vfd

VFDs and other industrial switch mode converters pull
current when the rectified voltage exceeds the voltage
on the filter capacitors. Since this occurs only at
the peaks of the ac voltage swing, there is an
incredible amount of harmonics in the at the kind of
frequencies that people notice. It's true that the
same effect occurs with linear dc power supplies with
transformers, but the transformers tend to damp it out
a little, i believe.

I don't know when active power factor correction is
going to make it down to the price range that we deal
in.

Eric


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