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Ok last post for the night.
The pulses sent out by the VFD need better insulation on the motor than a standard motor. The very square pulses, the ones that reduce the heat from the IGBT switcfhing, also stress the motor insulation since it was designed for the nice sine wave that the power company puts out. The dv/dt insulation rating is what gets you an inverter rated motor. The inverter rated motor insulation is normally about 1900V versus 1000V for a typical motor. That's because there is a weird physics issue with the impedance of the motor versus the impedance of the cable. It's hard to explain, but there is a reflected wave that makes its way back to the VFD for every pulse that is sent to the motor. The worst case magnitude is about 200%, so a 950 VDC pulse is reflected as 1900V. You can use a large filter to reduce this reflected wave, or a large inductor only to put a bit of slope on the pulse and reduce the stress.
The VFD vendors we use have their IGBT's rated at 2200V or so. So there are details somewhere that require the extra rating. These companies do not design in a safety factor if they can get away without it.
Sorry about the bad news on the motor insulation. If you stay below 500 VDC or 350 VAC a standard motor should take the voltage as long as you can round out the pulses a bit.
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