When independently repairing household appliances, technicians are often faced with the need to replace the start-protective relay or diagnose the engine itself. At this point, a logical question arises: how to identify the contacts, where exactly the starting winding is connected, and how it differs from the working winding. An error in the connection can lead to instantaneous burnout of the windings or jamming of the motor shaft, so understanding the principle of operation and the location of the terminals is a critical skill.
Compressors installed in domestic refrigerators are most often sealed asynchronous units. Inside the black metal “barrel” there is an electric motor, the stator of which has two main windings. One of them, working, provides the main rotation of the shaft, and the second, starting, is necessary only to create the initial torque and start the rotor. Without a clear understanding of where the starting winding is located in the compressor, further diagnostics are impossible.
In this article we will analyze in detail the electrical circuit of a single-phase motor, methods for determining the conclusions using a multimeter and visual signs that allow you not to confuse the contacts. You will learn how winding resistance helps identify their purpose and why wiring order directly affects the life of the refrigeration unit. The information will be useful for both novice craftsmen and experienced specialists who want to refresh their theoretical knowledge.
Principal structure of a single-phase motor
In order to figure out exactly where the starting winding is located, you must first understand the basic design of the electric motor hidden inside the sealed compressor casing. A single-phase asynchronous motor is not able to start on its own, since the magnetic field of one winding creates only ripples and not rotation. That is why an additional coil is placed in the stator, offset relative to the main one by a certain angle.
Working winding (main) occupies most of the stator slots and has less resistance. It is connected directly to the network and operates continuously while the compressor is turned on. Starting winding (auxiliary) has a higher resistance, since it is wound with a thinner wire and fewer turns, or has a special switching circuit. Its task is to create the phase shift necessary to start the rotor, after which it must be turned off.
In modern refrigerators, the starting winding is turned off automatically thanks to the starting relay. At the moment of switching on, voltage is applied to both windings, the rotor begins to accelerate, and the current in the working winding increases. When the current reaches a certain value, the relay opens the starting winding circuit, and the engine continues to operate only on the main one. If the leads are mixed up, the engine may hum but not start, or the relay will constantly click.
It is important to note that in some modern models of compressors with inverter control or capacitor start, the circuit may differ, but the classic division into the main and starting coils is preserved in most household units. Understanding the physics of the process helps to avoid fatal errors in diagnostics.
Location of contacts on the terminal block
The external interface for connecting electricity is the terminal block located on the compressor housing under the start-protection relay. It is here that there are three main terminals that lead inside the sealed casing to the stator windings. To determine where the starting winding is in the compressor, it is necessary to correctly identify these three contacts.
Usually the terminals are marked with letters or numbers, but over time the markings can wear off, oxidize, or be initially incomprehensible to a beginner. The standard marking is as follows:
- 🔵 C (Common) — the common wire to which one of the network wires is connected.
- 🔴 R (Run) —the output of the working winding.
- 🟡 S (Start) —the output of the starting winding.
In some cases, especially on old Soviet or European compressors, you can find a digital marking: 1, 2, 3 or 1, 2, 4. The arrangement of the contacts on the block also varies: they can be in a row, in a triangle, or in two rows. The geometry of the location does not carry a functional load; you need to rely only on resistance measurements.
The terminals pass through special insulators soldered into the housing and are hermetically sealed. Violation of the tightness around the terminals due to careless dismantling of the relay can lead to depressurization of the entire unit, so any manipulations require caution. Before starting work, make sure that the compressor is disconnected from the network and the capacitors are discharged.
Method for determining windings with a multimeter
The most reliable way to find where the starting winding is located is to measure the active resistance between the terminals using a multimeter. This method is universal and works even if there is no marking. To carry out the procedure, switch the device to the resistance measurement mode (Ohm) to a limit of 2 kOhm.
The measurement process consists of testing all three pairs of contacts. You need to measure the resistance between the first and second terminals, then between the second and third, and finally between the first and third. The obtained values will differ, and it is this difference that will indicate the purpose of each contact.
The measurement logic is based on the following rule: the resistance of the starting winding is always higher than the resistance of the working one. The total resistance between the two extreme terminals (common and one of the phase terminals) will be equal to the sum of the resistances of the working and starting windings. Thus, you will get three values: the smallest, the average and the largest.
The action algorithm looks like this:
- ⚡ Find a pair of contacts with the highest resistance. These will be the two extreme terminals (Working and Starting), and the third, left aside, is Common (C).
- 📉 Measure the resistance between Common and one of the extreme ones. If the value is less, it is Working winding (R).
- 📈 Measure the resistance between Common and the other extreme. If the value is greater, it is Start winding (S).
For example, if there is a resistance of 30 Ohms between contacts A and B, 15 Ohms between B and C, and 45 Ohms between A and C, then contact B is common. The pair A-C gives the sum, so A and C are the ends of the windings. Since the resistance of B-C (15 ohms) is less than B-A (30 ohms), then C is the operating winding and A is the starting winding. Thus, the starting winding always has greater resistance relative to the common wire.
Table of typical resistance values
Although the exact resistance values depend on the power and model of the compressor, there are approximate ranges that are typical for household appliances. Understanding these orders of magnitude helps you quickly navigate the instrument readings and weed out clearly faulty units where the winding could burn out or go into a short circuit.
Below is a table with approximate data for various types of compressors. Remember that actual values may vary depending on the engine temperature and the specific manufacturer.
| Compressor type | Working winding resistance (Ohm) | Starting winding resistance (Ohm) | Total resistance (Ohm) |
|---|---|---|---|
| Low power (up to 100 W) | 25 - 40 | 50 - 80 | 75 - 120 |
| Medium power (150-200 W) | 15 - 25 | 30 - 50 | 45 - 75 |
| Powerful (250+ W) | 8 - 15 | 20 - 35 | 28 - 50 |
| Fault (break) | ∞ (infinity) | ∞ (infinity) | ∞ (infinity) |
If the device shows a unit or an infinity sign in any section, this indicates an open circuit. In this case, repairing the compressor at home is impossible; the unit must be replaced. If the resistance is close to zero, this indicates an interturn short circuit, which is also a fatal malfunction.
It is worth considering that the resistance of copper depends on temperature. On a hot compressor the values will be higher than on a cold one. For accurate diagnostics, allow the unit to cool completely before taking measurements.
Connection diagram and role of the starting relay
After you have determined where the starting winding is in the compressor, you need to correctly connect the starting protection relay. This small unit is responsible for supplying voltage to the starting winding only at the moment of start and then turning it off. Incorrect assembly of the switching unit is a common cause of repeated breakdowns.
The relay is placed directly on the compressor terminals. It is important to observe the orientation: the relay has a top and a bottom, often indicated by a marking UP or an arrow. Inside the relay there is a movable core and contacts. When voltage is applied, current flows through the relay coil and the operating winding of the motor. The starting winding at this moment is powered through the normally closed contacts of the relay.
When the engine picks up speed, the current in the working winding drops (or increases, depending on the type of relay - starting or thermal), the magnetic field changes, and the contacts open, turning off the starting winding. If you mix up terminals C, R and S, the magnetic field will not form correctly, the shaft will not turn, and the relay will click endlessly, trying to start the motor.
⚠️ Attention: When installing the relay, make sure that it sits tightly on the terminals and is secured with a bracket. The vibration of a running compressor can weaken the contact, which will lead to burning of the terminals and melting of the plastic.
Some circuits also contain a capacitor that can be connected in parallel with the starting winding to increase the starting torque. The presence of a capacitor is typical for compressors operating at elevated ambient temperatures or for high-power units.
Diagnosing starting circuit faults
Understanding where the starting winding is located allows you to effectively diagnose a number of common problems. If the refrigerator hums but does not start, or starts for a few seconds and turns off, the problem often lies in the start circuit.
One of the common problems is sticking contacts of the start relay. In this case, the starting winding remains connected to the network constantly, although it should be turned off. This leads to rapid overheating of the engine and tripping of the thermal protection. The compressor in this mode operates with a characteristic hum and overheats.
Another option is an open circuit in the starting winding circuit. The engine will hum, trying to start on one working winding, but there will not be enough torque to take off. In this case, the multimeter will show infinite resistance in the section responsible for starting.
- 🔊 Humming without starting: Check the relay and the presence of voltage on the starting winding at the time of start.
- 🔥 Overheating: Check whether the starting winding turns off after the engine accelerates.
- ⚡ The machine knocks out: There may be a short circuit between the windings or to the housing.
It is also worth checking the starting capacitor, if it is provided for in the design. Swelling, electrolyte leaks or changes in capacity indicate its malfunction. Replacing the capacitor often solves the problem of difficult starting without replacing an expensive relay.
☑️ Diagnostics before replacing the relay
Safety measures when working with electrical equipment
Working with a refrigerator compressor carries a risk of electric shock as the voltage used is 220 volts. In addition, there is a risk of mechanical damage and depressurization of the system. Compliance with safety regulations is a must.
First of all, always disconnect the refrigerator from the power supply before starting any work. Don't just rely on the switch on the case - unplug the cord from the outlet. Capacitors in a circuit may retain a charge for some time, so be careful when touching metal parts.
When working with a sealed enclosure, remember that there is pressurized refrigerant inside. It is strictly forbidden to open the casing, saw it or punch holes. If you damage the tubes or the housing itself, the refrigerant can escape abruptly, causing frostbite or displacing oxygen in the room.
⚠️ Attention: Never turn on the compressor without the start relay connected or with a faulty protection. This may lead to a fire or explosion of the windings.
Use only a serviceable tool with intact insulation. If you are not confident in your knowledge of electrical engineering, it is better to entrust the repair to professionals. Electricity does not forgive mistakes, especially in combination with the metal body of household appliances.
Is it possible to start a compressor without a relay?
Theoretically, you can start a compressor without a relay by briefly applying voltage to the starting winding manually (via a button), but this is extremely dangerous and is not recommended for long-term operation. Without a relay, the engine may burn out when restarted under load.
Frequent errors when replacing and connecting
Even knowing where the starting winding is in the compressor, technicians often make mistakes that nullify all efforts. One of the most common is ignoring the relay type. Starting relays come in different types (RT, RTP, RTK and modern posistor), and they are not always interchangeable.
An attempt to install a relay that is not suitable for the operating current or type of contacts will either result in the compressor not starting, or in the fact that the relay will burn out along with the windings. Always select an analogue strictly according to the markings or technical characteristics.
Another mistake is poor contact. The terminals must fit tightly. If the contact is weakened, sparking and heating occurs at the junction, which leads to oxidation and an increase in resistance. As a result, the compressor stops starting, although the engine itself is working properly.
Diagnostics “by ear” is also an error. An experienced master uses instruments rather than relying on subjective sensations. A humming noise can be a sign of either a problem with the relay or a mechanical shaft jam or water hammer.
⚠️ Attention: Compressor specifications and relay types may vary depending on the year of manufacture and manufacturer. Always check the labeling of parts with documentation or compatibility catalogs.
Attention to detail and adherence to the sequence of actions is the key to successful repairs. Take your time, check every step and use a quality tool.
What to do if the resistance of the windings is the same?
In a working single-phase motor, the resistance The starting and operating windings cannot differ, since they are wound with different wires. If you see the same values (for example, 20 ohms and 20 ohms), this is almost guaranteed to indicate a turn-to-turn short in one of the windings, which has changed its resistance. Such a compressor must be replaced.
Is it possible to swap the starting and operating windings?
No, they cannot be swapped. The starting winding is designed for short-term operation and has other parameters. If you apply operating voltage to it constantly, it will burn out in a few seconds or minutes. The engine must operate on the working winding.
Why does the compressor hum but does not start?
There may be several reasons: jamming of the mechanical part of the compressor, a malfunction of the starting relay (does not supply voltage to the starting winding), an open circuit in the starting winding circuit or low voltage in the network. Diagnostics should begin by checking the relay and the presence of voltage at the terminals.
How long can the starting winding work?
The starting winding only works when the engine starts, usually from 1 to 3 seconds. Once the rotor reaches about 75% of rated speed, the relay should turn it off. Prolonged operation of the starting winding leads to its overheating and destruction of the insulation.