Repair of refrigeration equipment often begins with diagnostics electrical part, and the key unit here is the motor-compressor. Starting winding plays a critical role in starting the engine, creating the necessary torque. If the refrigerator hums but does not start, or the circuit breaker trips, the problem often lies in the starting circuits or the electric motor itself. Understanding how to find and check this element is a basic skill for any home craftsman.
In modern single-phase asynchronous motors used in household refrigerators, there are usually two main windings: working (main) and starting. They have different electrical resistance and number of turns, which allows them to perform different functions. The working winding is responsible for maintaining the rotation of the rotor, occupying most of the stator slots, while the starting winding is turned on only for a short time to overcome the inertia of rest.
Incorrect pin definition can lead to a short circuit or failure of the starting relay. In single-phase compressors, the starting winding always has greater electrical resistance compared to the working winding. This physical property, based on the use of a wire of a smaller cross-section and fewer turns, is the main guideline for diagnosis. Ignoring this fact or confusion in connecting the terminals is guaranteed to lead to overheating and jamming of the motor.
The principle of operation and the difference between the windings
The electric motor of the refrigerator is designed so to operate from a standard single-phase network. To create a rotating magnetic field, a phase shift is necessary, which is ensured by the difference in the characteristics of the windings. Working winding (or main) is wound with a thicker wire, has fewer turns and, accordingly, less active resistance. It is constantly connected to the network while the compressor is operating.
The starting winding is made of a wire of smaller diameter, which increases its resistance. It is included in the circuit only at the moment of start through a starting relay or capacitor. As soon as the rotor reaches the required speed (usually about 75% of the rated speed), the centrifugal mechanism or relay opens the starting winding circuit. If this element does not turn off, the motor will quickly overheat due to high currents.
Visually inside the stator it is difficult to distinguish them without special equipment, but the electrical parameters give a clear answer. The resistance of the starting winding can exceed the resistance of the working winding by 1.5–2 times, and sometimes more, depending on the model compressor. It is on this difference that the main diagnostic method using a multimeter is based.
⚠️ Attention: An attempt to start the compressor by applying voltage directly to the starting winding without a working winding will lead to instant overheating and an interturn short circuit. The starting winding is not designed for long-term operation under load.
Understanding the physics of the process helps to avoid fatal mistakes. When you measure resistance, you are actually testing the wire's integrity and gauge. High resistance means thin wire and fewer turns, which is typical for a starting circuit. Low resistance is a sign of a powerful operating circuit designed for direct current.
Necessary tools and safety measures
Before starting any work with electrical equipment, you must ensure your safety and prepare the tools. The main diagnostic device will be multimeter (tester), capable of measuring resistance in the range from 0 to 200 Ohms with high accuracy. It is advisable to use a device with an audio test to quickly check the integrity of the circuits.
In addition to the multimeter, you will need:
- 🔧 A set of screwdrivers for removing the protective casing and terminal box.
- 🧤 Dielectric gloves for protection against residual voltage.
- 🔦 A powerful flashlight, since the compressor area is often poorly lit.
- 📝 Marker or stickers for marking wires so as not to mix them up during assembly.
The first step should always be to completely disconnect the refrigerator from electrical networks. Remove the plug from the socket and make sure that the compressor is freely accessible. Remove the rear protective panel (casing) covering the motor-compressor. In some models refrigerators you may also need to remove the condensate collection tray.
☑️ Preparation for diagnostics
It is important to let the compressor cool if it has recently been running. A hot motor may have changed resistance parameters, and touching hot parts is dangerous. Also visually check the condition of the wires: the absence of melted insulation, blackening or a burning smell is a good sign.
Step-by-step instructions for finding terminals
The process of identifying terminals begins with a visual inspection of the contact group on the compressor housing. Typically there are three contacts there, forming a triangle. On modern models, they can be signed with the letters: S (Start - start), R (Run - work) and C (Common - general). However, on old or restored units, the markings may be missing or erased.
If there are no markings, proceed to measurements. Set the multimeter to resistance measurement mode (200 ohm limit). First find the general conclusion. To do this, call pairs of contacts. The contact relative to which the other two show the least total resistance is general terminal. The logic is simple: the resistance between the working and starting terminals is equal to the sum of their resistances relative to the common one.
Action algorithm:
- Touch any two contacts with the probes and record the readings.
- Move one probe to the third contact, leaving the second in place, and write down the result again.
- Compare the values obtained. The least resistance will be between the common and working terminals. The largest is between the working and starting ones.
- The pin that participates in the measurement with the least resistance (relative to the third) is the working one. The remaining one is the starting one.
What to do if the contacts are oxidized?
If the contacts are covered with oxide, clean them with fine sandpaper or a needle file until they have a metallic shine. A poor connection will give false resistance readings, leading to misdiagnosis. Do not use harsh chemicals for cleaning.
After identifying the leads, be sure to mark them with a marker or stick stickers. This will save time when connecting the new start relay. An error in connecting even one contact can lead to the compressor humming but not starting, or starting in the opposite direction (which is often fatal for piston models).
Methods for measuring resistance with a multimeter
Measurement accuracy is the key to successful repairs. When working with a multimeter, it is important not to touch the metal parts of the probes with your hands, since the resistance of the human body can introduce an error in the measurements, especially at high limits. To determine start winding we need to fix the exact resistance values between pairs of contacts.
Consider a typical measurement scenario on three contacts, conventionally named 1, 2 and 3.
Suppose we get the following data:
Between 1 and 2: 20 Ohms.
Between 2 and 3: 12 Ohms.
Between 1 and 3: 32 Ohms.
We analyze the numbers. The highest resistance (32 Ohms) is observed between contacts 1 and 3. This means that we measured the sum of the resistances of the working and starting windings. Therefore, pin 2 is general terminal (C). Now we look at pairs involving the common output (2). The resistance between 2 and 3 is 12 ohms (lower value), and between 2 and 1 is 20 ohms (higher value). The lower resistance corresponds to the working winding, the higher resistance corresponds to the starting winding. This means that 3 is the working terminal (R), and 1 is the starting terminal (S).
For clarity, let's summarize the data in a table of typical values for different types of compressors:
| Compressor type | Operating resistance windings (Ohm) | Starting winding resistance (Ohm) | Ratio Rstart/Rwork |
|---|---|---|---|
| Low power (up to 100 W) | 15–25 | 30–45 | ~1.5–1.8 |
| Medium power (150-200 W) | 10–15 | 25–35 | ~2.0–2.5 |
| Powerful (inverter/linear) | 5–10 | 15–25 | ~2.0–3.0 |
| Old models (USSR) | 20–30 | 40–60 | ~1.5–2.0 |
If the multimeter shows one (infinity) in any section, this indicates a winding break. In the case where the resistance is close to zero, an interturn short circuit is possible, although the multimeter is not always sensitive to minor short circuits of the turns. For deeper diagnostics, it is necessary to measure the current consumption with an ammeter.
Typical faults and their symptoms
Faults in compressor windings are divided into three main categories: open circuit, interturn short circuit and breakdown to the housing. Each of them has its own symptoms, which help in diagnosis even before taking measurements. Breakage of the starting winding manifests itself in the fact that the refrigerator turns on, a quiet hum is heard, but the compressor does not start. After a few seconds, the thermal relay is triggered and a shutdown click is heard.
An interturn short circuit in the starting winding leads to a sharp decrease in its resistance. The compressor may try to start, but will overheat, consume increased current and eventually burn out. A breakdown to the housing (when the resistance between the contact and the copper tube does not show infinity, but a specific value) makes the operation of the refrigerator deadly.
Often problems with starting are associated not with the winding itself, but with the starting relay. If the windings are intact, but the refrigerator does not start, check the relay. In starting relays with a posistor (thermistor), a common cause is the destruction of the ceramic element. In a relay with an inductance coil, the core may stick or the contact group may burn out.
⚠️ Attention: If, when testing, you find resistance between any contact and the compressor housing (copper tube), it is prohibited to operate such a unit! This indicates an insulation breakdown and the risk of electric shock.
It is also worth considering that in some modern inverter models refrigerators there is no classic circuit with a starting winding and a relay. There, the engine is controlled by an electronic unit, and diagnostics are performed differently. This article is focused on traditional asynchronous motors with a starting winding.
Connection diagrams and contact markings
Correct connection of the terminals to the starting relay is critically important. The relay body is often marked with a connection diagram that cannot be ignored. The contacts on the compressor can be arranged in a triangle, with the apex of the triangle usually pointing to the common wire, but this is not an axiom. Always rely on resistance measurements, not just geometry.
Standard contact markings on compressors of various brands:
- 🇪🇺 European/Asian: C (Common), R (Run), S (Start).
- 🇺🇸 American: C, S, R (often in that order).
- 🇷🇺 Soviet/Russian: P (Start), R (Operation), O (General).
The start relay is placed directly on the compressor contacts. It is important to orient the relay correctly: the arrow on the relay body (“UP” or “Top”) should point straight up. If you flip the relay, gravity will not allow the mechanism (or posistor) to work correctly, and startup will not occur. In a relay with a coil, incorrect orientation will result in the core not rising and closing the contacts of the starting winding.
When replacing a relay or thermostat, always check the connection diagram of the new element with the old one. Even if the connectors are shaped correctly, internal wiring may vary. An error in connecting the starting and working windings in places will result in the engine trying to start through the thin wire of the starting winding under full load, which will cause instant overheating.
FAQ: Frequently Asked Questions
Is it possible to start a compressor without a starting winding?
No, a standard single-phase asynchronous motor does not have a starting torque. Without a starting winding (or a capacitor that creates a phase shift), the rotor will only hum and heat up, but will not begin to rotate. Rotation can be created manually by turning the shaft, but in a hermetic compressor this cannot be done without disassembling, which will break the seal.
Why does the starting winding burn out more often than the working winding?
The starting winding is made of thinner wire and is not designed for long-term operation. If the start relay sticks and does not open the circuit after starting, current continues to flow through the thin wire, causing the insulation to quickly overheat and melt. The working winding is thicker and withstands prolonged heating better.
Is it possible to rewind a burnt winding at home?
Theoretically it is possible, but in practice, rewinding a refrigerator compressor at home is impractical. Special equipment is required for winding, drying and, most importantly, evacuation and charging with freon. It is easier and more reliable to buy a new or used compressor assembly.
What wire is used for the starting winding?
Enameled copper wire (PETV, PELSHO) is used with a smaller diameter than for the working one. For example, if the working one is wound with a 0.8 mm wire, then the starting one can be 0.5 mm or thinner. The exact diameter and number of turns depend on the specific motor model and its power.