Independently connecting a compressor from a household refrigerator is a task that many home craftsmen face when creating homemade compressors, airbrushes or vacuum systems. The main difficulty often lies in correctly identifying the three contacts coming out of the motor windings. An incorrect connection can lead to instant equipment failure or even a fire, so understanding the principle of operation of a single-phase asynchronous motor is critical.
Unlike industrial three-phase units, household ones compressors operate from a regular 220 Volt network and require a starting winding to create the initial torque. Three pins on the terminal box is a standard configuration that allows a capacitor or relay start circuit to be implemented. Before starting any work, you must make sure that the device is completely de-energized and the capacitors are discharged, since residual voltage can be dangerous.
In this article we will analyze in detail the process of marking contacts, methods for determining the starting and operating windings using a multimeter, as well as connection diagrams for various types of starting devices. You will learn how to distinguish starting winding from the working one by resistance and why the order of connecting wires to the relay terminals is crucial for the durability of the unit.
The design of a single-phase compressor and the assignment of terminals
Electric motor The household refrigerator is an asynchronous single-phase motor with a squirrel-cage rotor. To start it, it is necessary to create a rotating magnetic field, which cannot be done with only one working winding. That is why the design provides a second start winding, which is activated only at the moment the engine starts. The three terminals located on the terminal strip are usually designated S (Start), R (Run) and C (Common).
The terminal Common is common to both windings. One of the power supply wires is connected to it. The output Run is designed to connect the working winding, which ensures the main rotation of the motor shaft during operation. The third terminal, Start, is connected to the starting winding, which has a higher active resistance and a smaller wire cross-section. It is through this contact that the impulse to start the rotor is supplied.
It is important to understand that in different models of refrigerators, be it Atlant, Indesit or LG, the location of these contacts on the terminal box may differ. However, the physical principle of operation remains unchanged: the starting winding always has greater resistance than the working winding. This key knowledge allows you to determine the purpose of the contacts even in the absence of markings or if the factory designations have been erased.
Necessary tools and safety measures
Before you start connecting the engine, you need to prepare a basic set of tools and personal protective equipment. Working with 220 Volt electric current requires maximum concentration and compliance with safety precautions. Any carelessness may result in electric shock or equipment damage.
You will need multimeter to measure the winding resistance. This is the main device that will allow you to accurately determine which of the three terminals belongs to which winding. You will also need insulating tape, screwdrivers, pliers and, preferably, a current clamp to monitor the current consumption during the first start.
⚠️ Attention: It is strictly forbidden to supply power to the compressor if you are not sure of the integrity of the wire insulation or if the motor housing is damaged. Check the insulation resistance between the windings and the housing - it should be infinite.
For safe testing, it is recommended to use a connection circuit through a circuit breaker with differential protection (RCD). This device will instantly cut off power if current leaks into the chassis, which can save lives. It is also worth preparing a fire extinguisher suitable for extinguishing electrical installations under voltage.
Determination of the starting and operating windings with a multimeter
The most reliable way to identify contacts is to ring the windings with a multimeter in resistance measurement mode (Ohm). Remove the terminal box cover and make sure the wires are disconnected from the start relay. Attach the multimeter probes to any two of the three available contacts. Record the resulting values. You need to find a pair of contacts with the least resistance - this will be the sum of the resistances of the working and starting windings, if measured through a common wire, or the resistance of one of the windings.
The logic of the determination is based on the fact that the working winding (Run) always has less resistance than the starting winding (Start). Therefore, between the common contact (C) and the working winding contact (R), the resistance will be minimal. Between the common terminal (C) and the start terminal (S), the resistance will be medium. And the maximum resistance will be measured between the starting (S) and working (R) contacts, since in this case the current passes in series through both windings.
Let's consider an example. Let's say you received the following measurements between pairs of contacts:
- 🔹 Between contacts 1 and 2: 20 Ohm
- 🔹 Between contacts 2 and 3: 30 Ohm
- 🔹 Between contacts 1 and 3: 50 Ohm
Analyzing the data, we see that the maximum resistance (50 Ohms) is obtained between contacts 1 and 3. This means that contact 2 is Common (common), since it is not directly involved in this circuit as an end, but is a connection point. Now we look at the remaining pairs involving pin 2. The resistance of 20 Ohms (1-2) is less than 30 Ohms (2-3). Therefore, contact 1 is Run (working winding), and contact 3 is Start (starting).
☑️ Check before starting
Connection diagrams for starting devices
After identifying the contacts, you must correctly connect the starting device. There are two main types of starters used in residential refrigerators: the starter relay (PSR) and the solid-state starter relay (PTC). The connection diagram depends on the type of device.
In case of use induction relay (with a coil and contacts), it is important to observe the orientation. The relay body usually has an arrow indicating the "UP" direction. If you turn such a relay over, it will not work or will not work correctly. The relay contacts are connected in series with the working winding, and the starting winding is connected through normally open relay contacts.
If you use PTC relay (posistor), the circuit is even simpler. The posistor is connected in series with the starting winding. When cold, its resistance is low and current flows through the starting winding. When heated, the resistance of the posistor increases sharply, actually opening the circuit of the starting winding. It is a mistake to try to connect a posistor parallel to the windings - it should be located exactly in the open circuit of the starting wire.
| Relay type | Operation principle | Installation features | Position sensitivity |
|---|---|---|---|
| Start-protective (Induction) | Electromagnetic opening of contacts when speed increases | Requires precise vertical installation | High (strictly in the direction of the arrow) |
| PTC (posistor) | Change of resistance when heated | Easy installation, put on the contacts | Low (works in any position) |
| Capacitor | Phase shift to create torque | Requires selection of capacitor capacitance | Does not depend on position |
For compressors used outside the refrigerator (for example, for inflating tires), the circuit is often used s capacitor. The capacitor allows you to create a more powerful starting torque and improves engine performance under load. The capacitor capacity is selected experimentally, usually in the range of 30-50 µF for motors with power up to 200 W.
Nuances of working with capacitors
When using capacitors Before reconnecting or changing the circuit, be sure to discharge the capacitor by shorting its terminals through a resistor. Using capacitors with an operating voltage below 350-400V will lead to their explosion.
Step-by-step instructions for assembling the circuit
Assembling the connection circuit requires sequential operations. First, prepare pieces of copper wire with a cross-section of at least 1.5 mm². Strip the ends of the wires and tin them with solder for better contact. Connect the common wire (C) to one of the terminals of the power plug or to the input circuit breaker.
Next, connect the wire from the contact Run to the corresponding contact on the starting relay or capacitor. Connect the wire from the contact Start to the second terminal of the starting device. Make sure all connections are tight and insulated. Use cambrics or heat shrink tubing to insulate solder joints.
After assembling the electrical part, secure the compressor on a flat surface. The vibration of a running engine can be significant, so the use of rubber dampers or a wooden base is mandatory. Connect the hoses for air intake and injection if the compressor is used for pneumatics.
⚠️ Attention: When turning on for the first time, keep your finger on the emergency shutdown button or plug. If the motor hums but does not start within 2-3 seconds, or if the current is 1.5 times the rated current, turn off the power immediately. This may indicate a jammed rotor or an error in the connection diagram.
For ease of control, add a switch button and a pressure gauge to the circuit to control pressure. A pressure gauge will help you evaluate the performance of the compressor and notice in time a drop in power, which may indicate wear of the piston group.
Typical errors and troubleshooting
One of the most common mistakes is confusion between the starting and operating windings. If you mix up the contacts Start and Run, the engine may start, but will operate in the wrong mode, heat up and quickly burn out. The starting winding is not designed to operate for a long time under current; it should only turn on for a split second.
Another common problem is the use of an inappropriate starting relay. The relay is selected for a specific engine power and coolant (although for operation in air this is less critical). If the relay “clicks” constantly and the engine does not start, the contacts may be stuck or the coil has burned out.
Also, mechanics often forget about grounding. The compressor housing must be reliably grounded. If the insulation breaks down, the housing may be exposed to dangerous potential. Lack of grounding turns the compressor into a source of mortal danger, especially in damp rooms.
Noise and vibration can be a sign of not only improper fastening, but also problems with the valves. If the compressor operates with a metallic knock, the cylinder group mounting bolts may have become loose or the valve plates have collapsed. In this case, disassembly and troubleshooting of the mechanical part is required.
Frequently asked questions (FAQ)
Is it possible to start a compressor without a starting relay?
Theoretically, it is possible if you manually briefly apply voltage to the starting winding at the moment of switching on, but this is inconvenient and unsafe. For constant operation, the use of a starting device (relay or capacitor) is mandatory, otherwise the engine will not pick up speed and will burn out.
Which wire is thicker: from the starting or working winding?
The wire of the working winding (Run) is always thicker, since it works constantly and the main current flows through it. The starting winding wire (Start) is thinner, since it turns on only for a short time.
Why does the compressor hum but not spin?
This may be caused by jamming of the piston group due to lack of oil or moisture, or a malfunction of the starting relay, which does not supplies an impulse to the starting winding. An open circuit in the starting winding is also possible.
Do you need to change the oil when using an air compressor?
If you use a refrigerator compressor to pump air, oil mist will enter the pneumatic system. For airbrushing or an aquarium this is critical, so you will need to install an oil separator or use an oil-free compressor. Changing the oil inside the crankcase is not always effective, since the design is designed to work in a closed circuit with freon.
What resistance should be between the windings?
The exact values depend on the engine power, but usually the resistance of the working winding is 10-20 Ohms, and the starting winding is 20-40 Ohms. The main rule is: R(start) > R(running). The resistance between the case and any contact must be infinite.