Using an old compressor from a broken refrigerator is an excellent opportunity to get a powerful and relatively inexpensive source of compressed air for a home workshop. Many craftsmen assemble homemade compressors from such units for airbrushing, tire inflation or pneumatic tools. However, the connection process requires not only technical skills, but also strict adherence to electrical safety rules.
Before proceeding with installation, you need to make sure that the unit itself is in good working order. An old motor may have expired life or damaged windings, which will make its operation dangerous. In this article we will analyze all the stages: from testing functionality to final assembly of the electrical circuit and installing filtration.
Checking functionality and searching for contacts
The first step will be a visual inspection and initial diagnosis. The body should not have dents, and the tubes should not have any through damage. If the compressor was recently removed from the refrigerator, freon and oil may still remain in it, so you should not open the sealed casing right away.
First, you need to determine the purpose of the three contacts on the start relay. They are usually marked with the letters S (Start), R (Run) and C (Common), but on older Soviet models there may be no markings. You will need a multimeter in resistance measurement mode.
⚠️ Attention: Before any manipulations with the electrical part, make sure that the device is completely disconnected from the 220V network. Residual charge in capacitors can lead to electric shock.
The testing process is as follows:
- 🔌 Find the pair of contacts with the highest resistance - these will be the terminals of the starting and working windings, and the third contact will be common.
- 📏 Measure the resistance between the common contact and the two the remaining ones: a smaller value will indicate the working winding, a larger value will indicate the starting winding.
- 🔍 Check the resistance between the housing and any contact; the device should show infinity, which indicates the absence of a breakdown to ground.
If the measurements show a short circuit or open circuit, further assembly does not make sense. If the readings are normal, you can proceed to preparing the components for the pneumatic part of the system.
Preparation of the pneumatic system and receiver
The refrigeration compressor is not intended for direct pumping of air into the instrument; it requires a receiver - a container for accumulating gas and stabilizing pressure. Old fire extinguishers, gas cylinders or truck resors are often used as a receiver.
The most important step is to clean the inner surface of the receiver from rust and dirt. Even a small layer of oxides will eventually turn into abrasive dust, which will quickly damage the piston group of the compressor.
To connect metal parts, fittings and hoses are used that are designed for pressures higher than the intended operating pressure. The standard shutdown pressure of household compressors is about 6-8 atmospheres, but the safety margin should be at least 10-12 atm.
| Component | Function | Material |
|---|---|---|
| Receiver | Air accumulation, pulsation damping | Steel, aluminum |
| Check valve | Protection against reverse air flow | Brass, steel |
| Pressure gauge | System pressure control | Metal, plastic |
| Safety valve | Emergency pressure release | Brass |
⚠️ Attention: The use of plastic bottles or cans of household chemicals as a receiver is strictly prohibited. They cannot withstand pressure and explode with scattering fragments.
Electrical connection diagram
The most difficult moment for beginners is the correct connection of the starting relay. Refrigerators use different types of relays: posistor and induction. The connection diagram depends on the type of device, but the principle remains the same: current is supplied to the working winding, and the starting winding is turned on only at the time of start.
For stationary installation of the compressor, the relay is often removed separately or mounted on the housing, providing access for replacement. The wires must have a cross-section of at least 1.5 mm² in order to withstand starting currents.
Differences in relay types
Thermistor relays work automatically: when voltage is applied, the posistor heats up and opens the starting winding circuit. The induction ones open mechanically when the engine speeds up.
The connection is made in the following sequence:
- ⚡ The phase wire of the network is connected to one of the relay contacts (usually through a thermal fuse).
- 🔄 The neutral wire goes directly to the common contact of the compressor.
- 🛡️ Be sure to install a circuit breaker at the entrance to the system to protect against short circuits.
Installation of a filtration and oil separation system
Unlike specialized air compressors, refrigeration units operate in an oil bath. When air is compressed, some of the oil is inevitably carried away as a suspension. If you plan to use air for airbrushing or pneumatic tools, installing an oil separator is critical.
Oil getting into the paint creates coating defects, and in pneumatic tools it washes away the factory lubricant, leading to jamming. Therefore, a separator is installed at the outlet of the discharge tube.
Installation of automation and pressure switch
For automatic operation of the compressor a pressure switch (pressure switch) is required. This device opens the electrical circuit when the maximum pressure in the receiver is reached and closes it when the pressure drops to the minimum threshold.
Connecting the pressure switch requires care: it breaks the phase wire going to the engine. Some models have an additional terminal for connecting an unloading valve that bleeds air from the compressor head when stopped.
Setting the differential allows you to set the difference between the on and off pressure. A delta that is too small will lead to frequent engine starts, which is harmful for the windings; too large will create pressure surges at the output.
Assembly of units and first start-up
Final assembly involves mounting all components onto a single frame or into a housing. The compressor vibrates during operation, so it should be installed on rubber shock absorbers. The mounting bolts must be securely tightened, and the electrical contacts must be insulated.
Before the first full start-up, check the tightness of all connections with a soap solution. Bubbles will indicate leaks that need to be eliminated with fum tape or replacing fittings.
☑️ Final check before turning on
Maintenance and safety
The service life of a homemade compressor directly depends on operating conditions. Regularly drain the condensate from the receiver through the drain valve at the bottom of the cylinder. Moisture accumulating in the tank causes corrosion and can get into the pneumatic tool.
⚠️ Attention: If you notice that the compressor begins to work louder, gets hotter, or does not turn off for a long time, stop it immediately. Operation of a faulty unit can lead to a fire or explosion of the receiver.
What kind of oil to pour into the compressor from refrigerator?
You should only use oil that is compatible with the seal material and type of refrigerant if the system is closed. For open systems (air blowers), a special compressor oil is suitable, but many successfully use synthetic motor oils with low ash content. The main thing is not to mix mineral and synthetic oils.
Why does the compressor hum but does not start?
Most often the problem lies in a faulty start relay or a jammed piston. If you hear a buzzing sound and clicks when you turn it on, but the shaft does not rotate, try to carefully (following safety precautions!) rotate the shaft by hand. If the shaft is free, change the relay.
Is it possible to use a compressor without a receiver?
Technically, you can start it, but using it for work is highly not recommended. Without a receiver, the air flow will pulsate, which will ruin the quality of the painting. In addition, the engine will turn on and off too often, which will quickly lead to its combustion.