Re: DC, MOTs & PFCs

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Malcolm Watts by way of Terry Fritz <twftesla-at-qwest-dot-net>" <m.j.watts-at-massey.ac.nz>

Hi Marc,

On 28 Nov 2001, at 7:49, Tesla list wrote:

> Original poster: "Metlicka Marc by way of Terry Fritz
<twftesla-at-qwest-dot-net>" <mystuffs-at-orwell-dot-net>
> 
> 
> 
> Tesla list wrote:
> > 
> > Original poster: "S & J Young by way of Terry Fritz <twftesla-at-qwest-dot-net>"
> <youngs-at-konnections-dot-net>
> > 
> > List,
> > 
> > My MOT filtered DC supply with no load benefits from about 135 mFd of PFC
> > capacitance to drop the AC line current.  But I was surprised to discover
> > that at load, the PFC correction actually increased the current!  Why?
> > 
> > I think the answer is that with no load, the DC supply filter caps are
> > essentially disconnected from the secondary, via the diode, almost all of
> > the 60 Hz cycle.  But at load, they are effectively in parallel with
the MOT
> > secondary for most of the load cycle.  This capacitance (about a
microFarad)
> > gets reflected back to the primary as a rather large cap and acts as a PFC
> > cap.  So adding more on the primary doesn't help.
> > 
> > Engineers - am I interpreting the situation correctly?
> 
> I could be showing some more of my ignorance here, but couldn't an mot
> system use Pfc. on the secondary side?
> Take care,
> Marc M.

PFC is modifying the load the mains sees. Moving a PFC cap to the 
secondary side modifies what the transformer sees as well and if it 
if suitably sized, forms a resonant circuit on the secondary side. 
Sounds familiar?

Regards,
Malcolm