When the refrigerator stops working cool, and the diagnostics indicate a motor malfunction, the technician is faced with the question of further actions. Often the unit must be dismantled for a detailed inspection, oil change or circuit leak check. It is at this point that it is critically important to conduct a quality check of a removed compressorin order not to install a obviously faulty part back into the system.
The testing procedure requires care and compliance with safety precautions, since you are dealing with electricity and potentially dangerous gases. Incorrect diagnostics can lead to a false conclusion about the operability of the unit or, conversely, to the rejection of a serviceable unit. In this article we will analyze all stages of testing, from visual inspection to complex electrical measurements.
The main purpose of the test is to determine the integrity of the windings, the absence of interturn short circuits and the ability of the motor to create the required pressure. If you plan to use used compressor or restore an old one, these steps will help save time and money by avoiding having to re-repair the refrigerator a week after assembly.
Visual inspection and preparation for tests
The first step should always be a thorough external inspection. Even if the compressor is removed from the refrigerator, there may be signs of serious problems on its housing. Pay attention to the condition of the pipes: if oil leaks from the suction tube or traces of active corrosion are visible, this may indicate a leak or wear of the piston group.
Check the integrity of the electrical contacts and the absence of melting on the block. Often the cause of failure is not the motor itself, but a burnt start relay or thermal relay, which are easily replaced. It is also important to make sure that the casing does not have dents that could damage the internal motor suspension.
⚠️ Attention: If gas with a pungent odor or hissing escapes from the compressor pipes during removal, work should only be carried out in a well-ventilated area. indoors. Although modern refrigerants are often safe, it is better to be on the safe side.
occupied room. Although modern refrigerants are often safe, it is better to be on the safe side.
You will need a minimum set of tools to carry out the tests. Prepare a multimeter, a set of screwdrivers, pliers and, preferably, a pressure gauge manifold to check the pressure. Don't forget about personal protective equipment, especially gloves, as the oil may be hot or contain acidic impurities.
Checking the integrity of the windings with a multimeter
The very first and most important electrical test is checking the windings for breaks. To do this, switch the multimeter to resistance measurement mode (Ohms). There are usually three contacts on the compressor block: common (C), starting (S) and working (R). Their location may differ between different brands, such as Atlant, Indesit or Beko, so it is better to check the technical documentation.
Ring pairs of contacts. Resistance should be displayed between common and working, common and starting, as well as between working and starting. If the device shows one or infinity (∞), this means an open circuit. In this case, the compressor cannot be repaired and requires replacement.
Normal resistance values depend on the motor power, but are usually within the following limits:
- 🔹 Between the common and working contacts: from 10 to 30 Ohms
- 🔹 Between the common and starting contacts: from 20 to 60 Ohm
- 🔹 The sum of the resistances should be approximately equal to the resistance between the working and starting winding
It is important to understand that the starting winding always has greater resistance than the working winding, since it is made of thinner wire. If the values differ radically from the norm or are equal to zero, this is a sign of a serious malfunction inside the case.
Diagnostics for breakdown to the case (short circuit)
One of the most dangerous malfunctions is a breakdown of winding insulation on the metal body of the compressor. This may result in an electric shock when touching the refrigerator or permanent knocking circuit breaker in the electrical panel. The test is carried out in the megohmmeter mode or, as a last resort, at the maximum resistance limit of the multimeter.
One probe is applied to any contact on the block (or a cleaned place on the pipe), and the second is applied to a part of the metal body that has been cleared of paint and rust. The device should show infinite resistance. Any readings, even a few megaohms, indicate an insulation failure.
| Test parameter | Normal value | Fault symptom | Action |
|---|---|---|---|
| Winding resistance | 10-60 Ohm (depending on the pair) | 0 Ohm or ∞ (infinity) | Replacing the compressor |
| Breakage to the housing | ∞ (Infinity) | Any numerical value | Urgent disposal |
| Resistance between windings | Rp + Rp ≈ Rtotal | Significant discrepancy | Diagnostics of interturn |
If a breakdown to the housing is detected, the operation of such a unit is strictly prohibited. Attempts to isolate the breakdown site from the outside are useless, since the breakdown is located inside a sealed casing filled with oil.
Identification of interturn short circuit
The most difficult problem to diagnose is interturn short circuit. In this case, the insulation is broken between the turns of one winding, but the circuit is not broken and the housing is not pierced. The multimeter may show normal resistance, but the compressor will hum, quickly heat up and turn off by the thermal relay.
An indirect sign of an interturn short circuit is an abnormally low winding resistance compared to the reference values for this model. However, a more accurate method is to measure the current consumption at startup. To do this, the compressor must be connected to the network through an ammeter or current clamp.
If the starting current significantly exceeds the rated current (indicated on the nameplate, usually 0.3-0.8 A for household refrigerators) and the motor hums, but does not start or stalls immediately, there is an interturn short circuit. This is also indicated by rapid heating of the housing within 1-2 minutes of operation.
⚠️ Attention: Do not keep a compressor with signs of an interturn short circuit on for more than 2-3 minutes. This can lead to final combustion of the windings and a fire.
For accurate diagnostics, professionals use special stands that measure no-load current. At home, you can focus on the sound: a smooth hum turns into a strained hum if there are problems with the coils.
Why does the compressor heat up?
The compressor can heat up not only due to a short circuit. Overheating is often caused by insufficient refrigerant in the system, a faulty start relay, or a clogged capillary. Therefore, it is better to check the removed compressor in conjunction with other elements.
Checking performance and pressure
Electrical service is only half the battle. The mechanical part must create sufficient pressure to circulate the refrigerant. To check, put a hose with a pressure gauge on the outlet pipe (discharge). When turned on for a short time, the pressure should increase quickly.
It is considered normal to create a pressure of 6 atmospheres or higher in a few seconds. If the pressure gauge needle barely creeps or stands still, it means that the piston group is worn out, the valves do not hold, or the connecting rod has broken. In this case, the compressor “drives” the gas, but does not create compression.
Also check the suction pipe. When operating, a working motor should create a noticeable vacuum. If you bring your finger to the entrance, there should be a slight suction. The lack of suction indicates the same thing - a mechanical breakdown inside.
☑️ Checking the mechanics
In a real refrigerator system, the circuit resistance is much higher than when blown into the atmosphere. Therefore, even a good result on a removed unit does not provide a 100% guarantee of operation in the system.
Analysis of oil condition and internal noise
When checking a removed compressor, be sure to listen to the sounds of its operation. A working unit operates relatively quietly, with a uniform rhythmic hum. Knocking, clanging, metallic grinding or intermittent humming indicate mechanical damage.
Pay special attention to oil that may leak from the pipes. It should be transparent or slightly yellowish. If the oil is black, has a burning smell, or contains metal shavings, the compressor has leaked. The breakdown products of oil and metal will enter the system and clog capillary tube or the filter drier.
In such cases, simply replacing the compressor will not help - a complete flushing of the refrigerator system with nitrogen and solvents is required. Ignoring this stage will lead to failure of the new compressor in a short time.
| Oil color | System condition | Recommendation |
|---|---|---|
| Transparent, light | Normal | Compressor is suitable |
| Dark brown | Overheating, aging | System flushing required |
| Black, with a burning smell | Combustion of the windings | Complete replacement of the circuit |
| Cloudy, interspersed | Moisture or acid | Replacing the filter and oil |
⚠️ Attention: If the oil has a greenish tint or a sharp sour smell, this is a sign of freon decomposition and acid formation. Such an environment is aggressive and will destroy the new compressor in a matter of days.
Launch and testing under load
The final stage of testing is a test run with a simulated load. To do this, you can connect a hose lowered into a container with oil to the discharge pipe. The bubbles should flow in an intense stream. However, a more professional approach is to use a stand with output throttling.
When starting, pay attention to the starting current. It briefly increases by 3-5 times, but then should drop to operating values. If the current does not drop, and the motor hums and the relay clicks, the problem is mechanical jamming or a strong interturn short circuit.
Let the compressor run for 5-10 minutes. During this time, the body should not heat up to the point where it is impossible to hold your hand (above 60-70°C). Excessive heating is a sure sign that the resource of the unit has been exhausted.
If all stages are completed successfully, the compressor can be considered conditionally suitable for installation. However, remember that the resource of a removed used unit is always less than that of a new one. When deciding to install a proven old motor, you take on the risk of repeated failure.
Is it possible to check a compressor without a multimeter?
It is impossible to accurately determine the condition of the windings without instruments. However, performance can be indirectly judged by the startup sound and the generated air flow at the outlet. If the motor hums smoothly and blows strongly, the electrical part is most likely intact, but this does not exclude an interturn short circuit.
What to do if the resistance of the windings differs by 20%?
The variation in resistance between pairs of contacts is acceptable within small limits, but 20% is a lot. Most likely, there is a partial interturn short circuit. Such a compressor may work, but its reliability is questionable, and it will consume more energy.
Is it necessary to change the oil in a removed compressor?
If the oil is clean and its level is normal, replacement is not required. Hermetically sealed compressors are designed to last a lifetime without oil changes. You can add or change oil only when the system is completely vacuumed, which is extremely difficult to do at home.
Why does the compressor hum but does not start?
There can be three reasons: a jammed mechanical part, a faulty start relay, or an interturn short circuit. If during an artificial start (jump) the motor starts to work, the problem is probably in the starting device or initial wear of the bearings.