phase converters for big iron

[Paul Koning]

>>>>> "Simon" == Simon Fryer <fryers at gmail.com> writes:

 Simon> All, On 4/28/05, Paul Koning <pkoning at equallogic.com> wrote:
 >> >>>>> "Simon" == Simon Fryer <fryers at gmail.com> writes:

 Simon> [...]

 Simon> All the solid state phase converters I have come across use a
 Simon> three phase H bridge to generate three phases from a common
 Simon> intermediate DC rail. The incoming voltage is converted to DC,
 Simon> switched, PWM synthesising a sinusoidal at the required
 Simon> frequency.
 >>
 Simon> The PWM works pretty well for induction motors. I would expect
 Simon> big iron to have some issues.
 >> Why?  I would expect power supplies to be every bit as tolerand of
 >> distorted waveforms as motors are, even assuming that these
 >> converters produce major distortion.

 Simon> Two areas of initial concern.

 Simon> 1) Transformers in linear supplies seem to be very well suited
 Simon> for 50/60Hz. Trying to feed them with a square wave with
 Simon> frequencies of up to 30KHz + would probably not do them a lot
 Simon> of good. They may be able to handle it just fine, but then
 Simon> again they might not.

Yes, but do solid state converters put out square waves, or rounded
waves?  I'd expect the latter, or certainly nothing with realy fast
rise times.  Don't those things ofen use IGBTs?  Those don't switch
all that fast as I understand it.

In any case, I don't see why a transformer would be any less happy
about harmonics than a motor; they are both windings on laminated
cores.

Also, square waves have theoretical spectra out to infinity, but the
amount of energy above a few kHz for a 60 Hz fundamental is very low.
I don't have the Fourier transform memorized, so I can't be more
specific...

	paul