Diagnosing a compressor malfunction often begins with checking the integrity of its electrical windings. When the refrigerator stops freezing and the motor does not start or hums but does not turn on, the first thing the technician must do is understand which part of the electrical circuit has failed. Starting and working windings have fundamental differences in design and resistance parameters, knowledge of which allows you to accurately localize the problem without disassembling the unit itself.
Incorrect pin definition can lead to a short circuit or instant burnout of the motor when trying to start. In this article we will analyze the physical principles of operation single-phase induction motorsused in household appliances and learn how to accurately identify contacts using a conventional multimeter. This is a basic skill required for anyone who repairs refrigeration equipment.
Before you begin measurements, you must ensure safety and prepare your tools. Working with electricity requires care, and compressor diagnostics requires the accuracy of the instrument readings. We will consider not only the theory, but also practical nuances that beginners often miss, relying only on the color of the wires or markings.
The principle of operation of a single-phase motor and the role of windings
Compressors of household refrigerators operate from a single-phase alternating current network. To create the rotating magnetic field necessary to start the rotor, two independent coils are placed in the motor stator. Working winding (main) occupies most of the stator slots and is designed to maintain rotor rotation after starting. It has a larger wire cross-section and lower active resistance.
The second coil, known as starting winding, is switched on only at the moment of start. Its task is to create an initial torque, shifted in phase relative to the main one. As soon as the rotor reaches a certain speed, a centrifugal relay or starting device disconnects this winding from the network. If this does not happen, the engine quickly overheats and burns out.
The difference in the physical design of the coils directly affects their electrical parameters. The starting winding is wound with a thinner wire and has a higher number of turns, which leads to high resistance. The working one, on the contrary, is wound with a thick wire to pass large currents in continuous mode, so its resistance is minimal.
⚠️ Attention: An attempt to apply voltage directly to the starting winding without a working one or in violation of the connection diagram will lead to instant failure of the motor due to current overload.
Understanding this difference is critical. important in diagnosis. By measuring the resistance between the terminals, we are actually measuring the length and thickness of the wire inside the stator without seeing it visually. It is the ratio of these resistances that allows us to unambiguously say which output is responsible for what.
Necessary tools and preparation for diagnostics
To carry out high-quality diagnostics, you will need a minimum set of tools that any home craftsman should have. The main device is multimeter (tester), capable of measuring resistance in the range from 0 to 200 Ohms with an accuracy of tenths. Without this device, determining the windings “by eye” is impossible and dangerous.
You will also need screwdrivers to remove the compressor protective casing, pliers to carefully remove the terminals, and possibly a solvent to clean the contacts from oxides or carbon deposits. Before starting work, be sure to disconnect the refrigerator from the power supply and remove the rear protective panel to gain access to the motor.
The start-up relay usually has three contacts coming out of the compressor. They may be signed with letters C, S, R or have other markings, but you should not rely on them, since the relay could have been replaced earlier or the markings have worn off. Your task is to measure the resistance between each pair of these three contacts.
☑️ Preparing to check the windings
Step-by-step instructions: how to ring a compressor with a multimeter
The measurement process begins with a visual inspection of the contact group. If the contacts are heavily melted or oxidized, they must be cleaned, otherwise the instrument readings will be distorted. Switch the multimeter to resistance measurement mode (limit 200 Ohm or 2 kOhm) and make sure that the device shows zero when the probes are shorted.
Take two probes and apply them to any two of the three available contacts. Record the reading. Then move one probe to the third contact, leaving the second one in place, and record the result again. Repeat the procedure for the third pair. As a result, you should have three resistance values, corresponding to three possible connection combinations.
The sum of the two smaller resistances should always be equal to the largest resistance (taking into account the instrument error). This is an axiom for a serviceable engine. If there is no equality, it means that somewhere there is a hidden defect, a break or an interturn short circuit, which requires a more in-depth diagnosis.
Now we analyze the obtained numbers. The pair of contacts that gave the highest resistanceis the sum of the resistances of the starting and operating windings. Therefore, the third contact, which did not participate in this measurement, is total (C or Common). It is to this that one of the wires of the network (phase or zero) is supplied.
The remaining two contacts are the outputs of the working and starting windings. To understand who is who, you need to measure the resistance from the common contact to each of them. The output whose resistance will be lessbelongs to the working winding. The one where the resistance more (usually 1.5-2 times) is to the starting one.
Resistance table and interpretation of readings
For ease of understanding, we present specific numerical values, which can be encountered in practice. Although the exact numbers depend on the power and model compressor, the proportions remain the same. Below is a table with typical readings for a working motor.
| Measured pair | Connection type | Typical value (Ohm) | Characteristics |
|---|---|---|---|
| Pin 1 - Contact 2 | Starting + Running | ~45-50 Ohm | Maximum resistance (sum) |
| Common - Contact 1 | Working winding | ~10-15 Ohm | Minimum resistance |
| Common - Pin 2 | Starting winding | ~30-35 Ohm | Average value |
| Case - Any contact | Ground breakdown | Infinity | Isolation intact |
It is important to note that values may vary. For low-power refrigerators, the resistance of the working winding can be 10-15 Ohms, and for more powerful units - only 3-5 Ohms. The main rule: the working winding always has less resistancethan the starting winding.
Pay special attention to checking for breakdown. Press one multimeter probe to the cleaned area on the compressor housing (scratches), and touch each of the three contacts with the second. The device should show infinity. If the arrow deviates or numbers appear, the winding is shorted to the housing. Operating such a unit is deadly.
Typical faults and their symptoms
Knowing how to identify windings helps not only in assembly, but also in finding defects. The most common problem is turn-to-turn short circuit. In this case, the winding resistance will be lower than the nominal one. The engine may hum, but not start, or start and quickly turn off via a thermal relay.
Another common malfunction is a break in the starting winding. When measuring, the multimeter will show infinity between the common contact and the terminal of the starting winding. In this case, the compressor will hum quietly, but the rotor will not budge, since there is no starting torque.
- 🔥 Overheating: If after starting the motor body becomes hot within a minute, the windings may be mixed up or the starting relay is faulty.
- 🔇 Silence: Complete absence of sound when turned on the network may indicate an open circuit or internal protection has triggered.
- ⚡ Knocking out plugs: Instant shutdown of the circuit breaker indicates a short circuit of the turns or a breakdown to the housing.
Sometimes it occurs rotor sticking. Electrically, the windings may be intact, the resistance is normal, but mechanically the shaft is jammed. In such cases, an attempt to start gives a sharp electric shock and a buzzing sound, after which the thermal protection is triggered.
Connection diagram and assembly of the unit
After you have determined which winding is which, you need to connect the starting relay correctly. The relay body usually has markings: Run (or R) - for the working winding, Start (or S) - for the starting winding. The common phase wire is connected through a thermal fuse and a relay to the corresponding contact.
The connection diagram is as follows: the phase wire comes to the common contact of the compressor. The starting relay coil is powered from it. The working winding is connected directly to the network (in parallel with the relay coil), and the starting winding is connected through normally open relay contacts.
When voltage is applied, current flows through the working winding and the relay coil. The starting winding is energized through the relay contacts, which are closed due to the current passing through the series-connected starting winding (in a relay with a current coil) or are simply closed mechanically. As soon as the engine accelerates, the current drops and the relay opens the starting winding circuit.
⚠️ Attention: When installing the relay, it is important to observe its spatial position (the arrow UP must look up). In an inverted state, the contacts may not open, which will lead to burnout of the starting winding.
Use the original terminals to secure the wires. If they are lost or burnt, use new ones, ensuring tight contact. Poor contact causes sparking and heating, which ultimately melts the plastic of the relay.
Why can’t you swap the working and starting windings?
If you constantly apply operating voltage to the starting winding, it will burn out in a few seconds. It is designed for short-term operation (up to 3-5 seconds) and has a thin wire. The working winding, connected as a starting winding, will not create sufficient starting torque, and the engine will not start.
Frequently asked questions (FAQ)
Is it possible to start a compressor without a starting relay?
Theoretically, it is possible if you briefly apply voltage to the starting relay winding manually with a button at the moment of switching on, but this is dangerous and requires skills. For constant operation, the relay is necessary, since it automatically turns off the starting winding. Without it, the motor will burn out.
What to do if the resistance of the windings is the same?
In a working single-phase motor, the resistance cannot be the same. If the multimeter shows the same values (for example, 20 Ohms on all pairs), this may indicate a complex turn-to-turn short circuit or an error in the measurements (poor contact of the probes). Most likely, the compressor is faulty.
How to determine where is the “phase” and where is the “zero” on the compressor?
For the operation of the motor itself, the polarity is not important, since it is alternating current. However, for proper operation of the circuit (so that the phase and not zero is broken through the relay), the phase is usually supplied to a common contact. You can determine the phase in the socket using an indicator screwdriver.
Is it possible to use capacitor starting instead of a relay?
Yes, some circuits (CSIR, CSCR) use capacitors. However, in domestic refrigerators, a circuit with a starting relay and a starting capacitor (or without it, depending on the model) is most often used. Replacing the starting type requires recalculating the capacitance and changing the design.
Correct determination of the windings is the foundation for successful repairs. Using a multimeter and measurement logic, you can accurately diagnose the condition of the compressor and avoid costly mistakes when replacing starting equipment.