of such a refrigerator is life-threatening, since a 220V potential may appear on the housing.
⚠️ Attention: If the multimeter shows even a small resistance (for example, 1-5 MOhm) between the winding and the housing, such a compressor is considered faulty. Over time, this breakdown will progress, which will lead to a short circuit.
A more complex case is turn-to-turn short circuit. In this case, the winding resistance may be below normal, but there is no breakdown to the housing. The engine can start, but will become very hot, hum and consume increased current. It is difficult to accurately diagnose an interturn short circuit with a conventional multimeter; this requires special devices or indirect methods, such as measuring the current consumption with clamps.
Connection diagrams and contact markings
Once you have determined which terminal belongs to the working winding and which to the starting winding, you need to correctly connect them to the starting relay. The standard marking on the compressor terminal block (if it is preserved) looks like this: C (Common) - common wire, R or M (Run/Main) - working winding, S (Start) - starting winding winding.
In Russian and European practice, letter markings are often found: O (General), P (Working), P (Starting). The color coding of the wires coming from the relay can also suggest a solution, but you cannot rely on it, since the wires may have been soldered previously. Always double-check the connections based on resistance.
The connection is made in the following order: the common wire (C) goes directly to one of the phases of the network (via a thermal relay). The working winding (R) is connected through the contacts of a thermal protection relay. The starting winding (S) is connected through a starting relay (or a capacitor in some circuits), which opens the circuit after the start. An error in the connection, for example, applying voltage to the starting winding as the main one, will lead to the fact that the engine will not be able to develop the required speed and will burn out.
Frequent errors and recommendations specialists
One of the most common mistakes is ignoring the state of the starting relay. Even if you find the correct windings, a faulty relay will not allow the engine to start or will not disconnect the starting winding, causing it to burn out. Always check the relay for cracks, a burning smell and sticky contacts.
Also, technicians often forget that after prolonged operation or unsuccessful attempts to start, the compressor may be hot. A thermal relay in this state may not close the circuit, creating the illusion of a break. Allow the unit to cool completely (30-40 minutes) before re-diagnosis.
- 🚫 Do not clean the contacts on the block with a knife too aggressively - you can damage the threads or break off the copper pin.
- 🚫 Do not use ordinary PVC electrical tape for insulation in high temperature areas - it may melt and cause a short circuit.
- 🚫 Do not try to start the compressor “directly”, bypassing the thermal relay, even for a short-term check.
If after all the checks it turns out that the windings are intact, the resistance is normal, there is no breakdown on the housing, but the refrigerator does not work, the problem may lie in the refrigerant circulation system (clogging, leak) or in the electronic control module. In such cases, simply replacing the compressor will not help; comprehensive diagnostics of the system is required.
What to do if the winding resistances are almost the same?
In rare cases, especially in older or specific engine models, the resistances of the starting and operating windings can be very close. In this case, a visual analysis of the relay connection diagram will help: the contact that opens after the start (goes through the starting device) always belongs to the starting winding. You can also carefully start the engine for a short time and measure the current: the current through the starting winding will be less, but it is not intended for long-term operation.
Is it possible to swap the starting and operating windings?
Absolutely not. The operating winding is designed for constant voltage and has low resistance. Starting - for short-term current and high resistance. If you apply 220V to the starting winding as a working winding, it will burn out in a few seconds due to overheating. If you apply it to the working one as to the starting one, the engine will not be able to start and will hum.
Why does the multimeter show zero Ohm between the contacts?
A reading close to zero (less than 1 Ohm) indicates a short circuit of the turns inside the winding. This means that the insulation between the turns of the wire is destroyed, and the current flows along the shortest path, bypassing part of the turns. Such a compressor is faulty and requires replacement, as it will knock out the circuit breaker or burn out instantly.