Re: [CAD_CAM_EDM_DRO] VFD ?

Hugh Currin wrote:

> Knowledgeable Ones:
>
> I have a question regarding the inards of a variable frequency drive which I
> hope you can help me with.
>
> I understand a VFD rectifies line voltage to DC then inverts it at the
> required frequency. My question is how it then inverts the DC. Is this a
> chopped voltage like a pulse width modulation to build the AC wave form? Or
> does it generate varying DC levels? I would suspect the former but do not
> know.

Old VFD's used several crude techniques to supply mostly big square
waves to the motor, which caused a lot of heating due to the distorted
waveform. Newer units use a high frequency PWM modulation of some
sort to supply the equivalent of a sine wave. The inductance of the
motor smooths out any significant current fluctuation caused by the
PWM frequency.

i have a special (single-purpose) VFD at work that runs a turbomolecular
pump that does use a variable DC voltage. It has a switching buck
regulator before the simple square-wave inverter.

Jon