A sudden stop in the operation of the refrigeration chamber or, conversely, its continuous hum often becomes the first bells indicating problems with the heart of the unit - compressor. It is this unit that is responsible for circulating the refrigerant through the system, creating the necessary pressure to cool the products. When motor-compressor stops starting or works intermittently, the temperature inside the chambers rises rapidly, threatening the safety of reserves.
Before calling a technician or writing off the equipment, it is worth conducting an initial diagnosis, to understand whether the problem really lies in the engine or whether the malfunction is more superficial. Many owners immediately blame the engine, although in half of the cases the fault is starting relay or the thermostat. Understanding the principles of operation of an electrical circuit and the ability to use a simple measuring instrument will allow you to save a significant amount on diagnostics.
In this article we will analyze in detail the algorithm of actions that will help identify the true culprit of the breakdown. You will learn to distinguish a mechanical knock from an electrical hum, check the integrity of the windings and determine the interturn short circuit. A competent approach to the question “how to check the serviceability of a refrigerator motor” will save you from buying unnecessary spare parts and will help you make an informed decision on repairs.
Primary visual and acoustic diagnostics
Any professional diagnostics begin long before you get your hands on a multimeter. The first step is always a thorough external inspection and analysis of the sounds that the unit makes when attempting to start. If you hear a characteristic click, followed by a hum lasting 3-5 seconds, and then again a click and silence, this is a classic symptom that start relay is trying to start the engine, but is unsuccessful.
Listen carefully to the nature of the noise. A smooth, quiet hum from a running compressor is normal. However, if you hear a metallic clanging, knocking or intermittent humming noise that ends abruptly, this may indicate a mechanical piston blockage or failure of the suspension inside the housing. In such cases, further operation of the equipment is dangerous.
⚠️ Attention: If, when you try to start, you smell a strong smell of burnt wiring or see sparking in the area of the terminal box, immediately disconnect the refrigerator from the network. Continuing tests may result in a fire.
It is also worth inspecting the back of the refrigerator for visible damage to the wires, oxidation of contacts or traces of soot. Often the problem lies in a blown thermal fuse or a broken ground wire, which can be easily fixed without replacing an expensive motor. Inspect the tubes for oily stains - this is a sure sign of depressurization of the freon system, in which the compressor may run in vain.
Checking the starting relay and thermal protection
Before "climbing" into the motor itself, it is necessary to exclude malfunction of the controls. Start relay - this is a small plastic block that Mounted on the side of the compressor. It is this that supplies the impulse to the starting winding, helping the rotor gain momentum. If the contacts inside the relay are stuck or burnt, the engine simply will not receive the necessary push to start.
Remove the relay and shake it near your ear. If you hear a rattling sound similar to a rattle, the internal mechanical part is damaged and needs to be replaced. Also check thermal relay (thermal protection), which is often built into the structure or located nearby. Its task is to open the circuit when the windings overheat. If the thermal protection contacts do not “ring” with a multimeter in a cold state, the circuit is broken.
To check, use the dialing mode on the multimeter. The probes are applied to the input and output terminals of the relay. The presence of a sound signal indicates the integrity of the circuit. The absence of a signal indicates that the part is faulty. Replacing the relay is an inexpensive procedure and often solves the problem of a “won’t start” refrigerator, creating a false impression of a motor breakdown.
Sometimes the cause of failure is not the relay itself, but oxidized contacts on the connector. Cleaning the contacts with fine sandpaper or alcohol can restore the functionality of the unit. Remember that even a working relay will not start the engine with an interturn short circuit, so checking the compressor itself remains a mandatory step.
Testing the motor windings with a multimeter
The most reliable way to check the electrical part of the engine is to measure the resistance of the windings. To do this, you will need to remove the protective cover, disconnect the wires from the start relay and gain access to the three contacts on the compressor. They are usually located in a triangle and are designated as C (Common - general), R (Run - working) and S (Start - starting).
The measurement process requires a sequential approach. First, find the common point (C) by calling all pairs of contacts. The resistance between the common point and the working winding is always less than between the common and starting windings. If the multimeter shows infinity (one in the most significant digit) on any of the windings, it means in the circuit break. A motor with a broken winding cannot be repaired.
Below is a table of approximate resistance values for a working compressor of various powers. The values may vary depending on the manufacturer and model.
| Pair of contacts | Normal resistance (Ohm) | What does deviation mean |
|---|---|---|
| General - Working (C-R) | 10 - 25 Ohm | High resistance - open, low - short circuit |
| General - Starting (C-S) | 20 - 50 Ohm | High resistance - open, low - short circuit |
| Working - Starting (R-S) | Sum (C-R) + (C-S) | Amount discrepancy - insulation violation |
If the instrument readings differ greatly from the table ones or the arrow goes to zero (short circuit), the engine is considered faulty. It is also important to check that there is no “breakdown to the body”. To do this, one multimeter probe is applied to the cleaned area on the compressor housing (where there is no paint), and the second probe is touched to each of the three contacts. The device should be silent. The appearance of resistance or a sound signal means that the insulation of the windings is broken, and operating such a refrigerator is mortally dangerous.
☑️ Checking the windings
Diagnostics of interturn short circuit
The most insidious fault that is difficult to detect with a conventional multimeter is interturn short circuit. In this case, the turns of the winding close together, the resistance drops slightly, and the device can show the norm. However, when voltage is applied, the current in the circuit increases sharply, knocking out circuit breakers in the apartment or burning the wiring.
An indirect sign of an interturn short circuit is that the compressor hums, tries to start, but quickly heats up and is turned off by thermal protection. If you measure the resistance and it seems to be normal, but the engine still does not run or is blowing plugs, the likelihood of this malfunction is high. Accurate diagnostics are carried out using special testers that measure inductance, but at home you can use the exclusion method.
Try to start the compressor directly, bypassing the starting relay, by applying voltage to the working winding through a capacitor (for single-phase motors). If in this case the engine hums, but does not turn, and at the same time gets very hot, the diagnosis is almost confirmed. Also, the interturn may be indicated by black smoke or sparks flying out from under the casing at the moment of switching on.
⚠️ Attention: Experiments with direct voltage connection are dangerous. If you are not confident in your skills in working with 220V electricity, it is better to limit yourself to resistance measurements. The risk of getting an electric shock or causing a short circuit is very high.
A motor with an interturn short circuit most often requires replacement. Rewinding the windings in a workshop is possible, but it is not economically feasible for household refrigerators, since the cost of the work is the price of a new compressor. In addition, the quality of rewinding rarely matches the factory one, and such a motor will not last long.
Checking the mechanical part and compression
Electrical service is only half the battle. The engine may be intact, but not create the necessary pressure in the system. This occurs when the piston group is mechanically worn or jammed. If the compressor hums smoothly and does not turn off, but the refrigerator does not freeze, most likely the problem is lack of compression.
To check this, you need to disconnect the discharge pipe (copper tube) from the compressor. When the engine is turned on, a powerful flow of air should flow from the open pipe, which can be felt by the palm from a distance of 10-15 cm. If the flow is weak or absent at all, it means that the valves inside the compressor do not hold pressure or the piston rings are worn out.
Is it possible to restore compression?
There are traditional methods of adding oil or using special additives, but They are ineffective in hermetic refrigerator compressors. Opening the case will break the vacuum and lead to moisture entering, which will completely kill the system.
Another sign of a mechanical malfunction is a characteristic metallic knock or vibration transmitted to the refrigerator body. This may indicate that one of the springs on which the motor is suspended inside the casing has burst, or the destruction of the connecting rod and piston group. In such cases, the engine operates with enormous noise and will soon fail completely.
It is also worth checking the condition of the oil. If, when the system depressurizes, a black, viscous substance with a burning smell flows out of the tubes, this means that the compressor was overheating and the oil was coking. This situation is often accompanied by shaft jamming. Attempts to wedge the shaft with a hammer or blow usually give a short-term effect and lead to final failure.
Measurement of current consumption and starting current
The most accurate diagnostic method used by professionals is to measure the current strength. For this you will need current clamps. The normal operating current of the compressor is usually from 0.5 to 1.5 Amps, depending on the power and model (indicated on the nameplate). If the actual current is significantly lower than the nominal current, this is a sign of an interturn short circuit or mechanical problems.
Particular attention should be paid starting current. At the moment of start, the engine consumes current 3-5 times higher than the working one. If the starting current is too high and immediately knocks out the circuit breaker, it means that a short circuit has occurred in the windings. If the current is normal, but the engine does not start, the problem may be insufficient voltage in the network or a malfunction of the starting equipment.
It is better to carry out measurements on a separate line, connecting the refrigerator directly, without extension cords, to eliminate the influence of wire resistance. Voltage stability also plays a role: when the voltage drops below 190-200V, the compressor may hum, but not start, which leads to its overheating.
If all measurements show the norm, the engine starts, the current is within acceptable limits, there is compression, but the refrigerator still does not freeze, look for the problem in a freon leak or a clogged capillary tube. In this case, the engine is working, but the system does not work due to lack of refrigerant.
Frequently asked questions about compressor diagnostics
Is it possible to replace the compressor with a similar one from another refrigerator?
Theoretically, yes, if the power and type of refrigerant (freon) are the same 134a or 600a), overall dimensions and type of starting device. However, you cannot simply “re-solder” the motor. It is necessary to evacuate the system, refill the exact amount of freon and oil, which is only possible with special equipment in the service.
Why is the compressor hot to the touch, but the refrigerator does not freeze?
This may indicate several problems: a freon leak (the engine is idling), a blockage in the system, or a loss of compression by the engine itself. Overheating is also possible due to a faulty thermostat, which does not allow the motor to rest.
A quiet hum is heard, but the refrigerator does not turn on. What to do?
Most likely, the thermal protection has tripped or the starting relay is faulty. Let the refrigerator stand unplugged for 2-3 hours to cool. If after turning on the situation repeats, check the relay and windings as described above.
Can the motor burn out due to a voltage surge?
Yes, a sharp surge in voltage can break through the insulation of the windings or damage the starting relay. To protect expensive equipment, it is recommended to use voltage stabilizers or voltage control relays.
How to understand that the compressor is jammed?
A jammed compressor usually makes a loud humming sound immediately after turning on, then clicks (the protection is triggered) and goes silent. When testing, the windings may be intact, but the rotor cannot turn. Sometimes a short-term supply of increased voltage helps, but this is a risky procedure.