The refrigerator is the heart of a modern kitchen, and its compressor is the very motor that makes this system work. When you hear a characteristic humming noise coming from the back of the unit, this means that a complex physical process of refrigerant compression is taking place inside the sealed metal cylinder. Many owners of household appliances are wondering: what exactly is hidden behind the durable walls of this black cylinder and why can’t it just be disassembled and put back together?
The internal structure of the compressor is a high-precision mechanism that operates in extreme conditions. Inside there is not just an electric motor, but a whole system where electrical energy is converted into mechanical movement, creating the necessary pressure in the circuit. Understanding what is insidehelps to better diagnose malfunctions, such as overheating, failure to start, or increased noise during operation.
In this article we will examine in detail the design of a piston compressor, which is the most common type in household refrigerators. We will look under the casing to look at the interaction of the windings, piston group and valves, and also explain why the internal cavity is filled with oil and is under pressure.
General design and purpose of a sealed housing
Visually, the compressor is a black metal cylinder, often coated with enamel to protect against corrosion. This housing is not accidental: it is sealedthat fundamentally distinguishes refrigeration compressors from conventional air compressors. Inside this “black box” there is an atmosphere of inert gas or freon vapor, and the pressure can be either vacuum or excess, depending on the phase of operation and the type of refrigerant.
The main function of the housing is to hold refrigerant vapor and compressor oil, preventing their leakage into the atmosphere. Any depressurization, even microscopic, leads to loss of performance of the entire refrigeration system. Inside the cylinder, on special shock-absorbing springs, the unit itself is suspended - a combination of an electric motor and a compressor mechanism. Such a suspension is necessary to reduce the level of vibration and noise transmitted to the refrigerator body.
It is important to note that the internal space of the case also serves to cool the motor windings. The refrigerant vapors sucked through the pipe wash the hot parts of the engine before entering the cylinder. This design solution allows heat to be effectively removed without the use of additional fans, which makes the operation of the device almost silent (except for the sounds of mechanical operation).
⚠️ Attention: It is strictly forbidden to open the sealed compressor housing at home for the purpose of repair. There may be gas under pressure inside, and a violation of the factory seal will lead to instant entry of moisture and air, which will destroy the system.
The housing is equipped with three external tubes: two copper (for connection to the circulation system) and one (often steel or copper) for charging oil and refrigerant at the factory. It is through these “veins” that the working substance moves, but the magic of energy conversion itself is hidden deep inside.
Electrical part: stator, rotor and starting equipment
If you look inside the housing, the first thing that catches your eye is the electric motor, which is the drive for the compressor. Most household models use an asynchronous motor with a squirrel-cage rotor. It consists of a stationary part - stator, and a rotating part - rotor. The stator is a set of metal plates with copper windings wound on them.
The windings are divided into two main ones: working and starting. The working winding provides the main rotation, creating a magnetic field. The starting winding is turned on only at the moment of start in order to create the necessary torque to overcome inertia and pressure in the system. As soon as the rotor picks up speed, the starting winding is turned off. In old models, the starting relay was responsible for this, in modern ones - electronics or built-in thermal protective devices.
- 🌀 Stator - a stationary part of the engine that creates a magnetic field for rotation.
- ⚙️ Rotor - a “squirrel cage” that rotates inside the stator and transmits movement to the shaft compressor.
- 🔌 Starting relay - a device that briefly supplies current to the starting winding to start the motor.
The rotor is pressed onto a vertical shaft, which is integral with crank mechanism. The motor windings are impregnated with special varnishes that are resistant to freon and oil. When the windings burn out (turn-to-turn short circuit), the oil inside the compressor often becomes acidic, which requires a complete flushing of the entire refrigerator system, otherwise the new compressor will burn out very quickly.
Mechanical part: piston group and valves
The heart of the mechanical part is piston group. This is where the refrigerant gas is compressed. The engine shaft, rotating, drives the crank, which converts the rotational motion into the translational movement of the piston inside the cylinder. This process resembles the operation of an internal combustion engine of a car, but without a fuel combustion stroke.
The operating cycle is as follows: when the piston moves down, a vacuum is created in the cylinder, and freon gas is sucked in from the evaporator through the inlet valve. When the piston begins to move upward, the intake valve closes and the gas is compressed. When a certain pressure is reached, the exhaust valve opens and the high-pressure compressed gas is pushed into the condenser (radiator on the rear wall).
| Component | Function | Material |
|---|---|---|
| Piston | Refrigerant compression | Aluminum alloy / Steel |
| Connecting rod | Motion transmission from the shaft | Steel |
| Valve plate | Gas flow adjustment | Elastic steel |
| Cylinder | Working chamber | Hardened steel |
| Works in conjunction with cylinder | Connects the piston with the crank | Works like a reed valve |
| Withstands high pressure | Converts rotation into stroke | Opens/closes under pressure |
| Minimum clearance with the piston | Precision mechanics | High strength |
Particular attention should be paid to valves. In modern compressors they are most often used repetal valves. These are thin metal plates that act as check valves. They must be perfectly polished and fit against the seat to prevent gas from flowing back. Wear or breakage of the valve plate is a common reason that the compressor hums, but does not produce cold.
Lubricating system and the role of compressor oil
In the lower part of the compressor housing, at the bottom, there is always a supply of special compressor oil. Unlike automobile oils, it has unique properties: it does not freeze at low temperatures (which can occur in the suction pipe) and is chemically inert in relation to freon and sealing materials.
Lubrication of rubbing parts occurs by gravity and splashing. At the lower end of the crankshaft there is often a special scoop or screw thread, which, when the shaft rotates, captures oil and lifts it up to the bearings and piston group. Oil creates a thin film between the piston and cylinder, ensuring tightness and reducing friction.
The oil level is critical. If it is not enough, scuffing and jamming of the mechanism will occur. If there is too much oil or it foams, it may occur water hammerwhen liquid (oil) enters the cylinder, which is intended only for gas. This can lead to damage to the valves or connecting rod.
Why should you not mix different oils?
Mixing mineral and synthetic oils (for example, POE and MINERAL) can lead to the formation of sediment that will clog the capillary tube of the refrigerator. Always use only the type of oil specified by the compressor manufacturer.
Types of compressors: from classic to inverter
Although the principle of gas compression remains similar, the internal layout may vary. Classic piston compressordescribed above is the most widespread. It is reliable, repairable (in theory), but has a number of disadvantages: high levels of noise and vibration, as well as cyclic operation (on-off).
More modern ones are rotary i spiral compressors, which are often found in expensive models or No Frost systems. In rotary models, compression occurs due to the rotation of the roller in the cylinder, which makes the operation smoother. Scroll compressors use two spiral bowls, one of which is movable. They have the highest efficiency and low noise level.
- 🔊 Piston —noisy, vibrate, but cheap and easy to repair.
- 🤫 Rotary — quiet, compact, but sensitive to the quality of the voltage in the network.
- 📉 Inverter - work constantly, changing speed, which saves energy and preserves products better.
They deserve special attention inverter compressors. The same piston or rotary mechanism can be used inside them, but the engine is controlled not through a simple on/off relay, but through a frequency converter. This allows the shaft to rotate at different speeds, smoothly maintaining the temperature without sudden jumps.
Diagnostics of internal faults
Understanding the internal structure helps to suggest the cause of the breakdown based on external signs. If the compressor hums, but does not start and clicks, most likely the problem is in the start relay or a jammed piston group. If the motor runs but does not cool, the connecting rod may break (a metal ringing can be heard) or the valves may break.
Overheating of the housing often indicates a lack of refrigerant (the motor is not cooled by freon vapor) or an interturn short circuit in the windings. In the latter case, the oil inside often changes color to dark brown or black and takes on a burning smell. You can check the condition of the windings with a multimeter by ringing the terminals on the case (usually there are three of them: common, starting, working).
⚠️ Attention: If you smell burning or see blackening of the oil, simply replacing the compressor is not enough. It is necessary to flush the entire refrigerator system (condenser, capillary, evaporator) with a special solvent or nitrogen, otherwise the acid from the old oil will kill the new unit.
☑️ Symptoms of a faulty compressor
Why you can’t repair a compressor yourself
Many enthusiasts try to open the housing, replace valves, or even rewind the engine. This is theoretically possible, but in practice such compressors rarely last long. Factory assembly is carried out under sterile conditions, and the housing is welded in an inert gas environment. It is almost impossible to restore such tightness in a garage.
In addition, the balancing of the rotating parts is carried out with the highest precision. Any intervention upsets the balance, which leads to strong vibration and rapid destruction of the bearings. Therefore the only reliable solution when a compressor breaks down is to completely replace it with a new analogue.
When replacing, it is important to choose a model with similar characteristics: cooling capacity, type of refrigerant (R134a, R600a) and type of oil. Installation of a new unit requires evacuation of the system and precise charging with freon, which is only possible with the use of professional equipment.
Frequently asked questions (FAQ)
Why is the compressor hot to the touch?
This is normal. During the compression process, the gas heats up, transferring heat to the housing. In addition, the motor windings also generate heat. The case temperature can reach 60-80 degrees Celsius. However, if it is hot to the point where you cannot touch it, or smells like burnt wiring, this is a sign of a malfunction.
Is it possible to replace an R134a compressor with an R600a?
No, it is not possible. These refrigerants require different types of oil and have different operating pressures. A compressor designed for isobutane (R600a) has a different internal channel design and electrical parameters. Replacement is only possible with a similar type.
How long does a compressor last in a refrigerator?
The average service life of a modern compressor is 10-15 years. However, with frequent power surges, overheating or improper operation (for example, installing the refrigerator close to the wall without a gap for ventilation), the service life can be reduced to 5-7 years.
Why does the compressor turn on and immediately turn off?
This may indicate that the thermal protection relay has tripped. Reasons: malfunction of the starting relay, jamming of the mechanism, short circuit in the windings or critically high pressure in the system due to blockage or excess freon.