Re: 3phase PFC

On Thu, 15 May 2008 05:31:39 GMT, Jamie Morken <jmorken@xxxxxxx>
wrote:

Terry Given wrote:
Jamie Morken wrote:
Terry Given wrote:

Jamie Morken wrote:
legg wrote:


The Vienna rectifier is about the most expensive,complicated and lossy
version of PFC correction that a body could have picked.



LOL. too true. Im sure they could have added a few more diodes if
they had tried just a little bit harder


Thank you for modifying the sim to work! :)

What circuits would be better than the vienna rectifier for 3phase
PFC? I found two other ones:

http://www.rocketresearch.org/new/3phase%20PFC/3%20phase%20PFC%20stage.png

http://www.rocketresearch.org/new/3phase%20PFC/3%20phase%20PFC%20stage2.jpg


I would like to generate +220V and -220V rails, and use them to
generate
splitphase 120VAC sine waves using halfbridges and two LC filters.
What
would be the best 3phase PFC circuit to generate these two rails, or if
not using 3phase PFC, could a boost circuit off the 3phase 6diode
rectifier generate a positive and negative rail?

cheers,
Jamie


It sure can. use a pair of boost converters, with a half-bridge as
the switches, the center-tap of which becomes 0V. So its:


---nnnnnn---+--->|-----+--- V+
| |
3ph X [Cbig]
BR | |
+--0V------+
| |
X [Cbig]
| |
---nnnnnn---+---|<-----+--- V-


X is typically an IGBT half-bridge, but can be FETs etc.

Hi Terry,

I think this is called the "Minnesota rectifier" right? Can it
be used without the centertap neutral of the 3phase source? I think
there is no current loop unless the 3phase neutral is available, but
for my generator we don't have the neutral..

cheers,
Jamie



Hi Terry,

Thanks for the dual rail boost! ;)


the same trick used to make a single-phase boost PFC (force current
to follow envelope of full-wave rectified line voltage) also works
for three-phase systems; the 6-diode bridge automagically "unfolds"
the current waveform.


Is this an alternate circuit to the vienna rectifier for 3phase PFC?
Do you have a schematic of it?

I drew it, above. I didnt include the current sensing, or the controller
of course.

vienna rectifiers suck so bad its funny. Rob Legg outlined the reasons why.

I've never designed one of these, but I have used a 300kW version. done
with a single IGBT half-bridge, and some honking great inductors.



Here's an ltspice sim from Larry Carroll on the ltspice yahoo
group, it implements 3phase PFC with one cycle PFC control!

"http://www.rocketresearch.org/new/3phase%20PFC/lbcarroll backup.zip"


The existence of the generator's neutral terminal is something that
might be profitable to investigate further. Perhaps you could consult
the manual, or take a closer look at the actual hardware wiring. It's
your generator; you can do what you want with it.

You might want to think simple:

http://scholar.lib.vt.edu/theses/available/etd-2113131549741341/

The single-switch 3ph pfc circuit is probably best applied off-line,
where filtering requirement of the critical conduction current is set
by known hardware, and not paper standards.

With a generator as a source, there is no actual inrush problem, as
you are never going to be hot plugging the converter into a running
generator.

RL
.

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