The situation when the refrigerator stops cooling, and diagnostics indicate failure compressor, often confronts the owner with a choice: buy a new unit or try to revive the old one. In some cases, especially when used in garages, workshops or homemade cooling systems, it is required connecting the compressor directly, bypassing the standard electronics of the refrigerator. This is a complex technical task that requires a deep understanding of the principles of operation of electric motors and adherence to strict safety measures.
Before you begin, you need to understand that motor-compressor is a device that operates under high pressure and uses electricity, life-threatening. Improper circuit assembly can lead not only to damage to expensive equipment, but also to fire or electric shock. This article is for informational purposes only and is intended for people with a sufficient level of qualifications in the field of electrical engineering.
In the process, you will have to face the need to select the right one starting relay, determine the motor windings and organize the cooling system. Ignoring any of these steps will make starting the unit impossible or will lead to its immediate combustion. We will look at all the stages in detail so that you can understand whether it is worth taking on this work yourself.
⚠️ Attention: Working with 220V electrical circuits without proper qualifications is deadly. Make sure that you turn off the power before touching the wires in any way.
The principle of operation and design of the motor-compressor
In order to competently connect the compressor, you need to understand what is inside the sealed casing. Unlike open compressors used in industry, the refrigeration unit is a single unit, where the electric motor and compressor part are in a common oil environment. The motor here is asynchronous, operating from a single-phase network, which imposes its own characteristics on the startup circuit.
The key feature is the presence of two windings: working and starting. The working winding has a larger wire cross-section and lower resistance; it is designed for continuous operation. The starting winding is needed only to create the initial torque and should be turned off immediately after the rotor reaches a certain speed. If this does not happen, the engine will burn out in a matter of seconds.
The process of starting and overload protection is monitored by a special device - start-protection relay. It is this that switches the windings and opens the starting coil circuit when 75-80% of the rated rotation speed is reached. Modern systems also use a thermal relay that opens the circuit when the engine housing is critically heated, which is an essential element of safety.
Necessary tools and safety measures
Preparing the workplace and having the right tools is half the success. To carry out the work, you will need not only a standard set of screwdrivers and pliers, but also specific measuring equipment. Without multimeter carrying out diagnostics and searching for windings is strictly not recommended, since the “poke method” here is almost guaranteed to lead to failure of the equipment.
Particular attention should be paid to insulation. All connections must be made using terminal blocks or high-quality soldering, and then reliably insulated. The use of ordinary low-quality electrical tape is unacceptable, since vibrations, which are inevitable during compressor operation, can disrupt contact. You will also need a pressure gauge station to monitor pressure and a nitrogen cylinder to check the tightness if you plan to use the unit for its intended purpose.
- 🛠️ Multimeter — for testing windings and checking the integrity of circuits.
- 🔌 Start relay —necessarily original or a complete analogue in parameters.
- 🔋 Capacitor —may be required to improve starting characteristics in some circuits.
- 🧤 Dielectric gloves —a mandatory element of protection when working under voltage.
Do not forget that there may be residual refrigerant pressure left in the system. Make sure the pressure is released before biting into copper tubing. Otherwise, the oil may splash, and the refrigerant may get into the eyes or onto the skin, causing frostbite.
Identification of contacts and connection diagram
The most critical stage is the correct identification of the winding terminals. On the housing of most modern compressors, the terminals are marked with letters, however, on old or removed units, the markings may be erased or missing. The standard marking looks like this: C (Common) - common wire, R (Run) - working winding, S (Start) - starting winding. Some domestically produced models use the marking P (start), R (work) and O (general).
If there are no markings, you can determine the purpose of the contacts using a multimeter in resistance measurement mode. Between the common wire and the working winding the resistance will be minimal. Between general and starting - average. And between the working and starting windings - the sum of the two previous values, that is, the greatest resistance. This knowledge is critically important in order to connect the motor connect the motor correctly
What happens if you mix up the starting and working windings?
If you apply voltage to the starting winding as to the working winding, the engine may hum, but not will start, or will start in the opposite direction (which is fatal for the compressor). Long-term operation on the starting winding will lead to rapid overheating and an interturn short circuit, after which the compressor will have to be discarded.
The connection diagram assumes that the phase wire of the network goes through the starting relay. Inside the relay there is a coil connected in series with the working winding, and contacts that open the starting circuit. When starting, the current is high, the contacts are closed, and current flows through both windings. When the speed increases, the current drops and the contacts open, leaving only the working winding working winding.
| Marking type | General (C/O) | Working (R) | Launch (S) |
|---|---|---|---|
| European | C | R | S |
| Russian | O | R | P |
| Resistance | - | Low | Medium |
| Wire cross-section | - | Thick | Thin |
Selection and installation of a starting relay
Successful engine starting directly depends on the correct choice starting relay. These devices are induction (with a current coil) and posistor (thermal). Induction relays operate on the principle of an electromagnet: the starting current draws in the core, closing the contacts, and when the current drops, the spring opens them. PTC relays use the property of a ceramic element to sharply increase resistance when heated by current, actually breaking the circuit of the starting winding.
When replacing a relay, it is important to take into account the power of the compressor and the type of refrigerant for which it is designed (although for connecting “directly” this is secondary, the main thing is the current characteristic). If you install a relay from a less powerful compressor, the contacts may stick and the starting winding will burn out. If it is from a more powerful one, the engine may not start, since the relay will not have time to close the contacts of the starting circuit.
⚠️ Attention: The technical characteristics of the relay must strictly correspond to the passport data of the compressor. Using a relay "at random" is prohibited.
The relay is installed on the three pins of the compressor output. It is important to secure it in the correct position, indicated by the arrow UP or UP. An inverted relay will not work correctly: the contacts either will not close or will not open in time. For temporary connection in laboratory conditions, you can use a button, but for permanent operation, reliable fixation of the standard relay is required.
☑️ Check before starting
Organization of the cooling and ventilation system
The standard refrigerator compressor is cooled by freon, which circulates inside the system, and oil sprays inside the casing. When using the compressor outside the refrigeration circuit (for example, as an air blower) or during repairs when the circuit is open, normal cooling does not work. The engine heats up very quickly, and without heat removal thermal protection the unit will constantly turn off, and in its absence, insulation breakdown will occur.
To solve this problem, it is necessary to organize forced cooling of the housing. Most often, a fan is used for this, aimed at the lower part of the casing, where the oil is located. It is also important to ensure free flow and flow of air around the unit. In some cases, during prolonged operation under load, they even install an additional radiator on the outlet pipe.
If you use a compressor to pump air into the receiver, remember that the outlet temperature will be very high. This requires the use of heat-resistant hoses and, preferably, an intercooler (heat exchanger) so as not to damage the connected equipment or create a fire hazard.
Startup and testing systems
The first launch after connection is the moment of truth. Before connecting to the 220V network, double-check all connections. Make sure the wires do not touch copper tubing, which could vibrate. The connection is best made through an extension cord with a button and, preferably, through a leakage current circuit breaker (RCD) to protect yourself and the wiring.
When you turn it on, listen to the sound of the engine. It should be smooth, without strong vibrations and metal clanging. If you hear a hum, but there is no rotation, and after 2-3 seconds the relay clicks, it means the engine will not start. This may indicate a mechanical jam (rare) or a malfunctioning start relay. If the engine is running, but the housing heats up in 2-3 minutes, it means that the starting winding is not turned off - turn it off immediately!
Checking the pressure is the next step. If the compressor is used for a refrigeration circuit, a pressure manifold must be connected. A vacuum must be created at the suction, and a pressure corresponding to the type of refrigerant at the discharge. For modern R600a refrigerants and R134a, the characteristics may differ, so check the saturated vapor tables.
⚠️ Attention: Design details and electrical diagrams may vary depending on the manufacturer and year of manufacture of the compressor. Always check the technical documentation for a specific model before starting work.
Frequent errors and troubleshooting
Even experienced craftsmen sometimes make mistakes when working with compressor equipment. One of the most common is ignoring the condition of the oil. If the compressor was lying on its side for a long time or was turned over, oil could get into the circuit. Starting such a unit without preliminary flushing or settling can lead to water hammer, since liquid, unlike gas, is not compressed.
Another mistake is the use of wires of insufficient cross-section. The starting current of the compressor is 3-5 times the rated operating current. If the wiring is thin, there is a voltage drop across it, and the engine cannot develop the required power to start, which leads to humming and overheating. Use copper wires with a cross-section of at least 1.5 mm² for connection.
- 🔥 Overheating - often caused by lack of cooling or sticking relay contacts.
- 🔊 Noise and vibration - may indicate worn bearings or loose casing fastenings.
- ⚡ Knocking out plugs is a sign of a short circuit in the windings or a breakdown on the housing.
If the compressor hums, but does not start, you can try to briefly (no more than 1-2 seconds) apply voltage to the starting winding in parallel with the working one using a button. If this does not help, most likely the piston group is jammed. At home, it is almost impossible to revive such a unit; it is easier to replace it with a new one.
Is it possible to use a car capacitor for starting?
As a last resort, it is possible if its capacity and operating voltage (at least 400V) are suitable. However, specialized starting capacitors for electric motors have different characteristics of ripple currents and are better suited for this task.
Conclusion and final recommendations
Connecting a motor-compressor from a refrigerator is a task that requires precision, knowledge and caution. A correctly assembled circuit with a selected relay can extend the life of the unit and ensure stable operation of your cooling or blowing system. However, do not forget that hermetic compressors are not designed to operate forever in disassembled form or in abnormal conditions.
Regularly check the vibration level, case temperature and current consumption. Any deviation from the norm should be a signal to stop and re-diagnosis. Remember that the safety and security of your property depend on the quality of the work you perform.
Is it possible to connect a compressor without a relay?
Theoretically, you can start the engine by applying voltage to the starting winding manually through a button, and then turning it off. However, you cannot leave the compressor operating in this mode: the starting winding will burn out in a few minutes. For constant operation, a relay or its electronic analogue is necessary.
What kind of capacitor is needed for a refrigeration compressor?
Usually starting capacitors with an operating voltage of at least 300-450V are used. The capacity is selected based on the engine power, most often in the range of 20-50 µF. The exact value is indicated on the marking of the old capacitor or in the compressor passport.
Why does the compressor hum but does not start?
There may be several reasons: the mechanics are jammed, the starting winding is burned out, the starting relay is faulty (does not close the contacts) or the voltage in the network is too low. Oxidation of the contacts at the junction of the wires is also possible.
Is it dangerous to open the compressor casing?
Yes, it is extremely dangerous and impractical. Inside there is compressed gas (often flammable isobutane) and oil vapor. Opening the casing breaks the seal and requires vacuuming and filling, as well as professional welding. It is easier to replace the entire unit.