Using a compressor from an old refrigerator is a great idea for creating a homemade compressor, airbrush or tire inflator. Many craftsmen are faced with the question of how to properly power this unit from a regular outlet, because the standard starting relay often breaks down or is not suitable for new tasks. Unlike industrial motors, a single-phase asynchronous motor is used here, which requires a special approach to start-up and operation.
Before you begin assembling an electrical circuit, you need to clearly understand what type of motor you are dealing with. Most often, household refrigerators use units with a starting winding, which is turned on only when the shaft accelerates, after which it is turned off by a centrifugal switch or thermal relay. Incorrect connection can lead to overheating of the windings, humming and rapid failure of the device, so it is important to follow the sequence of actions.
In this article we will analyze all stages of connection, from searching for contacts to choosing capacitor capacity. You will learn how to distinguish the working winding from the starting winding and what safety measures must be observed when working with high voltage. Proper assembly of the circuit will ensure a long service life of your homemade compressor.
Determining the type of motor and searching for contacts
The first step is always a visual inspection and continuity testing of the windings with a multimeter. On the case of most compressors, such as Atlant, Donbass or Indesit, the winding terminals are hidden under the casing and have three contacts arranged in a triangle. To understand which wire goes where, you need to remove the cover of the start relay and carefully disconnect the wires coming from the plug.
Using a multimeter in resistance measurement mode, ring the pairs of contacts. You will find that between the two terminals the resistance will be greatest - this is the sum of the resistances of both windings. The third contact, which gives the least resistance with each of the other two, is common. It is to this that one of the power wires (usually zero) is connected.
- 🔍 Find the contact with the lowest total resistance - this will be the common terminal.
- ⚡ Measure the resistance between the common terminal and the two remaining ones: a lower value will indicate the working winding, a larger value will indicate the starting.
- 📏 Write down the obtained values, since the choice of capacitor depends on the resistance ratio.
⚠️ Attention: Before any manipulations with the electrical part, make sure that the compressor is completely disconnected from the 220V network. Residual voltage in capacitors (if they are in the circuit) can be dangerous.
In some modern models, there may be motors without an explicit starting winding, operating through a frequency converter, but for secondary use, classic options with relays are most often chosen. If you plan to use the compressor as part of a complex installation, it makes sense to keep the original starting device, if it is in good condition.
Schemes for connecting to a 220 Volt network
There are several main ways to power the compressor, and the choice depends on whether you will use a standard relay or assemble a circuit with capacitors. The classic version involves the use of a starting relay, which automatically opens the starting winding circuit after a set of revolutions. This is the most reliable method, proven over decades of operation of refrigerators.
If the original relay is lost or you want to assemble a universal circuit, you can use capacitor starting. In this case, the capacitor is connected in series with the starting winding. The capacity is calculated based on the motor power: usually 70 uF per 1 kW of power is taken for the starting capacitor and 30 uF per 1 kW for the working one. Working winding connects directly to the network, and the starting one through the capacitor.
For reversible motors or circuits where a change in the direction of rotation is required (although this is rare for compressors, since the direction of shaft rotation is fixed by design), the circuit can change. However, in 99% of cases of refrigeration compressors, the direction of rotation is determined by the design of the valves and cannot be changed electrically.
What will happen if the windings are mixed up?
If you apply voltage only to the starting winding, the engine will hum, get very hot and can burn out in a few minutes. If you confuse the working and starting circuits in a circuit with a capacitor, the engine may not start or operate with low efficiency.
When assembling the circuit, it is important to use wires with a cross-section of at least 1.5 mm² to avoid voltage drop and heating of the wiring. All connections must be made in terminal blocks or soldered; twisting in this case is unacceptable due to the vibrations created by a running compressor.
Selection of capacitors for starting
The correct choice of capacitor is the key to successful engine starting. Refrigeration compressors are characterized by a difficult start-up, since there may already be residual freon or oil pressure in the system. Therefore, the starting capacitor must have a sufficiently large capacitance to create the necessary phase shift and torque.
You should use capacitors designed for a voltage of at least 400V (preferably 450V or 500V), since at the moment of starting voltage surges exceeding the standard 220V are possible. The type of capacitor must be designed for operation in alternating current (AC) circuits, not direct current (DC).
| Motor power | Starting capacitance (uF) | Working capacitance (uF) | Capacitor type |
|---|---|---|---|
| 100 W | 6-8 µF | 3-4 µF | CBB60 / CBB61 |
| 150-200 W | 10-15 µF | 5-8 μF | CBB60 / CBB61 |
| 250-300 W | 18-25 μF | 10-12 μF | CBB60 / CBB61 |
| 400+ W | 30-50 µF | 15-20 µF | CBB60 / CBB61 |
| Data is given for motors with a frequency of 50 Hz and a voltage of 220V. | |||
If you cannot find a capacitor of the required capacity, you can connect several capacitors in parallel. In this case, their capacities are summed up. For example, two 10 µF capacitors will give a total of 20 µF. The main thing is that the operating voltage of each of them meets the requirements of the circuit.
Using a standard starting relay
The easiest and most reliable way to connect the engine is to use its original starting relay. This device already contains all the necessary elements: a coil, contacts and often thermal protection. The relay is placed directly on the terminals of the compressor windings and is latched with special brackets.
It is important to correctly orient the relay in space. The housing usually has an arrow indicating the "UP" direction. If you install the relay upside down, the contacts will not open under the influence of gravity after starting, which will cause the starting winding to burn out. Inside the relay there is a movable core that should move freely.
- 🔌 Connect the input wires (phase and zero) to the relay contacts marked L and N.
- 🛡️ Check the condition of the thermal protection: it should “ring” with a multimeter in a cold state.
- 🔩 Make sure that the relay sits tightly on the compressor terminals and does not loosen when vibrating.
Inside the relay circuit, the starting winding is connected through normally closed contacts, which open when the current increases or due to centrifugal force. The working winding is constantly energized. This circuit ensures that the engine automatically restarts in the event of a short-term power loss, which is important for stable operation.
⚠️ Attention: If the relay makes a loud crackling sound or the engine hums but does not start, immediately turn off the power. This may mean jamming of the piston group or a malfunction of the relay itself.
☑️ Checking the starting relay
Safety measures and grounding
Working with compressor equipment requires strict adherence to electrical safety rules. The body of the refrigeration compressor is made of metal and if the insulation of the windings breaks down, it may become energized. To avoid electric shock, grounding is a mandatory element of the connection diagram.
Use outlets with a grounding contact and make sure that the grounding conductor in your cable is actually connected to the building grounding loop. If you use an extension cord, it must also be a three-wire one. In a workshop or garage, it is recommended to install an RCD (residual current device) at the input.
In addition to electrical safety, it is worth remembering about mechanical safety. The compressor outlet (discharge) can develop a pressure of several atmospheres. If you connect a hose or receiver, make sure it is rated for pressure above operating pressure. Also, the compressor gets very hot during prolonged operation, so do not touch metal parts during operation.
When soldering wires, use rosin or special fluxes that do not cause corrosion. Acid residue can corrode the connection over time, causing sparks and fires. It is best to hide all electrical connections in sealed junction boxes.
Diagnostics and troubleshooting
After assembling the circuit and turning it on for the first time, you must carefully monitor the behavior of the motor. A normally operating compressor emits a steady hum that turns into a rhythmic hum. If you hear a metallic clanging sound, a strong vibrational hum or a whistle, this indicates a problem.
A common problem is the inability to start under load. The engine hums, but the shaft does not spin. In this case, installing a more powerful starting capacitor or checking the mechanical part helps. Sometimes compressors that have been idle for a long time require “swinging” - a short-term supply of voltage to break the piston group from its place.
If the compressor is working, but gets very hot, check the current consumption with a current clamp. If the current exceeds the rated current indicated on the nameplate, there may be an interturn short circuit in the windings or ventilation is impaired. In this case, further operation is dangerous.
Why does the compressor hum but does not start?
Most often the reason lies in a faulty starting relay or lack of starting torque. Check if the relay contacts open after starting. It is also possible that the piston group may jam due to thickened oil or the lack of freon, which in the original design lubricates the components.
Can a compressor be used without freon?
Yes, the use of freon is not required to pump air. However, in the original compressor, freon helped remove heat and lubricate rubbing parts. When operating in air, the compressor's service life may be reduced and the housing temperature will be higher than normal. It is recommended to take breaks during operation to cool down.
Which capacitor to choose: film or electrolytic?
Only special film capacitors (CBB60, CBB61, K78-17) are suitable for operation in 220V AC circuits. Conventional electrolytic capacitors (found in power supplies) will explode when connected to an alternating current network, since they are designed for constant voltage.
Is it necessary to change the oil in a compressor during refurbishment?
Changing the oil in a hermetic compressor is extremely difficult and is not recommended without special equipment and skills. Mineral oil poured inside works well with freon, but can oxidize when in contact with air. For short-term work, replacement is not required; for continuous operation in air, it is advisable to use compressors originally designed for this purpose, or put up with a shorter resource.
How to determine whether a compressor has burned out?
If the multimeter shows infinite resistance between any terminals of the windings, there is a break. If the resistance is close to zero, there is a short circuit. Also a sign of combustion is a burning smell and a black coating on the terminals. In some cases, the breakdown may occur on the housing: ring any terminal with a copper tube, the device should not show conductivity.