Usage old refrigeration equipment as a source of compressed air - a popular solution for the home workshop. Many craftsmen are looking for a way to connect a compressor from a 220-volt refrigerator in order to assemble a quiet and efficient receiver for painting, inflating tires or pneumatic tools. However, the standard operation scheme of the unit inside the refrigerator body differs from the requirements for autonomous operation.
The main difficulty lies in organizing the correct start of the electric motor and protecting it from overloads. In normal mode, the refrigerator operates cyclically, relying on thermostat signals, while the workshop compressor must start at the user's command and work until the desired pressure is reached. Incorrect connection threatens winding failure or even fire.
In this article we will analyze in detail the electrical circuit, methods for determining winding contacts and the nuances of selecting starting fittings. You will learn how to safely supply voltage 220 volts to the motor-compressor, bypassing the standard automatics of the refrigerator, and what precautions must be taken when assembling the system.
Design features and principle of operation
Before proceeding with the electrical part, it is necessary understand what type of engine you are dealing with. Domestic refrigerators use single-phase asynchronous motors with a squirrel-cage rotor. A feature of their design is the presence of two stator windings: working (main) and starting (auxiliary). To start the rotor, it is necessary to create a rotating magnetic field, which is impossible when voltage is applied only to the working winding.
Unlike industrial three-phase motors, the phase shift, which is created artificially, is critical here. In the classic refrigerator circuit, starting relayis responsible for this, which briefly connects the starting winding at the moment of start, and then opens the circuit. If this element does not work or is selected incorrectly, the engine will hum but will not start, which will quickly lead to overheating and burning of the insulation.
It is also worth noting the cooling system. In normal mode, the compressor is washed with refrigerant and freon, which helps remove heat. When converting to an air compressor, the load on the unit often increases, and cooling becomes less efficient. Therefore, understanding the internal processes is necessary for proper operation.
Why can’t you just plug it in?
Inside the compressor there is a complex system of valves and pistons that operates in an oil environment. A direct connection without a relay and protection will lead to the fact that the engine will not be able to turn the piston from rest (high starting torque) and will burn out in a matter of seconds.
Determination of contacts and continuity of windings
The very first stage of work is identifying the pins on the compressor terminal block. Usually there are three of them, sometimes four (if there is a case grounding). On older models, the markings may be erased, so you cannot rely on the color of the wires. You will need a multimeter set to resistance measurement mode (200 Ohm or 2 kOhm).
The dialing process requires sequential measurement of the resistance between each pair of contacts. The resistance between the working and starting terminals will be equal to the sum of the resistances of each windings relative to the common wire. The winding with the lower resistance is the working one, and the one with the higher resistance is the starting one. The common wire always has the least resistance relative to the other terminals.
Below is a table of typical resistance values for various capacities of motor-compressors. Please note that actual values may differ depending on the temperature of the windings and the specific model Atlet, Secop or Danfoss.
| Winding type | Resistance (Ohm) | Wire cross-section | Function |
|---|---|---|---|
| Working (Main) | 10 - 25 Ohm | Thicker | Main engine operation |
| Starting | 20 - 40 Ohm | Thinner | Starting the rotor (short-term) |
| Total (Sum) | 30 - 65 Ohm | - | Checking circuit integrity |
| To the body | Infinity | - | Checking for insulation breakdown |
If the multimeter shows a resistance close to zero between any terminals, it means that an interturn short circuit has occurred. If the device shows infinity (“break”) between all contacts, the engine has burned out. In both cases, resuscitation is impossible or economically impractical.
Selection and connection of the starting relay
The key element of the circuit is the starting relay. It is this that provides the phase difference for starting and turns off the starting winding after a set of revolutions. There are two main types of relays that you can find: posistor. (solid-state) and induction (electromagnetic). For self-assembly, new posistor relays are most often used, since they do not have moving mechanical parts and are less sensitive to position.
The connection of the relay is carried out strictly according to the diagram printed on its body or in the documentation. An error in the polarity of the connection for induction relays can lead to the fact that the posistor will not start in a cold state. low resistance and passes current through the starting winding. When heated by current, its resistance increases sharply, actually opening the circuit of the starting winding.
- 🔌 Posistor relay: requires cooling time (1-2 minutes) before restarting. If you start the compressor immediately after turning it off, it may not start.
- ⚙️ Induction relay: reacts to the current in the working winding. When the current drops (starting mode), the contacts open. Sensitive to vibrations and position.
- 🛡️ Thermal protection relay: it often goes in tandem with the starting one. It breaks the power circuit when there is an overcurrent. combustion.
During installation, it is important to ensure reliable contact. A poor connection in the relay terminals will lead to sparking, heating and melting of the plastic. Use original blocks or high-quality terminal blocks designed for a current of at least 10 Amps.
Connection diagram for a 220 Volt network
Having assembled all the components, we proceed to the final assembly of the electrical circuit. from the refrigerator to the 220V network looks sequential: network → thermal relay → starting relay → motor windings. It is important to use a wire with a cross-section of at least 1.5 mm² (preferably 2.5 mm²) for the supply line.
For ease of control, it is recommended to install a switch or circuit breaker in the phase wire circuit (L). This will allow the installation to be de-energized. without pulling the plug out of the socket. It is also highly desirable to use an RCD (residual current device) with a leakage current of 30 mA, since the compressor is a metal case under voltage in a humid workshop environment.
Here is the assembly procedure:
☑️ Assembling the electrical circuit
If you use a capacitor motor (less common, mostly in older models), the circuit changes. The capacitor is connected in series with the starting winding and remains in the circuit constantly or is turned off by a centrifugal switch. The capacitance of the capacitor is selected experimentally, usually within the range of 20-50 uF for every 1 kW of power.
⚠️ Attention: It is strictly forbidden to supply 220V voltage directly to the starting winding without a working or without a relay. overheating of the thin wire of the starting winding and fire.
Organization of cooling and lubrication system
When operating as an air compressor, the unit is subjected to heavy loads. The standard freon that cooled the engine is missing. The oil in the crankcase splashes and is partially carried away by the compressed air stream, which requires regular topping up. In addition, the air heats up during compression, and without heat removal the compressor will quickly reach critical temperatures.
To solve the problem of overheating, many craftsmen install an additional blower fan, directing the air flow to the motor housing. Another effective solution is to install heat exchanger on the outlet pipe in front of the receiver. This can be a copper tube, coiled and immersed in water, or simply a long metal hose.
Lubrication requires special attention. Unlike freon, air does not carry oil back into the crankcase as effectively.
- 🛢️ Monitor the oil level through the inspection window or dipstick (if provided by the design).
- 💨 Install an oil catcher at the outlet of the compressor so that oil does not fly into the spray gun or pneumatic tools.
- 🌡️ Control the body temperature with your hand (up to 70-80°C is normal, higher is dangerous).
The use of synthetic oils for compressors extends the service life of the unit and reduces the formation of carbon deposits on the valves. Mineral oils can coke during prolonged operation at high temperatures.
Can a compressor be used without oil?
There are oil-free compressors, but in converted refrigerators the piston group is designed to operate in an oil bath. Working “dry” will lead to lifting of the cylinder and piston in a few minutes.
Setting up automation and safety
Operating the compressor without pressure control is unacceptable. In a standard refrigerator, the thermostat was responsible for this, but in the air system it is needed pressostat (pressure switch). It will automatically turn off the engine when the maximum pressure in the receiver is reached and turn it on when it drops to a minimum.
The pressure switch usually has four contacts for connecting to the network and the engine, as well as a discharge valve that bleeds air from the discharge line when the engine stops. This makes restarting easier, since the motor does not need to immediately overcome the pressure in the system.
A safety valve is a mandatory element. If the pressure switch gets stuck or fails, the pressure in the receiver may exceed the permissible limit, which can lead to a cylinder explosion. The valve is configured to operate at a pressure slightly higher than the operating pressure (for example, 8 bar at an operating pressure of 6 bar).
⚠️ Attention: The receiver (air cylinder) is a vessel operating under pressure. Using rusty, damaged or uncertified cylinders (fire extinguishers, gas cylinders) without checking with hydraulic pressure is dangerous to life.
Also do not forget about the filter at the air inlet. The intake air contains dust, which acts as an abrasive, quickly wearing out the rings and cylinder walls. A simple car air filter will significantly extend the life of your homemade compressor.
Frequent errors and troubleshooting
Even with the correct design, problems can arise. Often, beginners confuse phase and zero when connecting a relay, especially if they use old-style induction relays. In such cases, the engine hums, jerks, but does not start. The solution is to swap the contacts for connecting the starting winding.
Another common mistake is ignoring the thermal regime. Operating the compressor for 30-40 minutes without a break for models designed for 10-15 minutes of operation per hour will lead to oil degradation and thermal expansion of parts, which will cause jamming.
If the compressor vibrates strongly, check the fastenings and condition of the rubber shock absorbers. Vibration is transmitted to metal tubes, which can burst due to metal fatigue. Use flexible high-pressure hoses to connect to the receiver to compensate for vibration.
Why does the compressor hum but does not start?
Most likely, the starting relay is faulty (the contacts are burnt or the posistor is burnt out) or the piston is jammed due to lack of oil. A break in the starting winding is also possible.
Is it possible to increase the pressure above 10 atmospheres?
Standard refrigeration compressors are not structurally designed for pressures above 10-12 bar. Trying to pump more will result in destruction of the valve group or failure of the seals. For high pressure, special compressors are needed.
How often should the oil be changed?
When actively used as an air compressor, it is recommended to change the oil every 50-100 operating hours. Signs of the need for replacement: darkening of the oil, the appearance of an emulsion or metal shavings.
Do you need a capacitor to start?
In most modern circuits with a posistor relay, a capacitor is not required. It is needed only in circuits with capacitor starting or to increase the starting torque on old inductive relays.
Is it safe to use such a compressor for airbrushing?
Only if high-quality fine filters are installed (carbon and moisture-oil separator). A refrigeration compressor can emit oil vapors that will ruin the paint job.