Falling Polarization Index on a 4.16 kV Motor
A 4160 V, 1500 hp TEFC induction motor comes out of service for its annual shutdown. Leads are disconnected and grounded for 10 minutes. The winding RTD reads 31 °C, ambient 32 °C. A 5 kV insulation tester records 30 s = 700 MΩ, 1 min = 620 MΩ, 10 min = 480 MΩ. Last year, at a winding temperature of 40 °C, the same test gave 1 min = 1500 MΩ and 10 min = 2400 MΩ. The space heaters were left off during this outage because the breaker feeding them was locked out for other work. The planner wants the motor back in service tonight because "620 MΩ is far above the 5.16 MΩ floor for 4.16 kV."
1. Compute the polarization index and dielectric absorption ratio, and interpret them against last year's data.
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Calculations
- PI = R(10 min) / R(1 min) = 480 / 620 = 0.77
- DAR = R(1 min) / R(30 s) = 620 / 700 = 0.89
Interpretation
- A healthy Class B machine should show PI well above 1.0 - field practice commonly treats PI ≥ 2.0 as good and PI < 1.5 as questionable, with PI near or below 1.0 indicating the insulation is not charging and is not drying out.
- A PI below 1.0 means the leakage current is dominated by a conduction mechanism (moisture, surface contamination, carbon tracking) rather than by the healthy capacitive charging current that makes resistance rise with time. Both PI and DAR here are below unity - the trend is downward, not flat.
- Compare to last year: PI last year was 2400/1500 = 1.6, acceptable. It has fallen to 0.77. The 10-minute value fell by 80% while the winding temperature also fell 9 °C.
Conclusion: the insulation has degraded or become wet. The motor should not be returned to service on this data, regardless of the absolute value.
2. The planner argues that absolute resistance is what matters and that 620 MΩ is 120× the minimum. Address temperature correction and why the comparison is misleading.
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Two separate errors in the argument.
- Absolute minimums are floors, not acceptance criteria. The "1 MΩ per kV + 1" rule (5.16 MΩ here) exists to catch gross failure - a winding that is essentially shorted to ground. Passing it tells you the machine will probably not flash over on the next start; it says nothing about whether the insulation is in good condition. Acceptance should rest on trend, PI/DAR, and where justified step-voltage or surge testing.
- The temperature comparison runs the wrong way. Insulation resistance falls roughly by half for every ~10 °C rise in winding temperature. Last year's reading was taken at 40 °C; this year's at 31 °C - about 9 °C cooler. Corrected to a common base, this year's readings should be higher than last year's by a factor approaching 2 if nothing changed. Instead they are 60–80% lower. Correcting for temperature makes the apparent decline worse, not better.
Also note the space heaters were off during this outage, which is exactly the condition that allows a winding to absorb moisture and produce a low, flat PI.
Action: dry the winding (heaters or external heat, with the machine isolated and grounded per procedure), clean the end windings and terminal box, and retest at a recorded temperature. Do not return to service until PI recovers and the absolute value is consistent with the corrected trend.
3. State the test conditions that must be controlled for these numbers to be trustworthy.
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Conditions to verify or record:
- Isolation. Motor leads disconnected from the cable and starter; the cable itself tested separately. A cable still connected puts the surge capacitors, PTs, or the starter's control wiring in parallel with the winding and corrupts the reading.
- Grounding and discharge. Ground for the full period (a 4.16 kV winding holds charge), and ground again after each test step. Never leave a tested winding floating.
- Temperature. Record the winding temperature at the time of test, ideally by RTD or by resistance measurement, and correct to a common base before comparing to prior results.
- Guard terminal. Use the guard connection to shunt surface leakage across dirty insulators and end-winding contamination, which otherwise depresses PI in exactly the way seen here.
- Test voltage. Apply a consistent test voltage (typically 500 V–1 kV for a 4.16 kV machine, per the manufacturer) and be consistent year to year; a change in test voltage changes the reading.
- Atmosphere. Record humidity and whether space heaters were energized; a winding that is 10 °C below ambient is absorbing moisture.
- Instrument. Verify the tester's calibration and that its leads are clean and properly rated; a leaking or contaminated lead produces a low, flat PI that mimics wet insulation.