NEMA MG-1-2023 Motors and Generators – Induction Heater Duty Cycle Derating Tables

Induction heater duty (IH) is a specialized service condition defined in NEMA MG-1-2023 for motors driving induction heating equipment—e.g., induction furnaces, billet heaters, or hardening systems—where operation involves repeated short-duration high-torque (or high-current) bursts followed by cooling intervals. Unlike standard continuous-duty (S1) or intermittent-duty (S2–S8) classifications, IH duty explicitly accounts for the asymmetry between rapid resistive heating during on-time and slower convective/radiative cooling during off-time, requiring motor thermal modeling aligned with IEC 60034-1 Annex D principles but adapted for North American practice. The derating tables in MG-1-2023 Section 12.42 provide discrete multipliers (e.g., 0.75×, 0.85×, 0.95×) applied to the motor’s base continuous horsepower rating, based on empirically validated combinations of on-time (t_on), off-time (t_off), and duty cycle ratio (t_on / (t_on + t_off)), with implicit assumptions about ambient temperature (40°C), enclosure type (TEFC), and insulation class (F or H). Engineers apply these tables during motor selection to ensure that cumulative thermal stress remains within safe limits over thousands of cycles—preventing hot-spot temperatures from exceeding insulation system endurance thresholds. Importantly, the tables assume sinusoidal current waveforms and do not cover non-sinusoidal drive effects (e.g., VFD-induced harmonics), necessitating additional derating or thermal modeling when inverters are used.