Faced with a failure of refrigeration equipment, many amateur craftsmen and equipment owners are wondering about independently diagnosing the core unit of the unit. The compressor is the most expensive and complex element of the system, and it is its electrical part that often becomes the object of close attention when searching for the causes of failure. Understanding how the terminals are located and which one is the starting one is critical for correctly replacing the starting relay or checking the integrity of the electric motor.
Inside the sealed casing is hidden an electric motor, which has two main windings: working and starting. Working winding ensures continuous rotation of the rotor during normal operation, while starting winding creates the necessary torque at the time of launch. If you confuse these pins during connection or diagnostics, you can instantly damage the new compressor or even cause a short circuit in the electrical network, so accurate identification of the contacts is a primary task.
In this article we will analyze in detail the methods for identifying pins, the use of measuring devices and typical mistakes made during repairs. You will learn how to distinguish one winding from another by resistance, what to do if the marking is erased, and what nuances exist among different manufacturers of refrigeration units.
The principle of operation of a single-phase compressor motor
Electric motors installed in household refrigerators belong to the class of single-phase asynchronous motors. To start them, it is necessary to create a rotating magnetic field, which cannot be done using only one winding connected to a single-phase network. That is why the design provides for the presence of two windings located in the stator at an angle to each other.
Working winding (or main) occupies most of the stator slots and has a thicker wire, which provides less electrical resistance. It is designed for constant operation and maintaining shaft speed. At the same time, starting winding is made of thinner wire and has a larger number of turns, which leads to a significantly higher resistance.
The starting winding is put into operation only for a short time - at the moment starting the engine. Once the rotor reaches about 75-80% of its rated speed, a special device called a starting relay opens the starting winding circuit. If this point is not met, the winding will overheat and burn out due to the lack of phase shift necessary to create torque.
⚠️ Attention: The starting winding is not designed for long-term operation under load. An attempt to start the compressor with the starting winding constantly on will lead to its instant burnout and failure of the entire unit in a matter of seconds.
In some modern models, instead of the classic starting relay with a bimetallic plate or posistor, electronic control circuits can be used, but the physical principle of separating the windings inside the motor remains unchanged. Understanding this physics helps to correctly interpret multimeter readings when testing contacts.
Visual identification and marking of leads
The first step in the diagnostic process is always an external inspection of the compressor. On many modern models, such as Atlant, Indesit or Beko, the winding terminals are covered with a plastic cover, which is often marked with a connection diagram. However, even if there is a cover or markings on the case, you cannot rely on them alone, since over the years of operation the paint could be worn off, and the cover could be replaced.
The standard marking of pins on the terminal block usually looks like this:
- 🔴 C (Common) - the common pin to which one of the power wires is connected.
- 🔵 R (Run) or M —working output (main winding).
- 🟡 S (Start)—starting output.
In some cases, especially on old Soviet compressors or Chinese-made models, letter designations may be absent or replaced by numbers. For example, on some motors Danfoss or Secop the circuit can be printed directly on the metal casing next to the terminals. If you see three contacts arranged in a triangle, then the top of the triangle is most often a common wire, but this is not an axiom.
If the marking is not readable or is in doubt, you need to move on to instrumental verification methods. A visual inspection should also include checking the condition of the contacts themselves: they should not be oxidized, melted or have traces of carbon deposits, which may indicate poor contact in the starting relay.
Method of measuring resistance with a multimeter
The most accurate and safe way to determine where the starting winding is is to measure the electrical resistance between the terminals. To do this, you will need a digital multimeter switched to resistance measurement mode (Ohms) at a limit of up to 2 kOhms. Before starting measurements, make sure that the compressor is completely disconnected from the power supply.
The measurement process is as follows:
1. Remove the start relay and protective cover, freeing the three contacts of the compressor.
2. Attach the multimeter probes to any pair of contacts and write down the resulting value.
3. Repeat the procedure for all three possible pairs of contacts (1-2, 2-3, 1-3).
As a result, you will get three resistance values. The logic here is simple: the resistance between the working and starting terminals will be equal to the sum of the resistances of each winding relative to the common wire. But there is a simpler way of analysis:
- 📏 The lowest resistance value (usually 10-25 Ohms) will show the circuit between common and working output.
- 📏 The average resistance value (usually 30-50 Ohms) will show the circuit between common and starting output.
- 📏 Highest value (the sum of the two previous ones) will be between working and starting conclusions.
Thus, the starting winding always has higher resistancethan working, if measured relative to the common wire. This is due to the fact that the starting winding is wound with a thinner wire and has more turns to create the necessary inductive reactance.
☑️ Check with a multimeter
Table of typical resistance values
To facilitate diagnostics, below is a table with approximate resistance values for various types of compressors. Remember that these values may vary depending on the engine power and manufacturer.
| Compressor type | Working winding resistance (Ohm) | Starting winding resistance (Ohm) | Total resistance (Ohm) |
|---|---|---|---|
| Low power (up to 100 W) | 15 - 25 | 35 - 50 | 50 - 75 |
| Medium power (100-150 W) | 10 - 18 | 25 - 40 | 35 - 58 |
| Powerful (No Frost, inverter) | 5 - 12 | 15 - 30 | 20 - 42 |
| Old models (USSR) | 20 - 30 | 40 - 60 | 60 - 90 |
If during measurement the multimeter shows one (infinity), this indicates a winding break. If the device shows zero or a value close to zero, it means that an interturn short circuit has occurred. In both cases, the compressor requires replacement or professional rewinding, which is practically impossible at home.
It is also important to take into account the temperature of the windings when measuring. Copper resistance depends on temperature: the hotter the motor, the higher the resistance. Therefore, measurements on a cold and hot compressor may differ by 10-15%, which is a normal physical phenomenon.
Why does resistance change with temperature?
Metallic conductors, such as copper, when heated, increase their electrical resistance due to increased thermal vibrations of atoms in the crystal lattice, which impedes the movement of electrons.
Features of connecting the starting relay
Once you have determined which pin is the starting one, you need to connect the starting relay correctly relay It is this component that is responsible for supplying voltage to the starting winding at the right time. An error in connecting the relay will result in the engine either not starting or burning out.
There are two main types of starting devices:
1. Posistor —they work based on the thermal expansion of a ceramic element. When current is supplied, the posistor heats up and opens the circuit.
2. Induction - use an electromagnetic coil and a movable core to open the contacts when the speed increases.
When installing a relay, it is important maintain orientation. There is always an arrow on the relay body indicating the "UP" direction. If you install the relay upside down, gravity will not allow the core (in induction relays) or contacts (in posistor relays) to work correctly, and the starting winding will remain on constantly.
⚠️ Attention: Never connect the compressor directly to the network without a starting relay, trying to “start it manually”. This is guaranteed to lead to burnout of the starting winding and possible electric shock.
The relay contacts must sit tightly on the compressor terminals. If the contacts are oxidized or loose, they should be carefully cleaned and tightened. Poor contact in the starting winding circuit causes sparking and heating, which is a common cause of failure of the entire starting system.
Diagnostics of winding faults
Incorrect identification of the starting winding or problems in its circuit can manifest themselves in different ways. If the compressor hums but does not start, and after a few seconds the thermal protection relay clicks, this is a classic sign that the starting phase is not working.
Possible reasons:
- 🔥 Breakage of the starting winding: The multimeter shows infinity. The engine cannot create an initial torque.
- 🔥 Interturn short circuit: The resistance is significantly below normal. The engine hums, heats up and knocks out the circuit breaker.
- 🔥 Malfunction of the starting relay: The windings are intact, but voltage is not supplied to the starting winding or is not turned off in time.
It is also worth checking for a breakdown in the case. To do this, one multimeter probe is applied to any terminal, and the second is applied to a stripped place on the copper tube or compressor housing. The device should show infinity. If there is any resistance, it means that the insulation of the windings is broken, and operating such a compressor is deadly.
In some cases, especially after a long period of inactivity, the compressor may not start due to souring of the oil or mechanical jamming of the piston group. In such a situation, even serviceable windings will not help to start the unit without the use of special wedging methods, which requires separate consideration.
Frequently asked questions (FAQ)
Is it possible to start a compressor without a starting winding?
No, a standard single-phase asynchronous motor will not start on its own without a starting winding or other device that creates a phase shift (for example, a capacitor). The shaft will only hum and vibrate, but rotation will not occur.
What to do if the resistance of the starting and working windings is the same?
In rare cases (bifilar winding), the resistances can be very close. In such a situation, you need to look for markings on the diagram or use the “poke” method with caution: if the connection is correct, the no-load current will be minimal, and if it is incorrect, the engine will hum and get very hot.
Why does the starting winding burn out more often than the working winding?
The starting winding is made of a thinner wire, since it works for a short time. If the starting relay sticks and does not open the circuit, the current continues to flow through the thin wire, causing it to quickly overheat and melt the insulation.
Is it possible to swap the starting and operating windings?
Absolutely not. If you apply operating voltage to the starting winding as the main winding, it will burn out almost instantly due to the high resistance and thin wire. If you connect the working one as a starting one, the engine may not start or operate with very low efficiency.