The Long Cable Inverter Drive Induction Motor Transient Over-voltage Mathematical Analysis

Modern Adjustable Speed Drive (ASD) systems consist of a PWM source inverter withIGBT power devices. IGBT switching speed (typically from50ns to300ns) allowsincreasing the carrier frequency to improve current waveform quality as well as to reduceaudible noise in motor laminations. However, in many applications, these systems requirelong motor leads; this is why motors often undergo over-voltages stress due to the reflectedwave. These over-voltages can cause the destruction of the motor insulation, in particular inthe early turns. They can also cause partial discharges in the drive-to-motor cable therebyreducing its lifespan. Besides, the reflected wave affects the common mode (CM) voltage. Asa result, the conducted EMI problem gets worse.A mathematical technique to compute incident and reflected waves in long cable PWMdrives has been developed. The mathematical formulas describing these waves are developedin the frequency domain. The inverse Laplace transform is then applied to obtain the timedomain sets of equations for their computation at any point along the cable. This techniquehas permitted to investigate over-voltage transients and the dv/dt not only at the motorterminals, but all along the inverter-to-motor cable. As a result, the oscillation frequency, thecritical cable length and the possible peak voltage could be determined. Over-voltageinvestigations has been done for i) inverter wide and spaced output voltage pulses ii)successive small and close pulses.Contrary to most previous papers, this work shows analytically that i) in the first caseover-voltage can go up to2p.u ii) in the second case over-voltage can go higher than theknown2p.u level and can reach up to4p.u in the worst conditions. This paper includessimulations which are compared with the obtained other models to validate the contribution ofestablished technique.