The situation when it is necessary to remove and start the compressor separately from the refrigeration unit itself arises quite often among repairmen and home craftsmen. Most often, this is necessary to check the functionality of the unit before installation, use the engine for compressor tasks (painting, inflating tires) or for in-depth diagnostics of cooling system faults. Refrigeration compressor It is a complex sealed unit operating under high pressure, so starting it requires a clear understanding of the electrical circuit and compliance with safety regulations.
Unlike conventional electric motors, refrigerator motors cannot operate directly from a 220 volt network without additional starting elements. They require a special trigger mechanism and often a thermal protector that prevents the windings from burning out when overheated. Ignoring these components when trying to “revive” the motor will lead to instant failure or even explosion of the starting capacitor. In this article, we will analyze in detail the connection process, the necessary tools and typical errors.
Before starting any work, make sure that the unit has been disconnected from the power supply for a long time and that you have completely discharged from static electricity. Working with electrical and mechanical components under pressure requires caution. If you plan to use the removed motor for household needs, remember that the oil in the refrigeration compressor crankcase is intended only for operation in a closed circuit and does not like contact with moisture from the air.
Design and principle of operation of the compressor unit
To successfully start the engine, you need to understand what is inside its metal casing. The sealed block contains the electric motor itself and the piston group, immersed in an oil bath. Three contact pins are placed on the body, forming a triangle. These pins correspond working windingto the starting winding and the common point (ground). It is the correct connection of these contacts with the starting device that ensures rotation of the rotor.
The key element of the circuit is the starting relay. In older models, relays with a posistor or electromagnetic were used, in modern ones, solid-state options are more common. Relay performs a critical function: it supplies a pulse to the starting winding only at the moment of start, and then turns it off, leaving only the working winding to work. If this cycle is broken, the engine will hum, but will not start, or the thermal protection will work.
⚠️ Attention: It is strictly forbidden to apply voltage directly to the windings without a starting relay. This will lead to humming, overheating and rapid burning of the winding insulation, since the initial torque is not created.
Also an important component is the thermal protection relay. It opens the circuit if the current consumption exceeds the norm or if the case heats up to a critical temperature. When assembling the test circuit, it is better to save this element and connect it in series so that it can perform its protection function in the event of a short circuit or jamming of the piston group.
Why can’t you spin the compressor for a long time without load?
If you start the compressor with open pipes (without freon and pressure), it will work, but the lubrication system may not function correctly. The oil sprays under the pressure of freon vapor, and in its absence, insufficient lubrication of the rubbing pairs is possible. For a short-term test (1-2 minutes), this is acceptable, but long-term idling is undesirable.
Necessary tools and materials for connection
To organize a mobile stand for testing compressors, you will need a minimum set of tools and electrical components. The basis will be a piece of electrical cable with a plug and a core cross-section of at least 1.5 mm², which will allow the safe passage of inrush currents. You will also definitely need a working one starting relaycorresponding to the power of the engine being tested (the rating is usually indicated on the relay body or in the compressor marking).
To connect the contacts you will need alligator clips or high-quality terminal blocks. The use of twists in this case is unacceptable due to the risk of sparking and poor contact. If you plan to use the compressor to pump air, you will need to prepare a hose and possibly a receiver in advance, but for the initial start-up it is enough to simply make sure that the suction and discharge tubes are open and not pinched.
- 🔧 Insulated pliers and side cutters for working with wires.
- 🔌 A working socket or extension cord with grounding for connection to the network.
- 🧤 Dielectric gloves for protection against electric shock during testing.
- 📏 Multimeter for testing the windings and checking the integrity of the circuit before turning on.
Special attention should be paid to the condition of the relay itself. If you are using a relay removed from the refrigerator, inspect it for cracks and melting. Often the reason for a non-working compressor lies precisely in sticky relay contacts, and not in the engine itself. Posistor The inside of the relay should be intact, without chips or signs of overheating.
Identifying contacts and checking the windings with a multimeter
Before connecting wires to the network, it is necessary to identify the terminals on the compressor housing. Three studs arranged in a triangle have connections hidden under the casing. To determine which winding is which, you need to “ring” them with a multimeter in resistance measurement mode. This will allow you to avoid connection errors and understand whether the motor is alive.
The resistance between the common point and the working winding is always less than between the common point and the starting winding. Between the starting and working windings the resistance will be total (maximum). If the multimeter shows infinity (open) or zero (short circuit) between any two contacts, the compressor is most likely faulty and requires replacement or complex repairs.
| Contact Pair | Expected Resistance (Ohm) | Value |
|---|---|---|
| Common - Working | 10–25 Ohm | Main winding |
| Common - Starting | 20–45 Ohm | Starting winding |
| Working - Starting | 30–60 Ohm | Sum of resistance |
| Any contact - Housing | Infinity | Checking for breakdown |
It is also important to check that there is no breakdown on the body. One probe of the multimeter is applied to any of the three contacts, and the second is applied to the stripped area on the copper tube or metal case. Resistance must be endless. If the device shows any value, it means that the insulation is broken, and the operation of such a unit is dangerous to life.
Connection diagram: step-by-step starting instructions
The process of assembling the test circuit requires care. First, identify the upper and lower pairs of contacts on the relay (usually they are signed or labeled S, R, C, where S is starting, R is working, C is common). The starting relay is placed on the compressor contacts in the correct position (arrow up), after which the wires are connected to its terminals.
The connection diagram is as follows: the phase wire from the network goes to the input of the thermal relay (if it is remote) or directly to the common contact of the circuit. From the thermal relay or common point, the wire goes to the working winding of the compressor. The starting winding is connected to the corresponding relay contact, which closes the circuit only at start. The neutral wire is connected to the second side of the working winding.
☑️ Check before first start
After assembling all connections, visually check the circuit again. Make sure there are no exposed wires sticking out anywhere. Plug the plug into the outlet. If everything is assembled correctly, you will hear the confident sound of the motor running and feel the air flow from the discharge tube. If there is a hum, but the motor does not spin, immediately turn off the power - perhaps the piston group is jammed or the relay is faulty.
For ease of use as a compressor for a workshop, the assembled circuit is often placed in a separate box or on a wooden stand, fixing the compressor itself through rubber shock absorbers. This reduces the level of vibration and noise, which can be quite high in older units.
Diagnosis of faults during startup
Even with correct assembly of the circuit, problems may arise. The most common one is that the compressor hums, but does not start, and after a few seconds the thermal relay clicks, turning off the power. This indicates that the starting impulse is too weak or absent, or the rotor is stuck. In 80% of cases, the starting relay is to blame, which is easier to replace than to look for an identical used one.
Another common problem is that the motor starts, but runs very quietly and does not create pressure. This may indicate a loss of compression due to wear of the piston group or, as is more common when using old refrigerators, a freon leak (although freon is not needed to work as an air pump, the compression mechanism itself is important). If the motor is silent and does not make any sounds, check the presence of voltage at the input and the integrity of the windings.
⚠️ Attention: If during operation the compressor makes a metallic knock, clanging or strong vibration noise, its further operation is prohibited. This indicates the destruction of internal mechanical components, which can lead to rupture of the case under pressure.
It is also worth paying attention to the temperature of the case. Normally, it should heat up, but not to the point where it is impossible to hold your hand. Excessive heating indicates overload, malfunction of the lubrication system or interturn short circuit in the windings. In such cases, the unit must be turned off urgently and allowed to cool.
Safety measures and operating features
Use of a refrigeration unit The use of a compressor outside of normal mode (for example, as an air blower) has its own characteristics. The oil inside will be partially ejected along with the air when operating in an open circuit. Therefore, it is recommended to install a moisture-oil separator or at least a filter on the outlet pipe, especially if you plan to use this air for airbrushing or pneumatic tools.
Electrical safety is priority number one. Since the circuit is often assembled in a “canopy” without a factory housing, the risk of electric shock is high. All connections must be securely insulated with heat shrink or insulating tape. Do not leave the appliance turned on unattended, especially if it is used for inflating tires or other tasks where pressure can increase uncontrollably.
The service life of the compressor in the “open cycle” mode (pumping air) may be less than in a sealed refrigerator circuit due to oxidation of the oil by oxygen in the air and moisture ingress. Regularly check the level and condition of the oil through the service pipe (if possible) or monitor the cleanliness of the outlet air.
If you use a compressor for systems that require clean air (for example, for an aquarium or pneumatic post), be sure to use carbon filters at the outlet. Oil vapors and a specific odor characteristic of the operation of refrigeration units can be toxic or simply unpleasant.
Frequently asked questions (FAQ)
Is it possible to start a compressor without a start relay?
Theoretically, it is possible if you manually short-circuit the circuit starting winding when turned on, but this is extremely dangerous and inconvenient. The risk of getting an electric shock or burning the winding due to untimely opening of the circuit is very high. The use of a relay is a prerequisite for safe and durable operation.
Why does the compressor knock out the machine in the panel?
This can happen for three reasons: a short circuit in the motor windings, an insulation breakdown on the housing, or a malfunction of the starting relay, which does not turn off the starting winding. Also, the reason may be that the motor is too powerful for weak wiring at the time of the starting surge.
What kind of oil should be poured into the compressor during maintenance?
Refrigeration compressors use special synthetic or mineral oil (for example, POE or Mineral), which does not conduct electricity and has a viscosity. Regular motor or transformer oil cannot be used, as it can cause insulation breakdown or destruction of seals.
How long can you run the compressor in test mode?
For initial diagnosis, 1-2 minutes of operation is enough. This time is enough to evaluate sound, vibration and air flow. Long-term operation without connection to the cooling system (condenser) is not recommended, since the cooling mode of the motor itself and heat removal are disrupted.
What to do if the compressor works, but does not pump?
If the engine hums, but no air flows, the filters may be clogged, the valves inside the cylinder head are damaged, or the connecting rod-piston is broken mechanism. In the case of using freon, the cause may be a complete gas leak, but for the air pump mode this is not critical - the problem is precisely in the mechanics of compression.