In situations where the standard electrical circuit of the refrigerator is broken, and calling a technician is impossible or economically impractical, it becomes necessary to start the motor-compressor directly. Most often, this need arises when checking the serviceability of the unit, using a dismantled motor as a homemade compressor for painting or inflating tires, or when the standard starting relay burns out. However, direct connection to a 220-volt household network requires a clear understanding of the physics of the process and strict adherence to safety precautions.
The main difficulty is that single-phase asynchronous motors used in household refrigerators are not able to start on their own without creating a rotating magnetic field. For this purpose, the design provides starting winding, which is turned on only during the start. If you apply voltage only to the main winding, the motor will only hum, but the shaft will not rotate, which will lead to rapid overheating and burning of the insulation. Therefore, the key task is to correctly simulate the operation of a standard starting relay.
Before starting any work with electrical circuits, it is necessary to completely de-energize the equipment. Working with 220V voltage poses a real threat to life, and incorrect switching of windings can lead to a short circuit or failure of expensive equipment. In this article we will analyze in detail the design of the contact group, methods for identifying pins and methods for safely starting the engine bypassing standard automation.
The design and principle of operation of a single-phase motor
The heart of any refrigeration unit is a hermetically sealed motor-compressor, inside which an electric motor and a pump part are combined. The electrical part is a classic asynchronous motor with a squirrel-cage rotor. A special feature is the presence of two types of windings on the stator: main (working) and additional (starting). The working winding occupies two-thirds of the stator slots and ensures rotation of the shaft in normal mode, while the starting winding is turned on only for a few seconds to create the initial torque.
The separation of the windings occurs due to the different physical characteristics of the wire. The starting winding is wound with a thinner wire and has high active resistance, which allows you to limit the current at the moment of start. The working winding is made of thicker wire and has low resistance. In the standard circuit, the starting relay is responsible for switching between them, which opens the starting winding circuit after the engine reaches approximately 75% of the rated rotation speed.
There are two main types of starting devices that can be found in old and modern models of refrigerators. In devices with a posistor-type start-up protection relay, the role of a switch is played by a thermoelement that changes its resistance when heated by current. Older models used relays with an electromagnetic breaker, where the current in the operating winding created a magnetic field that kept the starting circuit contacts closed. Understanding the type of relay is important for correct diagnosis, but with direct connections we often simulate their operation manually or using buttons.
⚠️ Attention: The compressor sealed casing is under pressure. It is strictly forbidden to open the motor housing, drill holes or damage the integrity of welds. Inside there may be a refrigerant, which, when mixed with air, forms an explosive mixture, as well as a special oil that is harmful to the skin.
Identification of contacts and checking windings
Before applying voltage, it is necessary to accurately determine which contact corresponds to which winding. On the compressor housing there is usually a terminal block with three terminals arranged in a triangle. There may be no standard markings, so you will have to focus on their relative position and electrical resistance. In most models, horizontally located contacts are the leads of the working winding, and the upper vertical contact is the common lead or output to the starting winding, but you cannot rely on geometry without continuity.
For accurate diagnostics, you will need a multimeter switched to resistance measurement mode (Ohm). First you need to find a pair of contacts with the lowest resistance - this will be the working winding. The resistance here usually varies between 10-30 ohms. Then the resistance is measured between the remaining third contact and each of the ones found. A pair with high resistance (usually 30-50 ohms) will indicate the starting winding. The sum of the resistances of both windings should be approximately equal to the resistance between the two extreme contacts of the triangle.
It is also important to check the motor for insulation breakdown (short circuit to the housing). To do this, one multimeter probe is applied to any contact of the block, and the second is applied to a place on the metal body of the compressor that has been cleared of paint. The device should show infinity (one in the most significant digit). If the multimeter shows any resistance value, it means that the insulation is broken, and operation of such a motor is strictly prohibited due to the risk of electric shock.
Table of typical winding resistances
|Power compressor|Working winding resistance|Starting winding resistance|
|100 W|15-20 Ohm|35-45 Ohm|
|150 W|10-15 Ohm|25-35 Ohm|
|200 W|8-12 Ohm|20-30 Ohm|
|300 W|5-9 Ohm|15-25 Ohm|
Direct connection diagram via a button
The most common and safest way to start a compressor without a standard relay is to use a regular button with normally open contacts (for example, a doorbell or a machine START button). The essence of the method is to manually apply voltage to the starting winding only at the moment of start, and then release the button, leaving power only on the working winding. This completely simulates the operation of a mechanical relay.
To implement this circuit, you will need a power cord with a plug, a button and two wires. One wire from the plug (phase) is connected directly to the contact of the working winding of the compressor. The second wire from the plug (zero) goes to the common motor contact. Key point: from the contact of the working winding, a tap is made to one contact of the button, and the second contact of the button is connected to the output of the starting winding. Thus, when the button is pressed, voltage is applied in parallel to both windings, the engine starts, and the button must be immediately released.
An alternative connection option involves using a two-key switch or two separate buttons for more convenient control, although this is redundant for one-time operations. The main rule here is not to keep the starting circuit on for more than 2-3 seconds. Prolonged supply of voltage to the starting winding will lead to its overheating and interturn short circuit, since it is not designed for constant operation. The engine operating time in this mode should not exceed 10-15 minutes in a row to avoid overheating.
☑️ Check before starting
Using a capacitor to start
In some cases, especially if the compressor is difficult to start or is used to operate as an air compressor under load, a simple button may not be enough. In such situations, a starting capacitor is used. This device allows you to create a phase shift of the current in the starting winding, which significantly increases the starting torque of the engine. The capacitor is connected in series to the starting winding circuit.
The capacitance of the capacitor is selected experimentally or according to the table, usually it ranges from 4 to 10 μF for household refrigerators, with an operating voltage of at least 400V (preferably 450V). The connection diagram remains similar to the push-button one: the capacitor is connected to the break in the wire going to the starting winding, and the button serves to briefly close this circuit. After starting the engine, the button opens and the engine runs only on the working winding.
The use of a capacitor is especially important if you plan to use the motor-compressor not for its intended purpose, but as a pump for aeration, painting or pneumatic tools. In such conditions, the load on the shaft is higher than when working with freon, and the additional starting torque helps to avoid rotor jamming. However, it is worth remembering that working capacitor (which remains in the circuit constantly) is usually not used for refrigeration compressors, since they are designed to operate without a permanent phase-shifting capacitance.
Organization of the cooling and lubrication system
When the compressor is operating in normal refrigerator mode, it the housing is washed with refrigerant vapor, which effectively removes heat from the windings. When connected directly to the network, without a closed refrigerant circuit, the engine will heat up significantly more. If you use a motor to create a compressor, you must ensure forced air cooling of the housing, for example, by installing a fan directed at the radiator fins or the casing itself.
The second critical point is lubrication. The compressor crankcase is filled with a special refrigeration oil that does not absorb moisture and has properties. When operating outdoors, a large volume of atmospheric air containing water vapor will pass through the system. Water, condensing in oil, turns it into an emulsion that loses its lubricating properties. This leads to accelerated wear of the rubbing pairs and eventual jamming.
For long-term operation as an air pump, it is recommended:
- ⚙️ Replace the standard oil with a more viscous, non-hygroscopic one (for example, diesel or special compressor oil).
- 🌬️ Install a moisture separator (oil and moisture separator) at the outlet of the compressor.
- 🌡️ Monitor the housing temperature, avoiding heating above 80-90 degrees Celsius.
- 🔧 Regularly check the oil level through the inspection window or dipstick (if the design allows).
If the compressor is used briefly to check the refrigerator, these measures are not necessary, but it is necessary to monitor the heating. When touched by hand, the body should be hot, but not scalding. If the engine overheats, operation should be stopped immediately and allowed to cool.
Table of typical startup faults
Even with correct connection, problems may arise that indicate hidden equipment defects. Below is a table that helps diagnose the condition of the motor-compressor based on external signs and instrument readings.
| Symptom | Possible cause | Action |
|---|---|---|
| Humming, but no rotation | Mechanical part jammed or starting winding broken | Check the resistance of the starting winding, try to rotate the shaft |
| Tripping of the circuit breaker | Short circuit in the windings or breakdown to the housing | Urgently turn off, ring the windings on the short circuit and on the housing |
| The engine starts and immediately stalls | Malfunction of the starting device or low voltage in the network | Check the button/relay, measure the voltage in socket |
| Strong vibration and noise | Destruction of the internal suspension or wear of bearings | Operation is possible, but the resource is very limited |
| Heating of the case until red hot | Operation no load or system clogging | Check the outlet pressure, ensure cooling |
⚠️ Attention: Technical characteristics of oils and permissible temperatures may vary depending on the compressor manufacturer (Danfoss, Secop, LG, Atlant). Always check the technical documentation for a specific model if you plan long-term operation.
Safety measures and final recommendations
Connecting the compressor directly is a temporary solution or diagnostic method, but not a method of permanent repair of a household refrigerator. The standard starter relay performs a critical function: it protects the motor from overcurrent. If during operation there is a power surge or jamming of the piston group, the relay will break the circuit, saving the windings from combustion. When operating via a button, this protection is absent, and you take control of safety yourself.
Never leave a running compressor unattended. If there is smoke, sparks, or an unusual burning smell, immediately unplug the plug from the outlet. All wire connections must be reliably insulated; it is advisable to use terminal blocks or heat shrink, and not just twists with electrical tape, which can slip due to heat.
If your goal is to restore functionality refrigerator, after successfully starting the engine, be sure to replace the starting relay with a new one that matches the motor model. The price of the relay is low, and the risk of re-burning of the windings when operating without it is too high. For homemade compressors, it is recommended to assemble a full-fledged panel with a circuit breaker, a pressure switch and a pressure gauge.
Frequently asked questions (FAQ)
You can Is it possible to connect a compressor without a capacitor?
Yes, most household refrigeration compressors are designed to start through a start relay without using a capacitor. With a direct connection, it is enough to briefly apply voltage to the starting winding using a button. A capacitor is needed only if the engine starts poorly or is used under heavy load.
How to determine whether the compressor has burned out?
The main symptom is the resistance of the windings. If the multimeter shows infinity (open) or zero (short circuit) on the operating or starting winding, the motor is faulty. Combustion is also indicated by a breakdown on the housing (the resistance between the contact and the housing is not infinite).
How long can you keep the start button pressed?
The start button should be held for no more than 2-3 seconds until the engine picks up speed. Holding it for a long time will lead to overheating of the starting winding and its combustion, since it is not designed for long-term operation under current.
Why does the compressor hum but does not start?
This may indicate jamming of the mechanical part (piston group) due to lack of oil or foreign objects. Also, the reason may be a break in the starting winding or a malfunction of the starting device, which does not supply a start impulse.
Is it dangerous to use such a compressor for painting?
You can use a refrigeration compressor for painting, but with caution. It produces little air, but under high pressure. The main danger is the presence of oil in the exhaust air, which can ruin the paint job. A high-quality oil and water separator is required.