Using old refrigeration equipment to create homemade compressors is a popular solution for garage workshops, tire inflation or painting work. However, before proceeding with assembly, it is necessary to clearly understand the electrical circuit of the unit. Refrigeration compressor is a complex electromechanical device that is not directly started, like a simple fan. It requires special starting equipment, without which the motor rotor will remain motionless, and the windings can burn out in a matter of seconds.
In this article we will analyze in detail the process of connecting the motor to a household network 220 volts. You will learn how to identify winding terminals, select the necessary capacitors and assemble a safe circuit. Important to understandthat incorrect assembly carries the risk of electric shock, so all work must be carried out in compliance with safety precautions and with the power turned off.
Modern models are often equipped with inverter motors, which require a frequency converter for operation. Classic piston units from the USSR or later series (for example, Atlant, Don, Biryusa) are launched through a starting relay. It is on them that we will focus our attention, since they are most suitable for conversion into compressor units.
Preparation of tools and testing engine performance
Before looking for a connection diagram, you need to make sure that the engine itself is working and does not require an oil change or repair. Visually inspect the housing for severe dents or oil marks, which may indicate depressurization. If the compressor was removed from a non-working refrigerator, there is a high probability that the problem was in it, and resuscitating it will be more difficult than buying a new one.
For initial diagnostics, you will need a multimeter. Switch the device to resistance measurement mode (Ohms) and check the integrity of the windings. The probes are applied to the contacts on the block hidden under the protective cover. If the resistance between any contact and the housing shows one (infinity), then the winding is intact. If the multimeter shows zero or a value close to it, a short circuit is possible.
It is also necessary to check whether the device is “jammed”. When turned on, a working motor should emit a characteristic hum and vibration. If the rotor does not turn, wedge it by applying voltage, but this must be done carefully. Often, a short-term supply of current through a starting device, which will be discussed below, helps. If this does not help, the unit most likely must be disposed of.
⚠️ Attention: Never start the compressor without a hose or receiver attached to the outlet pipe, if there is no filter at the outlet. Dust can get inside the motor, which will lead to rapid wear of the cylinders and piston group.
Identification of contacts and winding patterns
The most critical point in the process is the correct identification of the terminals. The terminal block usually has three contacts (sometimes four if there is a ground connection). They are responsible for starting winding, working winding and common wire. An error in the definition will lead to the engine humming, heating up, but not starting, or the starting relay will burn out.
Use a multimeter to dialing pairs of contacts. You need to find three resistance values. The greatest resistance will be between the starting and working terminals. The lowest resistance will indicate the working winding, and the average value will indicate the starting winding. For example, if there is a resistance of 30 ohms between contacts A and B, 10 ohms between B and C, and 20 ohms between A and C, then contact B is common.
On many modern engines, the markings are already applied to the relay body or cover. Look for the designations: C (Common) - general, R (Run) - working, S (Start) - starting. In Soviet models (HC, SK), there may be no markings, and then you will have to rely solely on resistance measurements. In some cases, a visual inspection helps: the starting winding wire is often thinner than the working winding wire.
Below is a table for quickly checking resistance compliance:
| Pair of contacts | Expected resistance | Connection type |
|---|---|---|
| Working - General | Low (10-20 Ohm) | Working winding |
| Starting - General | Average (20-40 Ohm) | Start winding |
| Working - Start | High (sum of the previous two) | Serial connection |
Once you have identified the pins, it is recommended to mark them with a marker or electrical tape so as not to be confused during the installation process. This will save time and nerves when assembling the final circuit.
Selection and connection of the starting relay
The heart of the starting system is the starting relay. It is this that supplies voltage to the starting winding for a split second, creating torque, and then turns it off, leaving only the working winding to work. There are two main types of relays: posistor (solid-state) and induction (electromagnetic). For older refrigerators, induction models with thermal contact were most often used.
When selecting a relay, it is important to take into account the engine power and the type of refrigerant for which it was designed, although for operation in air this is not as critical as for the refrigeration cycle. The main thing is matching the starting current. If you use the original relay from the refrigerator, there should be no problems. If the relay is lost, you will have to select an analogue based on the cut-off current.
The connection is made strictly according to the diagram printed on the relay body or in the documentation for the model. Typically the relay is placed directly on the compressor terminals. It is important to orient the relay correctly: it has an "UP" or "TOP" arrow that should face up. If you turn the relay over, gravity will not allow the contacts to open correctly, and the engine will burn out.
What is the difference between posistor and induction relays?
Posistor relays use a thermistor, which is heated by the current and changes resistance, breaking the starting winding circuit. They are more reliable, have no moving parts, but require time to cool down (3-5 minutes) before restarting. Induction relays operate via an electromagnetic coil and open when the current drops after the engine starts. They operate instantly, but have mechanical wear of the contacts.
To check the operation of the relay without installing it on the compressor, you can assemble a temporary circuit. However, remember that a relay with a thermal contact (pocket) requires time to cool down. If, after trying to start, the engine does not turn on, wait a few minutes until you hear a quiet click of the posistor cooling down.
Using capacitors for starting
In some circuits, especially when using powerful motors or when operating in conditions of reduced mains voltage, one relay may not be enough. This is where starting capacitorscome to the rescue. They create the phase shift necessary to create a rotating magnetic field in a single-phase network.
The capacitance of the capacitor is selected experimentally or according to a formula depending on the engine power. Compressors from household refrigerators usually require capacitors with a capacity of 2 to 5 μF with an operating voltage of at least 400-450 volts. Using capacitors with a lower voltage will lead to their explosion.
The capacitor is connected in parallel with the starting winding. Important: the capacitor only works during startup. If left in the circuit constantly, the motor will overheat and draw excess current. Therefore, the circuit must contain a device that turns off the capacitor after a set of revolutions (often this function is taken over by the starting relay itself or a separate time relay).
When working with capacitors, be especially careful. Even after unplugging, they can retain their charge. Before touching the leads, be sure to discharge them through a resistor or incandescent lamp.
Assembling the electrical circuit and safety measures
The final assembly of the circuit requires care and compliance with electrical safety rules. All connections must be made using terminal blocks or high-quality soldering. Twists in this case are unacceptable, since the vibration of the compressor can weaken the contact, which will lead to sparking and heating.
Be sure to use a circuit breaker or fuse in the power circuit. The tripping current must be adjusted with a margin (usually 10-16 Amps for household models) to withstand inrush currents, but protect the wiring in the event of a short circuit. Grounding the compressor housing is a mandatory requirement, since the metal tank is a potential source of danger in case of insulation breakdown.
☑️ Check before the first start
For ease of control, add a "Start" button to the circuit (normal open) and the "Stop" button (normally closed), if you are not using a ready-made control unit. This will allow you to control the startup and emergency stop of the device.
⚠️ Attention: The temperature of the housing of a working compressor can reach 90-100 degrees Celsius. Do not touch metal parts during operation and ensure good ventilation in the installation area.
Common mistakes and troubleshooting
Even after carefully studying the instructions, beginners often make mistakes. The most common of them is mixed-up winding leads. If the engine hums, but the shaft does not spin, and after a few seconds the machine knocks out, immediately turn off the power. Most likely, you applied voltage to the wrong winding or the relay does not turn off the starting circuit.
Another problem is insufficient voltage in the network. If the voltage drops below 190 volts, the start relay may not operate or the engine may not be able to produce the required power. In such cases, installing a stabilizer or using a more powerful starting capacitor (with caution) helps.
Noise and knocking during operation may indicate wear of internal mechanisms or improper installation. The compressor must stand strictly horizontally on vibration-isolating feet. Also check if the copper tubes are touching each other or the housing, creating a resonating ringing.
If the engine starts, but quickly heats up and turns off by the thermal relay, check the output pressure. The filter or outlet valve may be clogged and the motor is operating in overload mode. Also, the cause may be an interturn short circuit in the windings, which requires replacing the motor.
Is it possible to start a compressor without a relay, directly?
Theoretically, it is possible if you briefly apply voltage to the starting winding manually (via a button), and then turn it off, leaving it working. However, this method is extremely dangerous and is not recommended. The risk of getting an electric shock or burning the engine due to untimely disconnection of the starting winding is very high. Use at least the simplest homemade relay or a latching button.
Which compressor is better: from a Soviet refrigerator or a modern one?
For homemade compressors, old models are often preferred (for example, HC or SK). They have a cast iron body, are more repairable and often have a longer service life. Modern compressors (especially inverter ones) are more difficult to control, require frequency converters and often have aluminum housings, which are less susceptible to repair. However, modern models are quieter and more economical.
Why does the compressor run jerkily?
Jergs can be caused by a malfunction of the start relay (burnt contacts), low voltage in the network or mechanical jamming piston group. Also check to see if the air inlet filter is clogged. If the compressor is old, the oil may have run out or carbon deposits have formed on the valves.
Is it necessary to change the oil in the compressor during refurbishment?
If the compressor is removed from the working refrigerator and the oil is clean, it is not necessary to change it. However, if you plan to use a compressor for painting, regular refrigeration oil may produce a misty output. In this case, it is better to replace it with synthetic compressor oil that does not form soot, but this requires complete disassembly and flushing of the system.