To understand how to check the motor winding with a multimeter, you need to sequentially measure the resistance between the phases and the insulation resistance relative to the housing. This diagnostic takes 10–15 minutes, does not require a special stand and allows you to identify an interturn short circuit, phase failure or insulation breakdown on the housing in one step. Below is the working methodology used by Elektromotors service engineers in Tashkent during the initial acceptance of engines.
Preparing for measurements
Before you handle the probes, complete three mandatory steps. They protect both the operator and the device itself from erroneous readings.
- Turn off the power to the input circuit breaker and hang a sign “Do not turn on - people are working.” Check the absence of voltage with an indicator on all three phases of the terminal box.
- Disassemble the circuitin the terminal box. Remove the star or delta jumpers - otherwise the multimeter will show the total resistance of the parallel branches, and not each phase individually.
- Clean terminals U1, V1, W1, U2, V2, W2from oxides with fine sandpaper. The oxide gives a transition resistance of 0.3–1 Ohm - for low-power engines this is a critical error.
Allow the engine to cool to ambient temperature. Copper resistance increases by about 0.4% for every degree, and a hot winding will show higher values. A pyrometer or thermocouple on the bearing shield housing is suitable for temperature control.
Safety and choice of device
To check the resistance of the windings, a digital multimeter with a resolution of 0.01 Ohm in the lower range is suitable - for example, Fluke 87V, UNI-T UT61E or an analogue. A multimeter is not suitable for checking insulation resistance: its 9-volt battery will not penetrate hidden defects in the varnish. Here you need a 500 V megohmmeter (for motors up to 660 V) or 2500 V (for high-voltage machines 6 and 10 kV).
Be sure to work with dielectric gloves if voltage was previously applied to the motor: a residual charge may remain in the winding, especially after the frequency converter. Remove the charge by connecting the leads to each other and to the body through a 10 kOhm resistor for 2–3 seconds.
Step 1. Measurement of resistance between phases
Set the multimeter to resistance measurement mode (Ω), range 200 Ohms. Connect the probes one by one to the pairs of pins U1–V1, V1–W1, U1–W1. Write down three values.
For a serviceable asynchronous motor the difference between the phases should not exceed 2–5%. If one of the windings differs noticeably, this is a sign of an interturn short circuit (resistance is less than normal) or a break/poor contact (resistance is greater than normal or “infinity”).
Approximate standards for winding resistance
| Motor power | Voltage 380 V (connection Y) | Permissible variation between phases |
|---|---|---|
| 0.55–1.5 kW | 10–25 Ohm | ±5% |
| 2.2–5.5 kW | 3–8 Ohm | ±5% |
| 7.5–11 kW | 1.2–3 Ohm | ±3% |
| 15–22 kW | 0.5–1.2 Ohm | ±3% |
| 30–55 kW | 0.15–0.4 Ohm | ±2% |
| 75–132 kW | 0.05–0.15 Ohm | ±2% |
The table is a guide. Take the exact nameplate values from the engine datasheet or manufacturer’s catalog. For motors with a delta circuit, the phase resistance will be 1.5 times less than specified.
Step 2. Checking the insulation relative to the housing
Switch the megohmmeter to 500 V. One probe - to the housing grounding bolt, the second - alternately to U1, V1, W1. Measurement time - 60 seconds (reading R60).
- More than 100 MOhm - insulation in perfect condition.
- 10–100 MOhm is the norm for the engine in operation.
- 1–10 MOhm—the insulation is damp or dirty and requires drying.
- Less than 0.5 MOhm - breakdown, operation is prohibited, winding rewinding is required.
According to PUE standards, the minimum permissible insulation resistance for 380 V motors is 0.5 MOhm at a temperature of 75 °C. On a cold car the requirements are stricter.
Step 3. Search for interturn short circuit
The multimeter detects only rough interturn short circuits - when 10–15% of the turns or more are closed. Light interturn defects are almost impossible to detect with a “conventional” device: the difference in resistance amounts to hundredths of an ohm and is lost against the background of the error of the probes.
Indirect signs of interturn closure during the initial examination:
- smell of burnt varnish coming from the terminal box or ventilation holes;
- darkening, melting or swelling of the frontal parts of the winding;
- hum and strong vibration when starting at idle;
- unbalanced current in phases when operating under load (difference more than 10%).
If there is at least one sign, you need a complete diagnostics of electric motors on a stand with a surge tester.
What do typical multimeter readings mean
We have compiled into one table the most common scenarios that a service recipient encounters.
| What the multimeter shows | Probable diagnosis |
|---|---|
| All three phases are equal, insulation > 10 MOhm | The winding is OK, look for the cause in the bearings, power supply, load |
| One phase “OL” (infinity) | Break winding or poor contact at the output |
| One phase is noticeably lower than the other two | Interturn closure of this phase |
| All three phases are 20–40% higher than normal | The engine has not cooled down or there is poor contact on the probes |
| Insulation 0–0.3 MOhm | Breakage to the housing, repair required |
| Insulation “jumps” in time | Moistening the winding, requires drying for 8–12 hours at 90 °C |
When to call a specialist
Self-checking with a multimeter is the primary screening. It answers the question “is the engine alive or not,” but does not give the complete picture. Contact the service if:
- the spread of resistance between phases exceeds 5%, but visually the winding is intact;
- insulation resistance below 1 MOhm even after drying;
- crane, elevator or explosion-proof motor - this requires a certified methodology;
- power above 30 kW - the error of a household multimeter becomes comparable to the measured value.
In Tashkent, Elektromotors engineers perform measurements using certified equipment: microohmmeter, megohmmeter up to 5 kV and pulse tester MTC-1. The client receives a full winding report on the day of contact. We service industrial motors, crane drives and induction machines of all types.
Typical mistakes when checking yourself
Over the years of working with engines at industrial sites in Tashkent, we have seen the same mistakes from technical personnel. If you avoid them, the accuracy of diagnostics with a multimeter increases dramatically.
- Measurement without disassembling the star. When the neutral is closed, the multimeter does not show the winding resistance, but the parallel connection of two phases - the value is always underestimated and is not comparable with the passport.
- Using Chinese probes with bad tips. Transition resistance of 0.3–0.5 Ohm kills accuracy for motors above 11 kW. It is better to use probes with gold-plated tips or alligator clips with a screw clamp.
- Checking the insulation with a multimeter in the “continuity” mode. This does not provide information about the condition of the varnish - the multimeter produces only 9 V, while breakdown during startup occurs at 1500–3000 V.
- Ignoring the temperature correction. Measuring on a hot car after stopping gives a resistance 20-30% higher than the real one. Always wait until it cools down to +25…+30 °C or make an adjustment using the formula R₂₀ = Rₜ × 255 / (235 + t).
- Measurement without removing the power cable. If the cable remains connected, neighboring circuits “ring” through the motor - frequency converter, filters, capacitor banks. The readings will be chaotic.
Additional checks worth doing
After the main measurements, it is useful to walk through several adjacent points in 5 minutes - often the cause of the problem turns out not to be in the winding, but next to it.
- Ring the thermal protection - built-in PTC thermistors or bimetallic contacts should ring “short” when the engine is cold.
- Check the integrity of the cable entries in the terminal box. A charred oil seal or a melted terminal is a reason to inspect the entire harness up to the starter.
- Measure the resistance between the case and the ground loop - it should be less than 0.1 Ohm. Poor grounding increases the risk of breakdown.
- Rotate the shaft by hand - it should rotate without jamming or extraneous noise. Stiff rotation is often disguised as an “electrical” problem.
A short checklist before measuring
- Turn off power, check with indicator.
- Disassemble the star/delta in the terminal box.
- Clean the leads, let the engine cool.
- Measure the resistance of all three phases and compare.
- Measure the insulation with a 500 V megohmmeter.
- Compare the results with the table and passport.
- Record values in the PPR log to track dynamics.
Engineer's tip: record the measurement results in a journal at each PPR. The dynamics of insulation resistance over 6–12 months will tell you more about the condition of the engine than a one-time measurement.
Do you need help with diagnostics? Elektromotors engineers in Tashkent will conduct a full winding check, measure parameters on a bench and issue a conclusion with repair recommendations. Contact us or order urgent departure - response within an hour in the city.