Conductor Insulation Breakdown
As mentioned, PWM circuitry, which causes the high rate of voltage rise at the carrier frequency, can cause insulation breakdown of the end turns of the motor windings, as well as possible breakdown of the feeder cable insulation. This relates to the very high rate of rise of the voltage (rate of voltage change with respect to time) in combination with the very rapidly repeating voltage pulse caused by the VFD. Conductor insulation failures in motors have occurred because of this phenomenon. This subject is not completely understood and is presently being researched. The known facts about the matter are summarized as follows.
Switches in the inverter section of VFDs used today cause instantaneous turn-to-turn voltage inside a motor’s windings to be significantly higher than what an equivalent normal sine wave supply produces.
Each cycle of the fundamental voltage consists of numerous pulses of voltage.
Long distance between a motor and its VFD causes the turn-to-turn voltage to get even higher.
There are different approaches in explaining why there’s an increase of voltage at the motor terminals. Some explain it in terms of resonant capacitance/inductance (LC) circuits; others explain it in terms of standing wave theory. Both approaches end up with a similar result. When the distance between a motor and its VFD exceeds a critical distance (which may be as low as 30 ft), there is a voltage overshoot that may exceed twice the amplitude of the voltage pulse originally delivered at the VFD output terminals.
This higher voltage comes at the motor at such a high rate of change for each of the PWM pulses, from zero volts to its peak value, that it’s unevenly distributed across the winding, causing high turn-to-turn voltages in the turns connected closest to the power leads. The result places very high stress on the conductor insulation, which can cause early breakdown of the insulation.