16 years ago
The VFD sinewaves will be built up by modulating the polarity and duty cycle of a high frequency square wave. Depends on the controller but even with the motor stopped the windings are probably energised at a low level to keep control of the rotor. Every switching cycle of the drive waveform involves losses so it's a trade off between audible noise and switching losses when choosing the drive frequency. The noise is probably magnetostriction in the motor laminations which is drowned out when the motor is running at speed. Don't worry about it unless something else is obviously wrong. You could get a newer drive but it will probably drive your cats crazy.