Diagnostics of refrigeration equipment faults often rests on the need to check the electric motor, and here the master is faced with the question: which winding is the starting winding and which is the working winding for the refrigerator compressor. Understanding the separation principle of these circuits is critical for correct diagnosis, replacing the start relay, or connecting a new motor. An error in determining the terminals can lead to instantaneous burnout of the winding or failure of the starting device, so the theoretical basis here is inextricably linked with practice.
In a classic single-phase asynchronous motor used in household refrigerators, the stator has two main windings located in slots with a certain offset. Working winding (main) occupies most of the stator slots and is made with a wire of larger cross-section, which provides lower electrical resistance. Starting winding winding (auxiliary) has a smaller number of turns, is wound with a thinner wire and is often located on top of the working winding, occupying a smaller part of the perimeter.
The difference in design is due to their functional purpose: the main winding creates torque during operation, and the auxiliary winding is needed only to create the initial phase shift at start. After the rotor reaches about 75-80% of the rated speed, the starting winding must be turned off with a centrifugal switch or starting relay, otherwise it will quickly overheat and burn out due to the lack of sufficient inductance to operate in constant mode.
Physical and electrical differences of windings
To accurately determine the purpose conclusions, it is necessary to rely on the physical parameters of the winding. Working winding always has lower active resistance compared to the starting one. This is a fundamental rule that works for the vast majority of household compressors, be it Atlant, Indesit or Birius. The difference in resistance can be twofold or even threefold, which is easily recorded with a conventional multimeter.
The second important difference is the cross-section of the wire. Since the work winding carries the majority of the load current, it is wound with thicker wire to minimize energy loss and heat. The starting winding works for a short time, so the wire there is thinner and the number of turns is less. Visually, if it is possible to look inside the stator (which is rarely required in conventional diagnostics), this difference is striking, but in practice we rely on electrical measurements.
It is worth noting that in some modern models with inverter control or permanent capacitor motors (CSR), the circuit may differ, but the principle of dominance of the starting circuit resistance over the working circuit remains the same. It is also important to consider that three-phase compressors, which are sometimes found in industrial cold, do not have a division into starting and operating windings in the classical sense - all three windings are identical.
⚠️ Attention: Never try to determine the type of winding “at random”, applying 220V voltage at random. This is guaranteed to lead to a short circuit or irreversible damage to the motor insulation.
Method of testing with a multimeter
The most reliable method of identification is to measure the resistance between the terminals. To do this, you will need a digital multimeter set to resistance (Ohm) measurement mode with a limit of 2 kOhm. Before starting work, make sure that the compressor is completely de-energized and the capacitor (if there is one in the circuit) is discharged.
The measurement process is as follows:
- 🔌 Find the three contacts on the compressor terminal block (usually they are located in a triangle).
- 📏 Measure the resistance between each pair of contacts and write down the results.
- 📝 The sum of the two smaller resistances should be equal to the third (maximum) resistance.
- 🧐 Compare the values obtained: the smallest value is the working winding, the average is the starting winding.
Consider an example. Let's say we have contacts A, B and C. We measured: between A and B - 20 Ohms, between B and C - 50 Ohms, between A and C - 30 Ohms. In this case, the pair with the lowest resistance (20 ohms) indicates the ends of the working winding, but it is important to interpret the circuit correctly. Typically the pins are marked as C (Common), R (Run) and S (Start). The resistance between C and R will be the smallest, between C and S - average, and between R and S - total (highest).
☑️ Check with a multimeter
For this One multimeter probe is applied to any contact, and the second is applied to a copper tube or a cleaned area on the compressor housing. The device should show infinity (one in the most significant digit).
Resistance table for different types of compressors
Resistance values may vary depending on the engine power and manufacturer. Below is a reference table that will help you navigate the typical values for popular models.
| Compressor model | Operating resistance (Ohm) | Starting resistance (Ohm) | Total (Ohm) |
|---|---|---|---|
| AT-60 (Atlant) | 18-22 | 35-45 | 55-65 |
| FK 8126 (Danfoss) | 20-25 | 40-50 | 60-75 |
| QD35 (Secop) | 12-16 | 28-34 | 40-50 |
| SC10 (Tecumseh) | 10-14 | 25-30 | 35-44 |
Please note that the values may vary slightly depending on the temperature of the windings. A cold engine will show less resistance than a warm engine. If the multimeter readings “float” or are unstable, this may indicate poor contact inside the terminal box or poor soldering of the leads.
Why are the resistances summed?
The resistance between the outer terminals (R and S) is equal to the sum of the resistances of the working and starting windings, since they are electrically connected in series through a common wire (C).
Connection diagrams and pin markings
Understanding which winding is responsible for what is necessary for the correct assembly of the electrical circuit. On a compressor terminal block, the terminals are often not labeled, but their locations are standardized. If you look at the block from above, the contacts usually form a triangle.
The standard marking on the diagrams looks like this:
- ⚡ C (Common) — the common wire to which the phase or zero goes (depending on the circuit).
- 🔄 R (Run) or M (Main) —the output of the working winding.
- 🚀 S (Start) — output of the starting winding.
In the classic circuit with a starting relay, the current first flows to the working winding. In parallel with it, through the contacts of the starting relay, the starting winding is connected. When the engine starts and the current in the run winding drops (or the centrifugal mechanism is triggered), the relay opens and the start winding is turned off. If you mix up the leads and apply voltage to the starting winding as the main winding, the engine will hum, but will not start, and the winding will quickly fail.
⚠️ Attention: For compressors from different manufacturers, the arrangement of contacts on the block may differ (for example, for Danfoss and Atlant, the pinout patterns may be mirrored). Always check the technical documentation or the markings on the relay itself.
Diagnostics of winding faults
Knowing which winding is starting and which is working in a refrigerator compressor allows you to diagnose characteristic failures. If the multimeter shows a resistance close to zero, then an interturn short circuit has occurred. If the device shows infinity (open), then the circuit is broken.
Most often it is the starting winding that burns out, since it experiences high current loads at the moment of start. Signs of a malfunction may be:
- The refrigerator hums for 5-10 seconds, then a click is heard and it goes silent.
- The starting relay crackles or visibly heats up.
- When you try to start, the circuit breaker in the panel is knocked out.
In some cases, the break may not be internal, but at the place where the copper lead is soldered to the terminal. Visual inspection and “moving” of the wires during testing can help identify such a defect. If the working winding burns out, the compressor, as a rule, requires replacement, since rewinding the stator at home is not economically feasible.
Common errors when replacement
When doing independent repairs, technicians often make mistakes associated with incorrect identification of pins. One of the common mistakes is connecting the starting winding directly to the network without a relay. This leads to instant overheating and smoking.
Another mistake is ignoring phasing. Although the direction of rotation is usually not critical for the operation of a single-phase motor (it always spins in one direction due to the design of the rotor), incorrect connection can disrupt the operation of the built-in thermal protection or control circuit.
It is also important to check the condition of the starter itself. Even if the windings are intact, a faulty relay will prevent the engine from starting, creating the illusion of an open circuit. Always check the relay for burnt contacts and the integrity of the posistor (if we are talking about a starter with a posistor).
Is it possible to start a compressor without a starting relay?
Theoretically, it is possible if you manually apply voltage to the starting winding at the moment of switching on, and then remove your hand. However, this method is extremely dangerous and is not recommended, as it requires a high reaction speed and carries the risk of electric shock. For test runs, it is better to use a button or a special starting device.
Why can the resistance of the windings change?
The resistance of copper depends on temperature. As the engine heats up, the resistance increases. The readings may also be affected by oxidation of the contacts in the terminal box. If the values differ greatly from the tabulated ones (by more than 20%), this may indicate an interturn short circuit.
What to do if the winding resistances are the same?
In standard single-phase refrigerator compressors, the winding resistances cannot be the same. If the multimeter shows the same values between all pairs of contacts (for example, 20 ohms, 20 ohms and 20 ohms), this is a sign of a faulty motor (turn-to-turn short) or measurement error. In three-phase motors, the resistances are equal, but they are not used in domestic refrigerators.
How are the terminals on Chinese compressors marked?
On Chinese compressors (for example, QD series), the marking may be absent or may be printed in small print under the terminal cover. The wires inside the relay are often color coded: red - common, white/black - working, blue/yellow - starting. However, you cannot rely on the color of the wires; always check with a multimeter.