On 2012-03-04, Wild_Bill wrote:
The two basic cautions for VFD use are that the output of VFDs consists of a
fast risetime voltage waves, similar in appearance to squarewaves (as
opposed to the gradual rise/fall of sinewaves normally applied to a motor
without a VFD).

With a bit of a complication. Those square waves repeat quite a
few times per cycle, starting out on for a small percentage of the time
and off for the rest, then a bit more on and less off until it reaches
the time when you should have the peak voltage of the sine wave, at
which point is is on the full time before it starts decreasing again.
This is to approximate the shape of the sine wave, and is more gentle to
the motor than a square wave of the full width of the half cycle would
be. This is called PWM (Pulse Width Modulation).

I've already spent too much time with ASCII graphics elsewhere
in this thread, and I won't bother doing it again here. :-)

Enjoy,
DoN.

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