Thermal Characterization of Electrical Wires and Insulation Operated in Variable Frequency Mode

NFPA 70 provides guidelines on conducting wire size rating for different service current ratings. Additionally, an engineering calculation method is provided for unusual cases. While not apparent, these guidelines are valid only for 60 Hz type frequencies. Electrical conductors have increased Joule heating with higher frequency operation because of electromagnetic effects like skin and proximity effects and the increase of total reactance of conductors and appliances. Until now, those effects have not been a fire safety concern, but the large increase in high power applications using frequencies higher than 60 Hz in variable frequency drives, also called adjustable frequency drives or simply “AC drives”, to control the operation of large motors, changes this. While research has improved the performance of motors and AC drives, there has been relatively little research on the thermal effects and fire safety implications of these systems. In this paper we show that conductors used with common variable frequency power electronics have heat dissipation rates that can be up to 50% larger than what is calculated according to the standard formulas used for 60 Hz systems. Using accelerated aging models for the wire insulation it is shown that these increases in heat dissipation rate can reduce useful cable life by a factor of two or more.