How to connect a motor from a refrigerator with three outputs and a capacitor

Using an old refrigerator compressor as a source of compressed air for painting, inflating tires or pneumatic tools is a popular task among home craftsmen. Such units, despite their age, often have an impressive service life and are capable of delivering pressure up to 10-12 atmospheres, which is quite enough for garage needs. However, before starting in a new role, you need to properly understand the electrical part, since simply plugging a plug into a socket here will not work.

The main difficulty is that the standard switching circuit in a household refrigerator implies the presence of a starting relay, which automatically turns off the starting winding after the shaft accelerates. When creating a homemade compressor, this relay is often replaced with a forced circuit with a capacitor or a button, which requires accurately determining the purpose of each of the three contacts on the motor terminal block. An error in the connection will lead to humming, overheating and rapid failure of the electric motor.

In this guide we will examine in detail the process of identifying pins, selecting a capacitor and assembling a reliable starting circuit. You will learn how to distinguish the working winding from the starting winding using a multimeter and why the polarity of the connection does not matter in this case, but the sequence of circuit assembly is critical. A competent approach will extend the life of the unit and ensure safe operation of a home-made installation.

Design of a single-phase asynchronous motor

Compressors of household refrigerators, be they popular models Atlant, Indesit or Beko, are equipped with single-phase asynchronous motors. The peculiarity of such motors is that to create a rotating magnetic field, they do not need one winding connected to a 220 volt network. That is why two separate windings are located inside the stator: the main (working) and auxiliary (starting) windings. The working winding occupies most of the stator slots and has a thicker wire, which provides less active resistance.

The starting winding is made of thinner wire and has a higher resistance. Its task is to create an initial impulse for rotating the rotor. In the classic refrigerator circuit, the starting relay is responsible for temporarily connecting this winding, which opens the circuit when the engine reaches a certain speed. In a capacitor circuit, we emulate the phase shift required for rotation by permanently or temporarily including a capacitor in the starting winding circuit. Understanding the physics of the process helps to avoid mistakes when the motor hums but does not spin.

The terminal block usually has three contacts forming a triangle. The leads from both windings and the common wire are suitable for them. It is important to understand that the windings are not physically connected to each other inside the housing, they are isolated. The connection occurs only through an external electrical circuit. If you plan to use the motor for a long time, it is worth considering that constant operation of the starting winding (in circuits with a working capacitor) can lead to its overheating, so calculating the capacitance capacitor is a critical step.

⚠️ Attention: Before any manipulations with the electrical part, make sure that that the compressor is completely disconnected from the network. Residual voltage in capacitors can be dangerous, although in older models it rarely persists for long.

Identification of terminals: finding common, starting and working windings

The very first and most important step is to determine which of the three contacts is common, and which belong to the starting and working windings. On older engines, the markings may be erased, and the colors of the wires (if they are preserved) do not always correspond to the standards. The only reliable way is to test the contacts using a multimeter set to resistance measurement mode (ohmmeter). You need to measure the resistance between each pair of contacts. In total, we have three measurements: between the first and second, second and third, third and first.

The measurement results will give three resistance values. The logic here is simple: the sum of the resistances of the two smaller values ​​is always equal to the largest value. The highest resistance is the sum of the resistances of the starting and operating windings, measured in series. Therefore, the two contacts between which we got the maximum value are the ends of both windings. The third contact, which did not participate in this maximum measurement, is common wire (Common).

After finding the common wire, the task becomes simpler. You need to measure the resistance between the common wire and each of the two remaining contacts. The contact, when measured with less resistance, refers to the working winding. The contact with high resistance is the output of the starting winding. For clarity, you can use the following table of typical values (they may vary depending on the motor power):

⚠️ Attention: If the multimeter shows a resistance close to zero or, conversely, infinity between any terminals, this indicates a short circuit or winding break. It is impossible to operate such a motor.

It is worth noting that on some modern or specific models of compressors, for example, from refrigerators LG or Samsung inverter type, the circuit may differ, but classic three-terminal motors obey the rule described above. After you have marked the leads (for example, with stickers “O”, “P”, “P”), you can begin assembling the circuit.

Choosing a capacitor: calculation of capacitance and voltage

The capacitor in the connection diagram acts as a phase shifter. It creates a phase shift in the current in the starting winding, which allows the creation of torque. There are two main types of connection: with a starting capacitor (which turns off after the start) and with a working capacitor (which remains in the circuit permanently). For homemade compressors, a combined circuit or simply a starting capacitor is most often used, since the refrigeration engine is designed for short-term operation of the starting winding.

The capacitance of the capacitor is selected experimentally or according to a formula depending on the engine power. A rough but workable calculation for a starting capacitor looks like this: for every 100 watts of power, approximately 7-10 microfarads (uF) are required. However, since the power of compressors is rarely known exactly, it is easier to base it on current. Typical values ​​for domestic refrigerators range from 20 to 50 µF. Using too large a capacitance will cause the windings to overheat, and using too small a capacitance will cause the engine to be difficult to start under load.

A critical parameter is the operating voltage of the capacitor. It should be at least 450 Volts, and it is better to take it with a reserve - 500 Volts and above. 250 or 300 Volt capacitors in such a circuit will not last long and may explode due to voltage surges during self-induction of the windings. Specialized starting capacitors in a round black case are ideal, often labeled as Start or CD60.

Is it possible to use several capacitors?

Yes, you can connect capacitors in parallel. In this case, their capacities are summed up. For example, two 20 µF capacitors will give a total of 40 µF. The main thing is that the voltage of each of them meets the requirements of the circuit (at least 450V). A series connection reduces the total capacitance and is not used in this case.

Connection diagrams: step-by-step instructions

Let's consider the most common and safe connection diagram with a starting capacitor and a button. In this circuit, the starting winding is energized only at the moment of start, and then the circuit opens. This is as close as possible to the normal operating mode of the refrigerator and protects the engine. You will need: a starting capacitor, a button (without fixation or with fixation, but with the ability to open the circuit), wires and a plug.

First, we connect one end of the working winding and one end of the starting winding together and connect it to the neutral wire of the network (or one of the plug contacts, since for an asynchronous motor the phase and zero are interchangeable, but for safety it is better to adhere to the standards). We connect the second end of the working winding directly to the phase wire of the network. We connect the second end of the starting winding to one contact of the button. We connect the second contact of the button to the capacitor, and the second terminal of the capacitor is connected to the phase wire of the network (parallel to the working winding).

The startup algorithm looks like like this: you press a button, applying voltage to the starting winding through a capacitor. The engine starts to rotate. As soon as you hear a confident buzz and feel pressure at the outlet (usually after 1-2 seconds), the button must be released. The motor will continue to operate on the working winding. If the engine is powerful or the load is heavy, the button may have to be held a little longer, but not more than 3-5 seconds.

⚠️ Attention: Do not hold the start button pressed for more than 5 seconds. If the engine does not start, it means the problem is mechanical (jammed piston) or electrical (insufficient capacitor capacity, break). Holding the button for a long time will lead to the burning of the starting winding, which is not designed for direct current.

Nuances of operation and safety

When operating a homemade compressor Moreover, such motors require normal ambient temperature. If you plan to use the compressor in a cold garage during the winter, the oil in the crankcase will thicken and the engine may not turn over or burn out when you try to start. In such cases, preheating is required.

Particular attention should be paid to the cooling system. In the refrigerator, the motor is cooled by freon and air flow inside the housing. When working “in the air” it heats up much more. Be sure to install a copper tube several meters long, rolled into a coil, or a radiator, onto the outlet pipe. This will not only cool the air, but will also help remove some of the heat from the cylinder head. Control the body temperature with your hand: if it exceeds 70-80 degrees, you need to take a break.

It is also worth mentioning the oil. Some compressors (especially older ones) may release oil along with the air. For the cleanliness of pneumatic tools and painting work, an oil and water separator must be installed at the outlet. Without it, your equipment will quickly fail, and defects will appear on the surface to be painted. Regularly check the oil level through the inspection window, if provided by the design, or monitor the operation by sound.

Typical problems and methods for solving them

Even with the correct connection, difficulties may arise. The most common problem is that the engine hums but does not turn over. This means that the starting torque is insufficient. The reasons may be low voltage in the network, a malfunction of the capacitor (loss of capacity) or, worse, mechanical jamming of the piston group. If, after replacing the capacitor with a known good one of larger capacity (within reasonable limits), the motor does not start, it probably requires repair or replacement.

Another problem is severe vibration. It occurs due to the fact that the compressor is rigidly fixed to a metal base. In the factory version it hangs on springs. For homemade installation, use rubber gaskets, automotive silent blocks or pieces of thick rubber under the mounting feet. This will extend the life of the tube connections and reduce the noise level.

Overheating and knocking out plugs may indicate an interturn short circuit in the windings. If during operation the smell of burnt insulation intensifies and the current consumption (measured by current clamps) exceeds the rated value indicated on the nameplate, operation should be stopped immediately. Also monitor the integrity of the supply pipes: moisture or dust getting inside the compressor through the inlet filter is unacceptable and will quickly damage the valve group.

Is it possible to connect the engine without capacitor?

Theoretically, it is possible to start a single-phase motor by giving the shaft an initial impulse manually, but for a compressor this is impossible due to the high discharge pressure. Without a capacitor or a starting relay, it will not be possible to create torque; the engine will only hum.

Which capacitor is better: oil or electrolytic?

For starting purposes, special non-polar electrolytic capacitors (Start), which are compact and cheap, are best suited. Oil capacitors (CBB60, CBB61) are more reliable for continuous operation, but have large dimensions. For a circuit with a start button, both types are suitable, the main thing is a voltage of 450V.

Why does the compressor turn off after a while?

Most likely, the thermal protection relay built into the compressor housing is triggered. This occurs due to overheating (poor cooling, high ambient temperature) or overcurrent (clogged filter, high pressure in the receiver, faulty valves). Let the engine cool and check the system.

Is it necessary to change the oil in the compressor during refurbishment?

In hermetic refrigerator compressors, changing the oil is extremely undesirable and difficult, as it requires opening the housing or evacuating the system. If the compressor was taken from an old but working refrigerator, the standard oil should be enough for a long time. You only need to add special compressor oil; regular motor oil can destroy the rubber seals.