Effect of cabling and grounding configuration on surge voltages in inverter-fed motors

In inverter-fed motor systems, output voltage of the inverter has a rectangular waveform with rise time of a few tens of nanoseconds, and consequently the motor suffers from repetitive surge pulses which may have adverse effect on motor insulation. The behavior of surge pulses should be clarified to devise the optimum insulation design of a motor. We have investigated how the cable structure and grounding wire connecting the inverter and motor, affects the surge behavior with particular regard to peak voltage between phase terminal to the ground and the peak voltage across the first coil in serially connected multi coils consisting one phase. As a result, we discovered phenomena whereby, when the cross sectional structure of the cable is asymmetrical, the surge voltage of one specific phase can become higher than that of the other phases resulting from the asymmetry among the capacitances between the feeder line and ground line. We have confirmed these phenomena can be prevented by the following three methods: using cables that have symmetrical capacitance to ground among three phases (e.g., a three-phase shielded cable), providing the grounding wire as a separate cable to reduce capacitance between the feeder line and grounding wire, inserting capacitors at motor terminals or inverter terminals to cancel out capacitive asymmetry.