| Abstract |
Increasing efficiencies and improved performances in pulse width modulation (PWM) based adjustable speed drives (ASD) are obtained owing to the continuous enhancements in working frequencies and slew rates (dv/dt) of new generations switching devices, such as MOSFET or IGBT. These advancements have beneficial consequences particularly in ASD for high power, high voltage motors, as those adopted in traction motors. However, some undesirable side effects arise, such as shaft voltages and bearing currents, EMI, overvoltages and uneven voltage distribution in the coils. In particular, both thermal and electrical ageing of the motor insulation is faster due to the enhanced stresses associated with steep fronted waveforms, as compared to that arising under sinusoidal voltages.
In this paper the results of an experimental investigation carried out on some candidate insulation systems for adjustable speed drive, high voltage, traction motors are presented. The effects of the applied voltage wave-shapes parameters, such as the slew rate and frequency, are analysed. The characteristic parameters of partial discharges (PD) are analysed under sinusoidal and quasi square voltages wave shapes in order to compare the performances of the insulation systems. The results of PD test are examined and cross checked with the life curves of the insulation systems in order to gain a widespread knowledge of the degradation mechanisms affecting the motor insulation as close as possible to those taking place in real operating conditions. |
