Re: DC Supply

To: tesla-at-pupman-dot-com
Subject: Re: DC Supply
From: Greg Leyh <lod-at-pacbell-dot-net> (by way of Terry Fritz <terryf-at-verinet-dot-com>)
Date: Fri, 25 Sep 1998 02:39:52 -0600
Approved: terryf-at-verinet-dot-com
> Original Poster: Mark Finnis <mefinnis-at-medicine.adelaide.edu.au>
> 
> At 04:16 24/09/98 -0600, Greg Leyh wrote:
> >Remember, with center-tap grounded secondaries, the windings
> >cannot be connected directly together in either delta or wye,
> >since now you will have *six* phases to worry about (2 per
> >CT'ed secondary).
> >Each secondary HT terminal must go to its own full-wave
> >rectifier set (consisting of 2 stacks), and the DC outputs
> >of the rectifier sets can then be bussed together.
> >
> > ---------------------- +
> >             |_
> >             D^
> >             |
> >    HT ------o
> >(6 places)   |_
> >             D^
> >             |
> > ---------------------- -
> >
> >The resultant DC voltage will be symmetrical around ground.
> 
> Greg, Jim, Bert H, Malcolm & Others,
> 
> I don't get this.  Surely the two NST secondaries, centre-tapped are
> *in-phase* with each other.  There can only be 3 phases, to have 6 would
> imply phase shifting by the transformer ????

Sure!  Transformers make fine phase shifters, and if you compare 
the two HT secondaries (with respect to the case) on any NST, 
you will see that they are 180deg out of phase.  This is required, 
since if they were in phase, there would be no net voltage difference 
between their HT terminals.

Consider 3 NST's, with their 3 primaries connected in 'wye' to a 
3-phase feeder with neutral.  Each primary is referenced to the 
neutral, and is shifted 120deg from its neighbors.  
The center-tap/case of each NST is attached to ground.

If we consider just the three 'left hand' HT windings of the 
three transformers, we see that there are three 7.5kV windings
connected in 'wye'.  The center of the wye is the case ground.
Let's call the NST's 'A', 'B', and 'C'.  If we define the phase 
of the left hand (LH) winding of A to be 0deg, then the phases 
of B and C (LH) are 120deg and 240deg, respectively.

Now what are the respective phases of the right hand windings?
the RH windings are referenced to the same case ground at the LH's,
and as shown above, are always 180deg out from their LH counterparts.

A (LH) =   0 deg
A (RH) = 180 deg 
B (LH) = 120 deg
B (RH) = 300 deg  (120deg + 180deg)      
C (LH) = 240 deg
C (RH) =  60 deg  (240deg + 180deg)      
       
Presto!  Six identical phases, spaced 60deg apart, all referenced
to the case ground of the transformers!
This is a standard trick used in high current 3-phase power supplies
in order to get the ripple frequency up to 720Hz, which is *much*
easier to filter.  It's called a 12-pulse rectifier.


> The real question is "what to do with the centre taps".   Jim Lux, who
> started this question, thinks I should still ground them.

Yes, indeed!  With an NST, there isn't really a choice.


> To my simple medical mind.  The only effect grounding the centre-tap should
> have is to reference the rails to +/- 7.5 kV, as opposed to "15 kV floating
> in the breeze".

That's correct.  The ultimate voltage of the rails will be closer 
to 1.1 * 7.5kV, =  +/-8.25kV.  (16.5kV rail to rail).  
All of my coils have been 6-pulse type rectifiers.  Yours would the 
first to my knowledge that will run on 12 cylinders!
-- 


-GL
www.lod-dot-org