Cable characteristics and their influence on motor over-voltages

Overvoltage transients occur on AC induction motors when connected through a power cable of sufficient length to a variable speed AC drive consisting of a PWM inverter and IGBT power devices. Factors contributing to a motor overvoltage transient equal to twice the theoretical DC bus voltage are described using classical transmission line analysis. A critical cable distance (I/sub c/) is defined where twice the DC bus overvoltage occurs. However, literature is lacking on how motor transients greater than twice the DC bus voltage are generated. This phenomenon is observed on all PWM inverters with output cable lengths greater than the I/sub c/ distance. Theoretical calculations of cable frequency and damping are correlated with simulation and experimental results. Finally, the paper presents an external hardware apparatus (the Terminator), based on a cable's characteristic impedance, that clamps the motor's terminal voltage to levels far below twice the DC bus voltage. All calculations and proposed solutions are verified or demonstrated with experimental results.