Dismounted compressor from an old refrigerator often finds a second life in the hands of a home craftsman. This powerful and relatively quiet unit is ideal for creating homemade compressors, airbrushes, vacuum systems or pneumatic tools. However, simply removed from the refrigerator body, it is a pile of metal with protruding tubes and wires, unable to function without proper wiring.
The main difficulty lies in organizing the correct electrical connection, since refrigerator motors are of the asynchronous type with a starting winding. Unlike simple motors, they cannot start on their own without a special one, which supplies an impulse to the starting winding only at the moment of switching on. An attempt to directly connect to a 220 volt network without this part will only lead to humming and eventual burnout of the windings. starting relay, which supplies an impulse to the starting winding only at the moment of switching on. An attempt to directly connect to a 220 volt network without this part will only lead to humming and eventual burnout of the windings.
In this article we will analyze in detail the preparation process, connection diagrams and safety measures that must be observed when working with high voltage. You will learn how to identify contacts, select a working relay, and assemble a reliable system that can last for many years. The correct approach will avoid short circuits and ensure stable operation of your homemade device.
Diagnostics and preparation of the compressor
Before proceeding with the electrical part, you need to ensure the mechanical integrity of the unit. Visually inspect the housing for dents, oil marks, or corrosion. If the compressor has been removed from a running refrigerator, there may be refrigerant residue on its tubes, so it is best to carry out work in a well-ventilated area. Tightness is a key factor: if air and moisture get inside when removing the tubes, the inner surface may begin to rust, which will shorten the service life.
The next step is to check the motor windings using a multimeter. There are three contacts on the compressor housing, covered with a plastic cover or located under the start relay. You need to "ring" them to determine the resistance of each winding. Typically, the sum of the resistances of the starting and operating windings gives the resistance of the common winding. If the device shows an open circuit (infinity) or a short circuit (zero), this electric motor cannot be used.
⚠️ Attention: Never turn on the compressor if air comes out of its tubes with a characteristic smell of burnt oil or if the resistance of the windings is unstable. This may result in fire or electric shock.
It is also important to check that the piston mechanism is not jammed. When turned on briefly (literally for a second), a working unit should begin to work with a characteristic rhythmic hum. If you hear only a monotonous hum without vibration, an oil change or mechanical intervention may be required, but most often this is a sign of a malfunction.
Design and selection of a starting relay
The heart of the starting system is the starting relay. It is this small block that is responsible for supplying voltage to the starting winding in the first fractions of a second of operation, after which it turns it off, leaving only the main winding to work. There are two main types of relays: induction (with a coil) and posistor (thermal). For older models of refrigerators, induction ones were most often used, and for modern ones - posistors.
When choosing a relay for your project, it is critical to take into account the power of the compressor and the type of refrigerant for which it was originally designed. An incorrectly selected relay will either not start the engine or will constantly click when trying to turn it on, which will quickly damage the part. The relay body is usually marked with a marking indicating the power range in watts or the model of the refrigerator for which it is intended.
What is the difference between posistor and induction relays?
Induction relays operate on the principle of an electromagnet: current passes through a coil that draws in the core, closing the contacts of the starting winding. As the engine accelerates, the current drops and the spring opens the circuit. Thermistor relays use a thermocouple: when current is applied, the resistance of the posistor increases, it heats up and opens the circuit. PTC resistors are cheaper, but require time to cool down (3-5 minutes) before restarting.
If you are using a compressor from an unknown source, it is best to select a relay experimentally or buy a universal model with an adjustable power range. Visually inspect the contacts of the old relay: if black carbon deposits or oxidation are visible on them, the part must be replaced. Stripping the contacts in this case gives only a temporary effect.
Connection diagram and pinout of contacts
Identification of contacts is the most important step. The compressor usually has three terminals arranged in a triangle. To avoid confusion, use a multimeter in resistance measurement mode. Between the working and starting windings the resistance will be maximum. Between working and general - average, and between starting and general - minimal (or vice versa, depending on the design, but the summation logic remains the same). Most often, the common point is located separately, and the other two contacts are located next to each other.
The standard connection diagram is as follows: the phase from the network goes to one relay contact, zero - to the common contact of the compressor. Inside the relay, the phase branches: it is constantly supplied to the working winding and briefly to the starting winding through normally open contacts. After the engine starts, the contacts open and the starting winding is de-energized.
| Contact type | Designation on the diagram | Resistance (approximate) | Function |
|---|---|---|---|
| General (C). data-i="78">Starting (S) | Common | Sum Rstart + Rwork | Connection to network zero |
| Worker (R) | Run | Average value | Continuous engine operation |
| Launcher (S) | Start | Minimum/Maximum | Starting the rotor (short-term) |
For ease of installation, it is recommended to use color coding of the wires, although in old refrigerators it could be arbitrary. It is better to make the phase wire red or brown, the neutral wire blue, and the ground wire yellow-green. This will simplify further maintenance and troubleshooting of your system.
☑️ Checking the connection diagram
Necessary tools and materials
For high-quality assembly of the connection unit, you will need not only the compressor itself and the relay, but also a number of additional components. The basic set includes a power cord with a plug that has a grounding contact. The use of plugs without grounding when working with metal compressor housings is unacceptable from the point of view of electrical safety.
You will also need a circuit breaker or fuse for a current of 6-10 Amps. It will protect the wiring in the house from overload at the moment of starting the engine, when the starting current can be several times higher than the rated current. A thermal relay that will turn off the power if the windings overheat is useful, although in many modern compressors such protection is built into the housing.
- 🔧 Tools: multimeter, set of screwdrivers, side cutters, insulating tape or heat shrink, pliers.
- ⚡ Electrical equipment: cable with a cross-section of at least 1.5 mm², grounding plug, circuit breaker.
- 🛠 Materials: clamps for fastening wires, dielectric caps, mounting plate for fixing the relay.
All connections must be made reliably, preferably using soldering or high-quality terminal blocks. Twists, even well-insulated ones, oxidize over time and begin to heat up, which can lead to a fire. Pay special attention to the places where the wires exit from the relay housing and the compressor.
Step-by-step connection instructions
The assembly process begins by installing the compressor on a flat, stable surface. It is advisable to use rubber feet or an anti-vibration pad, since the unit creates strong vibration during operation. Attach the start relay to the compressor terminals. It is put on tightly until you hear a characteristic click, and is secured with a bracket or clip, if provided for by the design.
Next we proceed to electrical switching. Connect the neutral wire directly to the compressor common terminal (or to the corresponding relay terminal if the circuit requires it). Lead the phase wire to the input contact of the relay. From the output contacts of the relay, the wires go to the working and starting windings, respectively. If you use a capacitor (to improve starting characteristics), it is connected in parallel with the starting winding.
After assembling the entire circuit, check all connections again with a multimeter for short circuits. Make sure that no exposed wires are touching the metal housing. Only after a thorough check can you connect the device to the network. It is better to turn it on for the first time through an extension cord with a button, while being at a safe distance.
Safety measures and common mistakes
Working with electricity and mechanical devices under pressure requires strict adherence to safety rules. The most common mistake is ignoring grounding. The compressor housing is metal, and in the event of a breakdown of the winding insulation, it will contain a dangerous potential. Without grounding, touching such a unit can be fatal.
Another common mistake is installing the compressor in a closed space without ventilation. The engine gets hot during operation and requires heat removal. If you hide the compressor in a box to reduce noise, be sure to provide ventilation holes and possibly active cooling.
⚠️ Attention: It is strictly forbidden to pinch the suction or discharge tubes during operation. This creates excess pressure, which can rupture the compressor housing or lead to an explosion of oil mist inside the crankcase.
Also, do not forget about the temperature regime. If the compressor overheats (becomes hotter than 90-100 degrees), it is necessary to take a break from operation or install an additional fan. Long-term operation at extreme temperatures leads to coking of the oil and jamming of the piston group.
Maintenance and extension of service life
For your homemade compressor to work long and stably, it requires regular maintenance. First of all, this is monitoring the level and condition of the oil. Although many refrigeration compressors are considered “maintenance-free,” under intense use in DIY projects, the oil can lose its properties or burn out.
Every six months, it is recommended to check the tension of the fasteners and the condition of the electrical contacts. Vibration gradually weakens bolts and screws, which can cause the motor to misalign or cause electrical contact to occur. You should also clean the housing of dust, which acts as a heat insulator, interfering with cooling.
If you notice that the compressor has become louder, takes longer to build up pressure, or is often switched off by the thermal relay, this is a signal that diagnostics are needed. It may be necessary to change the oil, clean the valves, or replace the start relay, the life of which is also not endless.
Is it possible to use a compressor without a receiver?
Technically, you can start the compressor without a receiver (air tank), by directly connecting the hose to the outlet. However, this is highly not recommended. The receiver stabilizes the pressure, cools the air and collects condensate. Without it, you will get a pulsating air flow, overheating of the compressor due to frequent starts and moisture getting into the pneumatic tool.
Why does the compressor hum, but does not start?
Most often, the reason lies in a faulty start relay or jammed piston group. It is also possible that there is low voltage in the network or a faulty capacitor (if there is one in the circuit). Try replacing the relay - this is the cheapest and most common cause of the problem.
What kind of oil should be poured into the compressor?
Refrigeration compressors are usually filled with synthetic POE (polyester) or mineral oil, depending on the type of refrigerant for which the unit is designed. For air compressors, it is better to use specialized compressor oil that does not form soot. It is strictly forbidden to mix different types of oils.
How long can a compressor operate without interruption?
Refrigeration compressors are not designed for 100% continuous operation (duty cycle). Typically their duty cycle is around 50-70%. This means that after 10-15 minutes of work they need a break to cool down. For continuous operation, special modifications or an enhanced cooling system are required.