Understanding the Negative Temperature Coefficient in Motor Insulation
In electric motors, the insulation typically exhibits a negative temperature coefficient (NTC). This characteristic means that as the motor warms during normal operation, the insulation’s resistance to electrical current diminishes; conversely, when the motor cools after shutdown, the resistance increases.
Key data points:
- Every 10 °C increase in insulation temperature cuts resistance by ~50 %.
- Every 10 °C decrease in temperature doubles the resistance.
Because of this pronounced temperature dependence, it is essential to correct insulation‑resistance measurements for temperature before drawing conclusions about trends or degradation. A practical way to assess insulation health during temperature swings is to perform a polarization‑index (PI) test; the PI curve can reveal changes that raw resistance values alone might miss.
For a detailed case study illustrating how an insulation‑integrity issue can develop, visit the PdMA Corporation website: PdMA Corp. and click the “Insulation Fault Zone” button.
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