Motor Insulation Resistance Test Log Worksheet
A megger reading on its own does not say much. The trend, the temperature and the shape of the curve do. This log records the three timed readings, applies the temperature correction and calculates the two ratios technicians ask for.
| Test date | Motor | Test voltage (V) | Winding temperature (C) | IR at 30 s (MOhm) | IR at 1 min (MOhm) | IR at 10 min (MOhm) | DAR (1 min / 30 s) | PI (10 min / 1 min) | 1 min IR corrected to 40C | Result | |
|---|---|---|---|---|---|---|---|---|---|---|---|
1.4118 | 2.5833 | 689.219 | |||||||||
1.25 | 1.6 | 65.2913 | |||||||||
1.05 | 1.0476 | 59.397 | |||||||||
1.5455 | 2.5882 | 1,202.0815 |
Test only de-energized, isolated and discharged windings under your electrical safety procedure. The 40C correction here uses the common rule that insulation resistance halves for every 10C rise. Acceptance values depend on the standard (for example IEEE 43), the insulation class and voltage, so use your procedure for pass limits.
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How to use the log
Isolate and discharge the motor, record the winding temperature, apply the test voltage and note the reading at 30 seconds, 1 minute and 10 minutes. Enter the values in a row and the worksheet calculates DAR, PI and the one minute value corrected to 40C. Pick a result, add the test voltage you used and export to Excel to chart the corrected IR by motor over the years.
- Use the same test voltage each time for comparable trends
- Record winding temperature, not just ambient
- Discharge windings fully after the test
DAR, PI and the corrected value
DAR is the ratio of the one minute to the thirty second reading. PI is the ratio of the ten minute to the one minute reading. Clean dry insulation keeps charging, so the readings rise and the ratios climb. Wet or dirty insulation leaks, so the readings flatten and both ratios approach one. The temperature correction brings readings to a common 40C reference, because insulation resistance falls sharply when warm. A minimum 1 minute value in the order of 1 MOhm per kV plus 1 MOhm is a long-standing rule of thumb, verify with the standard.
Worked example
A fan motor tested at 32C reads 1200 MOhm at one minute. Correcting to 40C gives about 1200 times 0.5 to the power of minus 0.8, which is roughly 2090 MOhm, the winding is cooler than reference so the corrected figure rises. With 10 minute reading at 3100, PI is about 2.6. That is a healthy trend unless the history shows a fall. A motor that was 5000 MOhm last year and 2100 now deserves a look even though it passes.
Common mistakes
Common errors include testing a warm motor and forgetting the correction, testing with surge arresters or cables still connected, skipping the discharge, and treating a single pass as proof of health. Surface moisture can be dried out and the test repeated. Keep the leads and the instrument calibrated.
Frequently Asked Questions
What is a good polarization index?
Many standards and practitioners treat a PI of 2.0 or more as acceptable for most insulation classes, and a PI below 1 as a serious concern. Exact limits depend on the standard and class, so check IEEE 43 or your motor manufacturer.
Why correct to 40C?
Insulation resistance changes strongly with temperature, so readings taken in different seasons cannot be compared directly. Correcting to a standard temperature, typically 40C, makes trends meaningful.
Can I use this for transformers or cables?
The ratios and correction work in the same way, but acceptance limits and test voltages differ. Rename the columns and use the limits from the relevant standard for the equipment.
How often should motors be tested?
Annually is common for important motors, and before and after any rewind or long storage. Wet environments justify more frequent tests.
Netray can help link motor test results to equipment records in your ERP so trends appear alongside work order history.
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