Winding Temperature Rise Calculator
Use the resistance method to find the average winding temperature of a motor or transformer after a heat run, and compare the rise with insulation class limits.
- T2 = (R2 ÷ R1) × (k + T1) − k = (1.6 ÷ 1.25) × (234.5 + 25) − 234.5 = 97.7 °C
- Rise = T2 − ambient = 67.7 K
Rise limits by resistance method, 40 °C ambient (IEC 60034-1)
| Insulation class | Maximum rise | This winding |
|---|---|---|
| B (130 °C) | 80 K | Within limit |
| F (155 °C) | 105 K | Within limit |
| H (180 °C) | 125 K | Within limit |
Resistance Method
Rise = T2 − Tambient
k = 234.5 for copper and 225 for aluminium (the inverse of the temperature coefficient at 0 °C). Measure R2 as quickly as possible after shutdown, or extrapolate back to the moment of shutdown.
Worked example
A copper winding reads 1.25 Ω at 25 °C and 1.60 Ω hot: T2 = 1.28 × 259.5 − 234.5 = 97.7 °C. With 30 °C ambient the rise is 67.7 K, within class B.
Frequently Asked Questions
What is the resistance method for temperature rise?
It uses the known temperature coefficient of copper or aluminium to calculate average winding temperature from the change in DC resistance.
Why is k = 234.5 for copper?
Copper resistance would extrapolate to zero at −234.5 °C, so resistance is proportional to (234.5 + T).
What is the class F temperature rise limit?
105 K by resistance at 40 °C ambient for most machines under IEC 60034-1.
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