How to independently check the compressor windings: from theory to practice

The compressor is the heart of any refrigeration system, be it a car air conditioner or a household refrigerator. Motor windings Over time, compressors wear out, overheat or become damaged, leading to failure of the entire unit. But how do you understand that the problem is in the windings, and not in the mechanical part? This article will help you understand the nuances of diagnostics without contacting a service center.

Many car owners and amateur repairmen are afraid to check the windings for fear of damaging the equipment or getting an electric shock. In fact, if you follow basic safety rules and have a minimum set of tools (multimeter, megohmmeter), the procedure becomes simple and understandable. We will consider 5 verification methods, including visual inspection, resistance measurement, breakdown testing and current consumption analysis. We will pay special attention typical mistakesthat beginners make, and we will give recommendations for interpreting the results.

Before starting the test, make sure that the compressor is completely de-energized. Even if it is not connected to the network, capacitors can retain charge for a long time. To discharge, use a screwdriver with an insulated handle to briefly short-circuit the capacitor terminals. Do not neglect this procedure - the voltage in the compressor circuits can reach 380 V in industrial models and 220 V in household.

1. Visual inspection: the first signs of winding malfunction

Start the diagnosis with an external inspection of the compressor. Burnt contacts, melted insulation or traces of oil on the housing are sure signs of problems with the windings. Please note:

• 🔥 Darkening or blackening on terminals and wires - indicates overheating due to increased resistance or short circuit.
• 💧 Oil leaks from under the flange - may indicate a breakdown of the winding to the housing if the oil is mixed with soot.
• 🧲 Rust or oxidation on metal parts - increases the risk of current leakage to the housing.
• 🔌 Loose or burnt contacts - often cause intermittent connections and power surges.

If the compressor hermetically sealed (as in most refrigerators), it is not possible to visually inspect the windings. In this case, pay attention to indirect signs: for example, if the thermal relay or knocks out the machine in the shield, there is a high probability of an interturn short circuit.

⚠️ Attention: Don't confuse traces of oil with a refrigerant leak. The oil in the compressor circulates along with freon, and its presence on the housing does not always indicate a malfunction. However, if the oil has a dark color or a burning smell, this is a reason for a more in-depth diagnosis.

For automotive air conditioning compressors, inspection is simplified - often it is enough to remove the protective cover to see the terminal block. Please note the condition power cables and pressure switch: their malfunction can simulate problems with the windings.

2. Checking the winding resistance with a multimeter

This is the most accessible diagnostic method that can be carried out using a conventional multimeter. Winding resistance must correspond to the passport values (usually indicated on the compressor nameplate or in the technical documentation). If there is no data, focus on average values:

• 🔋 Start winding (Start, S): 15–40 Ohms (depending on power).
• 🔄 Working winding (Run, R): 5–20 ohms.
• 🔌 Total resistance (S + R): must be equal to the sum of the resistances of the individual windings.

Verification algorithm:

1. Disconnect the compressor from the network and discharge the capacitors.
2. Remove the terminal block cover (soldering iron may be required on hermetic compressors).
3. Measure the resistance between the terminals:
   • S → R (starting + working windings),
   • S → body (check for breakdown),
   • R → body (check for breakdown).

Compressor type	Resistance R (Ohm)	Resistance S (Ohm)	S + R (Ohm)	Permissible deviation
Household refrigerator (up to 150 W)	8–12	20–30	28–42	±10%
Car air conditioner	2–5	10–15	12–20	±15%
Industrial (380 V, 1–3 kW)	3–8	8–15	11–23	±5%

If the resistance between the windings tends to zero - this is short circuit. If it shows infinity - break. A breakdown to the case is diagnosed if the resistance between any winding and the metal case is less 0.5 MOhm.

⚠️ Attention: In some compressors (for example, Danfoss or Embraco) the starting winding can be connected via PTC relay. Before taking measurements, make sure that the relay is turned off, otherwise the readings will be incorrect.

3. Checking for breakdown with a megohmmeter

The multimeter is not always able to detect hidden breakdown of the winding on the body, especially if the insulation is slightly damaged. For an accurate diagnosis, use megohmmeter (insulation tester) with voltage 500 V or 1000 V.

Procedure:

1. Connect one megger probe to compressor housing (clean the contact area).
2. Connect the second probe one by one to the terminals of the windings (S, R, general).
3. Apply test voltage for 1 minute.
4. Take the readings.

Normal insulation resistance values:

• ✅ For domestic compressors: > 20 MOhm.
• ✅ For industrial: > 50 MOhm.
• ❌ Critical value: < 0.5 MOhm (requires winding or compressor replacement).

If the insulation resistance drops below 10 megohms, but the compressor is still running, this does not mean that it is working properly. Such windings can break at any time, especially with high humidity or power surges.

For automotive air conditioning compressors, a megger is used less frequently due to the difficulty of accessing the housing. In this case, you can get by with a multimeter, but be sure to check for leakage current (we'll talk about it below).

4. Checking current consumption (for experienced ones)

This method requires current clamps or a multimeter with a current measurement function up to 10 A. It allows you to identify turn-to-turn short circuit, which is not always noticeable when checking resistance.

Algorithm:

1. Connect the compressor to the network using a current clamp.
2. Start the compressor in operating mode (for refrigerators, turn the thermostat to maximum).
3. Measure the starting and operating current.

Compressor type	Rated current (A)	Starting current (A)	Symptoms of a problem
Household refrigerator (100–150 W)	0.8–1.2	2.5–4.0	The current is higher than normal - interturn short circuit; below normal - breakage or wear
Car air conditioner	5–10	15–25	Current surges - unstable operation of the windings

If the operating current exceeds the rated current by 20% or more, this indicates:

• 🔥 Interturn closure (load increases).
• 🛠 Bearing wear (the compressor is overloaded).
• ⚡ Breakdown to the housing (part of the current goes to the ground).

⚠️ Attention: In automobile air conditioning compressors, the current may fluctuate due to changes in system pressure. For accurate diagnostics, measure the current during stable operation (2-3 minutes after startup).

5. Checking windings without instruments (emergency method)

If you don’t have a multimeter or megohmmeter at hand, you can use indirect methods diagnostics They are not 100% accurate, but they help identify obvious faults.

Method 1: Check phase balance (for 3-phase compressors)

• Turn off the power.
• Rotate the compressor shaft by hand (it should rotate smoothly, without jamming).
• If the shaft rotates jerkily or with force, there may be a malfunction of the winding or bearings.

Method 2: Heat Test

• Run the compressor for 10-15 minutes.
• Touch the housing in the area of the windings (without touching live parts!).
• If the body evenly warm - everything is fine.
• If there is local overheating — there is a high probability of an interturn short circuit.

Method 3: Capacitor Test

Always works when paired with a compressor starting capacitor. If it is faulty, the compressor may not start or may hum, simulating problems with the windings. Test the capacitor with a multimeter (in resistance mode): a working capacitor will first show low resistance, and then it will gradually increase to infinity.

What to do if the compressor hums but does not start?

This is a typical symptom of either a faulty capacitor, a jammed shaft, or a breakdown of the winding to the housing. First check the capacitor (replace with a known good one), then manually rotate the shaft. If the shaft does not rotate, the problem is mechanical. If it rotates, but the compressor does not start, look for a breakdown in the windings.

6. Typical mistakes when checking windings

Even experienced professionals sometimes make mistakes that lead to incorrect conclusions. Here are the most common:

• 🔌 Test under voltage - this is not only dangerous, but also distorts the results. Always turn off the power!
• 📉 Ignoring temperature — winding resistance depends on temperature. Take measurements at room temperature (20–25°C).
• 🔧 Incorrect connection of probes - for example, measuring the resistance between S and R instead of S → body.
• 📊 Comparison with invalid data — not all compressors have the same parameters. Always check the documentation.
• 🔍 Neglecting visual inspection — sometimes the breakdown is visible to the naked eye (soot, melting).

Another common mistake is ignore compressor type. For example, in inverter compressors (used in modern air conditioners) winding resistance may be lower than standard values due to control features. Such compressors require specialized diagnostics.

7. When does winding repair make sense?

In most cases, rewinding the compressor windings unprofitable, especially for household and automotive models. However there are exceptions:

• 💰 Expensive industrial compressors (the price of a new one is 3–5 times higher than the cost of repair).
• 🚗 Automotive air conditioning compressors for rare models (if a new one is difficult to find).
• 🔧 Manifold compressors (their windings are easier to rewind).

The cost of rewinding in the service is 30–70% of the price of a new compressor. However, there is no guarantee that after repair the service life will be the same as that of a new unit. If the compressor has worked for more than 10 years, it is more advisable to replace it with a new one.

There is one more nuance for car air conditioners: when replacing a compressor, it is often necessary to flushing the system and replacement receiver-dryer, which increases the cost of repairs. In this case, rewinding the windings may be justified if the remaining components of the system are in good condition.

FAQ: Frequently asked questions about winding diagnostics

Is it possible to check the compressor windings without removing it from the car?

Yes, but with reservations. For a car air conditioning compressor you can:

1. Turn off the power (remove the fuse or disconnect the battery).
2. Disconnect the compressor power connector.
3. Test the windings with a multimeter right on the spot.

However, visual inspection and megger testing will require dismantling.

What insulation resistance is considered critical for a car compressor?

For automotive air conditioning compressors, the critical value is - less than 0.5 MOhm. With insulation resistance 0.5–2 MOhm the compressor may still work, but its life is significantly reduced. Optimal value - more than 10 MOhm.

Why does the compressor heat up but not pump?

There are several reasons for this:

• 🔥 Interturn closure - the windings heat up, but the magnetic field is not enough to rotate the shaft.
• 🛠 Bearing jamming - the shaft does not rotate, current is consumed, but no work is performed.
• ⚡ Valve failure — the compressor works, but does not create pressure.
• 🔌 Low voltage in the network - the windings overheat due to increased current.

First check the mechanical part (turn the shaft by hand), then the windings.

Is it possible to repair the windings of a refrigerator compressor with your own hands?

Theoretically yes, but in practice it is extremely difficult. You will need:

1. Depressurize the compressor (at risk of moisture ingress).
2. Carefully remove the old windings without damaging the core.
3. Wind a new wire (you need a special winding wire and a machine).
4. Soak the windings with varnish and dry.
5. Fill the system with freon and oil.

Without experience, the likelihood of success is minimal. For refrigerators, it is cheaper to buy a new compressor.

How to distinguish a winding breakdown from a capacitor breakdown?

Ring the capacitor separately:

• If the capacitor is broken, the compressor will not start, but the windings will be fine.
• If the capacitor is broken, the compressor may hum, but not rotate (or rotate jerkily).
• If the winding is broken, the compressor either does not start or the circuit breaker/fuse blows out.

The most reliable way is to disconnect the capacitor and apply voltage directly to the windings (for a short time!). If the compressor starts, the capacitor is to blame.