Using an old compressor from a failed refrigerator is a great way to get a powerful and relatively inexpensive air pump for your garage needs. Many craftsmen use them to assemble compressors for paint booths, pneumatic tools, or even airbrushes. However, before the device starts working, it must be correctly connect the electrical circuit, since it cannot be plugged directly into the socket.
The main difficulty is the presence of a starting and protective relay, without which the asynchronous motor will not be able to start or will burn out in a few seconds. In this article we will analyze the physical principle of operation, methods for determining contacts and the order of assembling the working circuit. You will need minimal skills in working with electricity and a basic set of tools.
It should be noted right away that safety comes first here. Working with mains voltage 220 Volts requires extreme caution and compliance with all electrical safety standards. Any error when switching wires can lead to a short circuit or electric shock.
Design and principle of operation of the compressor
The electric motor installed inside the sealed casing of the refrigerator is an asynchronous single-phase unit. It consists of a stator with two windings and a rotor. To start rotor rotation, a phase-shifted second winding is required, which is called starting. The main winding is responsible for the further operation of the engine.
Unlike industrial three-phase motors, household compressors cannot start on their own. They require a short-term impulse to the starting winding, which creates the necessary torque. This process is controlled by a special starting relaywhich automatically turns off the starting winding after the rotor accelerates.
There is also a thermal protection relay in the circuit. It opens the contacts if the current in the windings exceeds the permissible values, for example, when the piston group is jammed or the network is overloaded. Without this element, the risk of fire or failure of the windings in the event of a compressor malfunction is extremely high.
- 🔧 Main winding —works continuously during shaft rotation.
- 🔧 Starting winding —turns on only at the time of start (1-3 seconds).
- 🔧 Starting relay —switches the current to the starting winding.
- 🔧 Thermal relay —protects the motor from overheating and overloads.
⚠️ Attention: There is freon and oil inside the compressor casing. It is strictly forbidden to open the sealed housing or saw through the tubes under pressure without special equipment. We consider only the electrical part of the external terminals.
Identification of contacts and marking of terminals
The first step before connection is to identify the terminals on the motor terminal block. Typically, the compressor has three terminals arranged in a triangle, and sometimes a fourth that goes to chassis ground. Modern models are often marked, but on older units it may be erased or missing.
The standard marking is as follows: the letter C (Common) indicates the common wire, R (Run) - the working winding, and S (Start) - launcher. In Soviet and Russian models, the designations O (general), P (working) and P (starter) may be used. If there is no marking, you will have to use a multimeter.
The dialing process takes several minutes. It is necessary to switch the multimeter to resistance measurement mode (Ohms) and measure the resistance between each pair of contacts one by one. The greatest resistance will be between the ends of the starting and working windings (total resistance). The resistance between the common wire and the working winding will be average, and between the common and the starting winding will be the smallest.
Below is a table that helps identify the conclusions based on the measurement results:
| Pair of contacts | Resistance (Ohm) | Description |
|---|---|---|
| Pin 1 - Pin 2 | High (sum) | Working + Starting windings |
| Pin 1 - Pin 3 | Average | Common - Working winding |
| Pin 2 - Pin 3 | Low | Common - Start winding |
| Terminal - Housing | Infinity | Checking for insulation breakdown |
If the multimeter shows infinity between all terminals, then the winding is burned out and the motor must be disposed of. In the case when the resistance is close to zero, an interturn short circuit has occurred.
Connection diagram via a standard relay
The most reliable and correct way to start a compressor is to use its standard starting relay. It is selected by the manufacturer specifically for the characteristics of this engine. The relay is usually mounted directly on the side or rear wall of the compressor and fits over the contacts.
To connect, you will need an electrical cord with a plug. One end of the cord (phase) is connected to one of the contacts of the thermal relay, and the second (zero) is connected to a free contact on the engine block, which is not directly involved in the starting circuit, or through the relay body, if the design allows. The exact circuit depends on the type of relay: posistor or induction.
In posistor relays, which are most often found in modern models, the starting current passes through a thermistor. It heats up, its resistance increases, and current stops flowing to the starting winding. In induction relays, opening occurs mechanically due to a drop in current in the main winding when the rotor accelerates.
☑️ Check before starting
It is important to orient the relay correctly. There is always an arrow on its body indicating the direction "Top". If you install the relay upside down, the mechanism will not work and the starting winding will burn out. After installing the relay, a protective cover is put on.
Starting without a relay: using a capacitor
Sometimes a standard relay fails, and an analogue cannot be found. In such cases, you can force start the engine using a button or a capacitor. This method is considered temporary or experimental, since it deprives the engine of automatic protection when restarted.
To implement a circuit with a capacitor, it is necessary to apply voltage to the working winding constantly, and to the starting winding briefly through a button. The capacitor is connected in series with the starting winding. The capacitance of the capacitor is selected experimentally, usually in the range from 4 to 10 µF for refrigeration compressors.
The starting process looks like this: you apply power to the working winding, then press the button to supply voltage to the starting winding (through the capacitor). The engine hums and begins to rotate. As soon as it picks up speed (in 1-2 seconds), the button must be released. If you hold the button longer, the winding will overheat.
⚠️ Attention: When using a push-button circuit, you take on the protection function yourself. Forget to release the button in time - the engine will fail. Use this method only if installing a relay is not possible.
Calculating capacitor capacitance
Accurate calculation requires knowledge of the motor current, but empirically for refrigerator compressors the rule is: 1 µF for every 100 W of power. However, starting capacitors must be designed for a voltage of at least 350-400 Volts, otherwise they will explode.
There is also a scheme with a constantly switched on capacitor, but it reduces the starting torque and engine efficiency, so it is not recommended for compressors that need to overcome pressure in the system.
Organization of the cooling and safety system
After the electrical part has been assembled, the issue of cooling needs to be resolved. In the refrigerator, the motor is washed with freon vapor and oil inside the casing. When operating on “air” (as a compressor), it will get very hot, since there is no standard cooling.
For long-term operation, it is necessary to organize forced airflow. You can use a computer fan or an industrial cooler, directing the air flow directly to the compressor housing. Overheating of the oil inside will lead to its coking and jamming of the piston group.
It is also worth taking care of filtering the intake air. An air filter should be installed on the inlet pipe (the shortest tube), for example, from a car or homemade from a carburetor. This will prevent dust and abrasives from getting inside the cylinders, which will quickly damage the unit.
Do not forget to ground the housing. The metal casing must be connected to the grounding circuit of your wiring to prevent electric shock in the event of a breakdown of the winding insulation on the housing.
Typical errors and troubleshooting
When assembling a circuit, beginners often make mistakes that lead to the motor humming without rotation or instantly burning fuses. One of the most common mistakes is mixing up the starting and operating winding wires. If you constantly apply 220V to the starting winding, it will burn out in seconds.
Another common problem is using a capacitor that is too weak or lack of load at the start. It is difficult for the compressor to take off if there is already pressure in the receiver. In such cases, installing a check valve and a more powerful starting capacitor helps.
- 🔥 The engine hums, but does not spin —check the starting relay or capacitance of the capacitor.
- 🔥 The machine knocks out —there may be a short circuit in the windings or a breakdown in the housing.
- 🔥 Strong vibration —check the fastening of the compressor to the frame, the rubber dampers may be worn out.
If the compressor operates jerkily and constantly clicks the relay, it means either there is not enough voltage in the network, or the starter is faulty chain In this case, it is necessary to check the voltage with a multimeter and the integrity of the contacts.
⚠️ Attention: Technical characteristics of compressors may vary depending on the manufacturer and year of manufacture. Always check the markings on the nameplate of a specific device before assembling the circuit.
Frequently asked questions (FAQ)
Is it possible to use a compressor from a refrigerator to inflate tires?
Theoretically it is possible, but it is ineffective. Refrigeration compressors are designed to create a vacuum and circulate gas, and not to quickly pump large volumes of air under high pressure. Their performance is low, and the process will take a very long time.
What capacitor is needed to start without a relay?
Usually they use starting capacitors (CBB60 or analogues) with a capacity of 4 to 10 uF and a voltage of at least 400 Volts. The exact value is selected experimentally: the engine should start confidently without a strong hum.
Why does the compressor get very hot during operation?
In the refrigerator, cooling occurs due to the sucked freon vapors. This effect is absent in air. Without forced blowing by a fan, the case will heat up to 80-90 degrees, which is dangerous for the oil and insulation.
Is it necessary to change the oil in the compressor during refurbishment?
There is no need to change the oil if the compressor was working properly. However, if you plan to use it for painting (where the absence of oil mist is important), then you need to install a powerful oil separator at the outlet, since the compressor will throw out part of the oil along with the air.
How long can such a compressor work?
With proper organization of cooling and air filtration, as well as compliance with the operating mode (no more than 20-30 minutes continuously with interruptions), the compressor can last for years. The service life depends on the quality of assembly and operating conditions.