Reliability through increased safety insulation systems: the effect of high-speed switching on the motor insulation system

The dominant electrical machine in industrial general-purpose drives continues to be the efficient, robust, reliable inexpensive induction motor. Continuous development ensures they can accommodate electronic waveforms, although less than 10% are currently used with drives. Higher PWM switching frequencies combined with faster switching devices in the last ten years have caused a ten-fold increase in the rate of change of voltage applied to a motor. This can be accompanied with an increased peak voltage due to the pulse train propagation to the motor and reflection, dependent on the rise time and cable length. Although these waveforms age the insulation system, the reduced reliability is overcome through modern increased safety insulation systems. This article summarises the principles that provide user confidence in modern improved insulation systems. It provides a challenge to drive manufacturers to review their approach to future products.