Inverter/motor inter-compatibility with particular bias to insulation

Variable speed AC motor drives are used in ever-increasing numbers because of their benefits for energy efficiency and flexible control of processes and machinery using low-cost maintenance-free AC motors. Virtually all AC drives, at low voltage (up to 690 V AC) are voltage-source PWM inverters with IGBT power devices and consequently generate high rates of change of voltage. The fast-changing voltage generated by such a drive causes some increase in the voltage stress on the motor winding insulation-although today, in most cases, the additional stress is well within the motor's capability. For some years there have been occasional reports of motor winding insulation failures, which appear to have been caused by PWM drives. Most of the failures reported have been in systems using AC supply voltages in the region of 525 V to 575 V and above-mainly used in North America and South Africa. There is little evidence of such effects occurring with European LV supplies. The number of failures reported is small, but sufficient to cause concern in some application areas. Consequently, some users may be specifying countermeasures that are costly and are often unnecessary. This paper aims to give an explanation of the effects involved and provide advice on what precautions are required to avoid problems.