Independent use of a compressor from an old refrigerator opens up wide opportunities for a home craftsman. This device can become the basis for creating an airbrush, a spray gun, a compressor for inflating tires, or even a vacuum pump. However, for the unit to work, it is not enough to simply apply voltage to the contacts, since an asynchronous motor requires correct phasing and a starting mechanism.
The process of starting the motor depends on the type of starting system used, which can be relay or capacitor. Most household refrigerators use exactly starting relay, which automatically turns off the starting winding after the rotor accelerates. Incorrect connection of the wires can lead to a short circuit, burnout of the windings, or even injury, so you must strictly adhere to the electrical diagram and safety rules when working with mains voltage.
In this article we will look in detail at how to determine the purpose of the contacts, connect the protective and starting relay, and also where to put the capacitor if it is present in chains. You will learn how to distinguish the working winding from the starting winding and how to check the operation of the unit before installing it in a homemade pneumatic system.
The principle of operation and structure of the compressor
The heart of the refrigeration unit is a sealed one compressor, inside which the electric motor and piston group are located. The engine is an asynchronous type with a squirrel-cage rotor, which ensures its reliability and durability. To start such a motor, it is necessary to create a rotating magnetic field, which cannot be achieved by applying voltage to only one winding.
That is why the design provides two stator windings: the main (working) and auxiliary (starting) windings. The working winding has a larger wire cross-section and lower resistance; it is designed for continuous operation. The starting winding, which has a thinner wire and high resistance, is turned on only at the moment of start to create the initial rotation impulse.
A critical element is the starting winding disconnect system. If it remains turned on after the engine reaches operating speed, the current in it will increase to critical values, which will lead to overheating and destruction of the insulation. In older models, this was done capacitor paired with a relay, but in modern units, only a starting relay is most often used, and the capacitor may be absent or serve as an interference filter.
⚠️ Attention: There is oil inside the compressor housing. If transported or stored incorrectly (eg upside down), oil may enter the refrigerant circuit. Before starting for the first time after long-term storage, it is advisable to let the unit stand in a vertical position for at least 24 hours.
Why can’t you turn on the compressor without a load?
Turning on the compressor without a connected receiver or hose at the outlet can lead to the fact that it will not create sufficient back pressure for the correct operation of the thermal protection, however, it is more dangerous to work with an open inlet in the in dusty rooms - the compressor will begin to suck dust and abrasive particles into the crankcase, which will lead to rapid wear of the piston group.
Determining the type of starting system
Before proceeding with the installation of the electrical circuit, it is necessary to visually inspect the box protruding from the compressor housing. This is the starting relay, which is often combined with a thermal protector. The design of this node directly dictates the connection diagram. Depending on the model of the refrigerator and the year of manufacture, there may be different types of mechanisms inside.
The most common are induction-type relays, the operation of which is based on the current passing through the coil. When starting, the current is high, the magnetic field draws in the core, closing the contacts of the starting winding. As the motor accelerates, the current drops and gravity (or a spring) forces the core down, breaking the circuit. Such relays are position sensitive - they are often marked "UP" or "UP".
There are also posistor (solid state) relays that have no moving mechanical parts. They use a thermistor, which, when heated by current, sharply increases its resistance, essentially turning off the starting winding. After turning off, such a relay requires time (several minutes) to cool down before it can start the compressor again.
- 🔍 Induction relay: has a movable core, makes a characteristic click when starting and stopping, requires strict orientation in space.
- 🔌 Posistor relay: looks like a plastic cylinder with a tablet inside, operates silently (no opening click), requires a pause between switching on.
- ⚡ Capacitor circuit: in some old or specific models, the capacitor is connected in series with the starting winding permanently or only for the start time through a separate time relay.
If you plan to use a compressor in a homemade device, it is best to keep the original “compressor + relay + thermal protection” combination. This ensures that the engine will operate in the normal mode provided by the manufacturer's engineers.
Contact markings and connection diagram
The most difficult step for a beginner is to understand the protruding contacts. On most compressors, the winding terminals are arranged in a triangle. To understand which contact is responsible for what, you need to remove the cover of the start relay and study the internal circuit or use a multimeter.
Usually the contacts are marked with Latin letters or numbers. The standard marking is as follows: C (Common) - common wire, R (Run) - working winding, S (Start) - starting winding. Sometimes there are designations L (Line) instead of C, or color coding of the wires going from the relay to the compressor.
To accurately determine the assignments, use a multimeter in resistance measurement mode. Between the contacts of the working and starting windings the resistance will be greatest. The resistance between the common contact and the working winding will be average, and between the common and starting winding will be minimal (or vice versa, depending on the design, but the working winding always has less resistance than the starting winding in total with the relay circuit, however, between the terminals on the motor the working winding has less resistance than the starting winding).
Consider a typical contact correspondence table for popular manufacturers:
| Manufacturer | General (C) | Worker (R) | Starter (S) |
|---|---|---|---|
| Atlant | Top left | Bottom left | Right |
| Danfoss | Top | Bottom Left | Bottom Right |
| LG | Top | Left | Right |
| Indesit | Top | Left | Right |
⚠️ Attention: Do not blindly rely on the color of the wires coming from the old relay, as the previous master may have mixed them up or replaced them. Always double-check the assignment of the contacts with a multimeter or according to the diagram of a specific relay.
The role of the capacitor in the starting circuit
The question of the capacitor often causes confusion. In the classic circuit of a household refrigerator, a capacitor can perform two functions: to be a starting one (increase the starting torque) or to be an interference suppressor. In most modern household compressors with a power of up to 200 W, a starting capacitor is not used - the engine starts due to the phase difference created by the design of the windings and the starting relay.
If your circuit contains a capacitor connected in parallel to the relay contacts or in series with the starting winding, its capacitance is usually from 2 to 5 μF (microfarads) at a voltage of at least 250-450 Volts. Using a capacitor of a smaller capacity can lead to humming and the inability to start under load, and a larger capacitor can lead to overheating of the windings.
For homemade installations, if the compressor is difficult to start (a hum is heard, but the shaft does not rotate), you can experiment with adding a starting capacitor. However, if the standard relay is working properly and the windings are intact, an additional capacitor is most often not required. It is important to use specialized starting capacitorsdesigned for operation in alternating current circuits.
In some circuits, a capacitor is installed at the power input (parallel to the network) to smooth out ripples and reduce radio interference. Removing it will not affect the operation of the motor, but may increase the level of electromagnetic noise.
Step-by-step instructions for assembling the circuit
Connecting the compressor requires care and adherence to the sequence of actions. Before starting work, make sure that the device is disconnected from the 220V network. To assemble a working circuit, you will need a piece of cable with a plug, a terminal block or insulating tape, and possibly a new power cord if the old one has become unusable.
First you need to secure the compressor on a flat surface. It is advisable to use vibration-isolating rubber gaskets, since the unit produces significant vibration during operation. Then connect the wires from the power plug to the contacts of the thermal protector and the start relay. Typically, one phase wire goes to the thermal protection, and then to the relay, and zero goes directly to the common terminal of the compressor or also through the protection.
Next, you should connect the relay terminals to the contacts on the compressor. The starting relay is put on the side pins, and the thermal protection (tablet or bimetallic plate) must fit snugly to the motor body or be placed on the contact terminal in order to correctly read the temperature.
☑️ Checklist before turning on for the first time
After assembling all the electrical circuits, check the circuit again. Apply power briefly. The compressor should start 1-2 seconds after the characteristic click of the relay. If you hear a hum, but there is no start, or the circuit breaker knocks out, immediately turn off the power and check the circuit.
⚠️ Attention: When the compressor operates without a cooling system (condenser and radiator), as in a refrigerator, the outlet pipe will heat up to high temperatures (80-100°C and above). Do not touch metal parts at the outlet and ensure good ventilation of the work area.
Checking performance and safety
Successful startup is only half the battle. For full operation, especially as a compressor for tools, you need to ensure its performance and tightness. The best way to check is to connect a hose to the outlet pipe and lower its end into a container of water. Bubbles will indicate a leak, and their intensity will indicate performance.
Particular attention should be paid electrical safety. The compressor housing is metal and if the insulation of the windings is damaged, it may become live. If you use the unit outdoors or in conditions of high humidity (for example, in a garage), the connection must be made through a residual current device (RCD).
It is also worth considering that refrigeration compressors are not designed for long-term operation at 100% filling (PV=100%). In the refrigerator they work cyclically. When converting into an air blower for airbrushing or painting, it is recommended to install a receiver (storage tank) and a pressure switch, which will automatically stop the motor when the set pressure in the system is reached.
- 🛡️ Grounding: Be sure to connect the grounding wire to the compressor body, if it has the appropriate terminal.
- 🌡️ Thermal control: Monitor the case temperature. If your hand does not tolerate touching for more than 5 seconds, let the unit cool down or organize forced airflow.
- 🔇 Sound insulation: Compressors from refrigerators are quite noisy. For comfortable work, consider the option of making a sound-insulating box.
Following these simple rules will allow you to safely and effectively use the old refrigeration unit for new tasks, saving money on the purchase of specialized equipment.
Is it possible to connect the compressor directly, without a relay?
Technically, you can start the motor by applying voltage to the working and starting windings at the same time, and then manually turning off the starting winding. However, this method is extremely dangerous and is not recommended. Without precise time control (0.5-2 seconds), there is a high risk of burning the starting winding or getting an electric shock. Using a standard or similar relay is the only correct solution.
Which wire is best to use for connection?
To connect a compressor with a power of up to 200-300 W, a copper wire with a cross-section of 1.5 mm² is sufficient (for example, PVS 2x1.5). It is important that the wire insulation is resistant to oils and temperatures, as it can be hot near the compressor. The length of the wire should not be excessive to avoid a voltage drop.
Why does the compressor hum but does not start?
Main reasons: jamming of the piston group due to lack of oil or long inactivity, faulty start relay (burnt contacts or the core is stuck), interturn short circuit in the windings or insufficient voltage in the network. It is also possible that the contacts of the starting and working windings are mixed up.
Is it necessary to change the oil in the compressor during refurbishment?
Changing the oil in a hermetic compressor is extremely difficult and is usually not required if it was removed from a working refrigerator. However, if the compressor has been opened (cut), the oil must be replaced with a special compressor oil, since regular motor oil may not be suitable in terms of viscosity and evaporation properties.