Motor Winding Resistance Calculator | Temperature Correction

Published by Nikolai Shelkovenko on

Free Engineering Tool

Motor Winding Resistance Calculator

Correct winding resistance measurements for temperature difference. Supports copper and aluminum windings. Includes 3-phase unbalance check — >2% between phases indicates a problem.

Copper & Aluminum IEEE / IEC Phase Unbalance
3-Phase Unbalance Check (optional — enter all three)
Phase A–B (Ω)
Phase B–C (Ω)
Phase C–A (Ω)

Results

Corrected Resistance at Target Temperature
—
Temperature Coefficient α
—
Resistance Change
—
Correction Factor
—

Temperature Correction Formula

The resistance of a conductor changes linearly with temperature. The standard correction formula is:

  • R₁ — measured resistance at temperature T₁
  • R₂ — corrected resistance at target temperature T₂
  • k — material constant: 234.5 for copper, 225.0 for aluminum

Material Properties

Materialα at 20°C (1/°C)k constantρ at 20°C (Ω·m)
Copper (annealed)0.00393234.51.724 × 10⁻⁸
Aluminum0.00403225.02.650 × 10⁻⁸

Phase Resistance Unbalance

For 3-phase motors, compare resistance between phases:

Unbalance % = (R_max − R_min) / R_avg × 100%
  • < 1% — Excellent, normal condition
  • 1–2% — Acceptable, monitor
  • > 2% — Problem: possible shorted turns, poor connections, or winding degradation
  • > 5% — Severe: motor should be removed from service for repair

⚠️ Important: All three phase readings must be taken at the same temperature. Allow the motor to reach thermal equilibrium (at least 30 minutes idle for large motors) before measuring.

Tip: For motors measured hot after running, you can back-calculate the winding temperature: T₁ = k × (R_hot/R_cold − 1) + T_cold. This helps assess insulation thermal class loading.

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