Motor Heat Dissipation
Calculate motor heat dissipation, temperature rise, and operating temperature from input power and efficiency.
Formula
How It Works
This calculator determines motor heat dissipation and winding temperature rise from efficiency and thermal resistance parameters. Thermal engineers, motor designers, and reliability engineers use it to ensure winding temperatures stay within insulation class limits. Excessive temperature degrades insulation life—per Arrhenius equation, every 10°C above rated temperature halves motor life expectancy.
Per IEC 60034-1, heat dissipation equals input power minus mechanical output: P_loss = P_in × (1 - η). For a motor operating at 85% efficiency, 15% of input power becomes heat. Loss distribution per IEEE 112: copper losses (I²R) comprise 30-60% of total, iron losses (hysteresis + eddy current) 15-25%, friction and windage 10-20%, and stray load losses 10-15%.
Thermal limits are defined by insulation class per IEC 60085: Class A (105°C), Class B (130°C), Class E (120°C), Class F (155°C), Class H (180°C). Modern industrial motors predominantly use Class F insulation with Class B temperature rise (105°C rise above 40°C ambient = 145°C maximum). The thermal equation is: T_winding = T_ambient + P_loss × R_θ, where R_θ is thermal resistance in °C/W. Typical values: 0.5-2°C/W for small brushed motors, 0.1-0.5°C/W for industrial motors with forced cooling.
Worked Example
Verify thermal performance of a 1.5 kW servo motor in an enclosed cabinet. Operating efficiency is 88%, thermal resistance (winding to ambient) is 0.35°C/W, cabinet ambient is 50°C, and motor has Class F insulation.
Step 1 — Calculate input power and losses: P_in = P_out / η = 1500 / 0.88 = 1705W P_loss = P_in - P_out = 1705 - 1500 = 205W
Step 2 — Estimate loss breakdown per IEEE 112: Copper losses (50%): 102W Iron losses (25%): 51W Mechanical (15%): 31W Stray (10%): 21W
Step 3 — Calculate steady-state winding temperature: ΔT = P_loss × R_θ = 205 × 0.35 = 71.8°C T_winding = T_ambient + ΔT = 50 + 71.8 = 121.8°C
Step 4 — Verify against Class F limit: Class F maximum: 155°C Margin: 155 - 121.8 = 33.2°C Per IEC 60034-1, minimum 10°C margin recommended for reliability
Step 5 — Calculate life impact if cabinet overheats to 60°C: T_winding = 60 + 71.8 = 131.8°C (still within Class F) T_winding = 70 + 71.8 = 141.8°C (only 13°C margin—derate or improve cooling)
Result: At 50°C ambient, winding reaches 122°C with 33°C margin to Class F limit—acceptable. If cabinet temperature exceeds 60°C, add forced-air cooling or derate motor output to maintain 20,000-hour design life.
Practical Tips
- ✓Per IEEE 1415 motor diagnostics, use thermal camera to measure steady-state temperature in actual mounting—datasheet R_θ assumes free-air convection; enclosed mounting increases effective R_θ by 30-50%
- ✓Derate continuous power by 3-5% per °C above 40°C ambient per NEMA MG-1-14.35; at 60°C ambient, a 100W motor should be limited to 60-80W continuous to maintain rated life
- ✓For servo applications with frequent starts/stops, calculate RMS power over the duty cycle: P_rms = √(Σ(P_i² × t_i) / T_total); use P_rms for thermal analysis, not peak power
Common Mistakes
- ✗Assuming case temperature equals winding temperature: Per IEC 60034-1, winding hot-spot is typically 30-60°C above measured case surface—use embedded thermistors or resistance method for accurate winding temperature
- ✗Running motors at stall without time limit: At zero speed, self-cooling fan stops; thermal resistance increases 3-5× per motor manufacturer data; continuous stall causes winding damage in 5-20 seconds depending on motor size
- ✗Ignoring duty cycle in thermal calculations: Per IEC 60034-1 duty types S1-S10, a motor may handle 150% rated current for 10-second intervals if followed by adequate cooling time—model thermal time constant (τ = R_θ × C_th) for intermittent duty
Frequently Asked Questions
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