Dr. Erik Baigar
erik at baigar.de
Wed May 5 13:39:54 CDT 2021
>> Note that VFDs are designed to run motors exclusively. They
>> approximate a sine wave with pulse width modulated 400 V pulses. DON'T
>> EVER try to run electronic gear with a VFD, at least without a massive
Yes, that is what I suggested; these filters are called "Sinus Filters"
in the field of driving even motors over longer cables. One does not want
to transmit the 10kHz PWM over longer distrances.
> I've heard that. But why? It's not like the electronics we're talking about actually runs on AC.
(a) Input filters often contain caps...
With 50Hz or 60Hz they create a little current which does not hurt.
Applying a square wave of 15kHz has harmonics up to 100kHz and more.
That means, that the current in these is more than 100 times bigger - BAM!
> Instead, it goes right into a transformer (an inductive load not much
> different from a motor)
(b) Yes, a transformer usually has an optimum frequency range. Above
it does not deliver the energy to the output. Why? (1) Because of stray
inductivity. That is what helps you in filtering ther higer frequencies.
But there is also (2) capacity between windings shorting the higher
frequencies -> Heat and stress for the insulation. And finally (3) each
time you are changing the magnetic field in the core, it causes loss
and heat. The more often one tries to do that, the more loss ->
Summary: Depends on the transformer what happens. May be OK, may
fail after some time duer to heat or the insulation may break
> and after that into a ripple filter. That
> filter IS the "massive smoothing filter" you're talking about.
(c) After the rectifier, you have got the caps. They are no
problem, but the rectifiers (e.g. 1N4007 or similar) are optimited
for lower frequencies. They to not like higher frequencies: Esp.
seitching them off causes some current in the wrong direction
eventually heating the diode. May be a problem, but have not
had trouble with this case yet...
> The voltage issue is a different one. I've never seen a VFD that
> offered anything other than frequency change -- indeed, it produces the
> same RMS output voltage as what you feed it, and it isn't insulated.
Look to the link in my other email. The device there is around
USD100 and you can select frequency (10..400Hz) and Uout (10%...100%)
and also go ramps etc.
Again good luck ;-)
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