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Re: Ganging 1256D's



Original poster: "Bert Hickman by way of Terry Fritz <twftesla-at-uswest-dot-net>" <bert.hickman-at-aquila-dot-net>

Bill, Jim and all,

Like many engineering problems, there are often many ways to arrive at
technically sound approaches. The root of the problem is that, due to
manufacturing or assembly variations, the relative positions of the wipers
between ganged variacs can vary slightly. This causes slight differences in
the output voltage between units - while one variac is at turn "N", one of
its neighbors may be at turn "N + 1" or "N + 2" while another may be at
turn "N - 1" or "N - 2". Under this situation, simply ganging the variac
outputs together results in significant circulating currents between
variacs which can create excessive no-load power consumption, and uneven
current sharing between variacs under load. 

Using paralleling chokes or transformers is one way to solve this problem.
However, as Bill and Scot indicate, for other than 2 paralleled variacs,
Superior recommends using the same number of paralleling chokes as variacs.
And, if the ganged variacs don't already come with these, at $122 apiece
for Powerstat T5587 chokes, the chokes may actually end up costing as much,
or even more, than the surplus cost of the ganged variacs themselves! A
Major Ouch! 

So what should a cost-conscious coiler do? Improvise!!
Either "reduce" the variation in the stack, or build your own balancing
chokes. 

Reducing the variation:
======================
The good folks at Superior want to sell you paralleling chokes because they
can't guarantee consistent coalignment within a stack-up during the life of
the product (or even after shipment). But YOU can! All it takes is an low
voltage AC voltmeter, a long Allen wrench, and some patience. Each variac
wiper assembly is secured to the shaft by four Allen screws. If you've got
an enclosed unit, you may need to remove the outer screens in order to gain
access to the Allen screws. The key is to make sure that the CCW limit
positions for all variacs line up, and that all variac wipers are squarely
on the very first turn. Once coaligned, you can then retighten the Allen
screws and you should now be very close. 

You can confirm proper alignment by powering up the stack with the wiper
terminals disconnected and then measuring the output voltage between pairs
of wipers. At any position, the output voltage of a given variac should be
within a volt or so of any other. Once you've confirmed proper position,
you can then tie the outputs together without worrying about excessive
circulating currents. 

Designing/Using home-brew balancing chokes:
================================
Suppose you want to use chokes, but not spend $122 on them? What should we
plan for? Let's look at the 1256D in detail to get a clue. The wound core
is about 12" in diameter, and along the outside it has about 8 turns/inch
(TPI) of #10 AWG wound around all but 4.25" of the periphery. So the total
outer periphery of the core that's covered by a winding is about (Pi*12 -
4.25) or about 33.5". At about 8 TPI, this means that we've got about 268
turns or so on the total winding. If the unit is configured for 0-280 volt
operation, the variac develops around 1.05 volts/turn. 

If we can limit the wiper variation between variacs to no more than +/-1
turn, then the worst-case voltage difference between variacs would be about
2.1 volts, but if we can only guarantee +/-2 turns, then this will rise to
about 4.2 volts. So, worst case, each paralleling choke must be capable of
handling at least the maximum expected output current, and the core size
and number of turns on the chokes must be sufficient to support at least
2.1 volts across each half of the choke. The paralleling chokes can simply
be center-tapped windings wrapped around a common core. If we make two
chokes using wire sufficient to handle 30 amps, and a third choke capable
of handling 60 amps, then we can actually get by using only three chokes
for four variacs as shown in the diagram below. Not that making these
chokes is MUCH easier than making ballast inductors - fewer turns are
needed on a much smaller core. 

Let's plug in some nunbers... we can use the standard transformer equation
to determine the volts/turn for a given core area and operating frequency
(from Eric Lowdon - "Practical Transformer Design Handbook", 2nd ed., Tab
Books, 1989, 389pp - a great book!):

   V/N = 4FfaB*1e-8 = volts/turn
 
Where:
   V = Volts
   N = Number of Turnn
   F = 1.1 (waveshape factor for a sine wave)
   f = 60 Hertz
   a = core cross sectional area (in square inches)
   B = Maxwells (or lines)/square inch = 80,000

Then, for a core with a cross section of 1 square inch:
   V/N = 4*1.1*60*1*80,000*1e-8 = 0.213 volts/turn

We need our chokes to be able to handle 2.1 volts on each leg, so:
   N = 2.1/0.213 = 9.85 turns - use 10 turns on each "leg" of the choke.

Let's allow 800 circular mils (CM)/ampere for wire sizing for intermittent
duty - at 30 amps, this means we need about 24,000 CM or about #6 AWG. In
order to handle twice the current we can use two #6 AWG wires wound in
parallel or a single #3 AWG winding. If you used a larger core area, you
could get by with fewer turns. The cores from small junk NST's, small
variacs, or even taped cores from surplus current transformers should be
more than adequate. 


                                |----> Vout = [V1-(V1-V2)/2]
                                |
                                |   
To Wiper of            N turns  |  N turns            To Wiper of
 Variac 1  <-----------OOOOOOO--o--0000000--------->   Variac 2
           V1          -------------------        V2
                       -------------------
                       -------------------
                           
           I1
  W1  <-----------
                 O       
          Choke  O   I1+I2
           #1    o-----------
                 0          |
           I2    O          | 
  W2  <-----------          O
                     Choke  O  I1+I2+I3+I4
                       #3   o--------------> Vout
                            O  
           I3               O
  W3  <-----------          |
                 O          |
         Choke   O   I3+I4  | 
           #2    o-----------
                 0
           I4    O
  W4  <-----------


Hope this helps save you some $$$... $490 for Superior's technically
correct answer is probably too painful for most coilers... :^) 

-- Bert --
-- 
Bert Hickman
Stoneridge Engineering
Email:    bert.hickman-at-aquila-dot-net
Web Site: http://www.teslamania-dot-com


Tesla list wrote:
> 
> Original poster: "by way of Terry Fritz <twftesla-at-uswest-dot-net>"
<wysock-at-ttr-dot-com>
> 
> To Jim Lux, all.
> 
> I've watched this thread for some time; even replied to one participant off
> line about two weeks ago.  The (technically) correct way to balance the
> output of a 4-stack of Powerstats(tm) or Variacs(tm), is to use one
> current averaging transformer ON EACH stack.  Four stacks; four
> balancing transformers.  You can find the schematics of how this
> is correctly polarized and done in a Superior Electric catalog, or
> Technipower (for the former GenRad Variac line) catalog.  Hope this
> helps.
> 
> Best regards,
> Bill Wysock.
> 
> > Date:          Sat, 17 Mar 2001 17:22:52 -0700
> > From:          "Tesla list" <tesla-at-pupman-dot-com>
> > To:            tesla-at-pupman-dot-com
> > Subject:       Re: Ganging 1256D's
> 
> > Original poster: "Jim Lux by way of Terry Fritz <twftesla-at-uswest-dot-net>"
> <jimlux-at-earthlink-dot-net>
> >
> > Or... build yourself a 4 winding balancing choke..
> >
> > ----- Original Message -----
> > From: "Tesla list" <tesla-at-pupman-dot-com>
> > To: <tesla-at-pupman-dot-com>
> > Sent: Saturday, March 17, 2001 11:28 AM
> > Subject: Re: Ganging 1256D's
> >
> >
> > > Original poster: "BunnyKiller by way of Terry Fritz
<twftesla-at-uswest-dot-net>"
> > <bigfoo39-at-telocity-dot-com>
> > >
> > > Tesla list wrote:
> > >
> > > > Original poster: "D.Wightman by way of Terry Fritz
> > <twftesla-at-uswest-dot-net>"
> > > <dwightman-at-mmcable-dot-com>
> > > >
> > > > Ok, since were on the subject of ganging 1256-D powerstats I too have a
> > > > question. I have a stack of 4, 1256-D powerstats, and also have 2
of the
> > > > paralleling chokes from superior electric. Since 1 choke is good for 2
> > > > variacs, and i will be using a stack of 4 variacs total, and have
only 2
> > > > chokes. Then would it be fair to say that I should get a third choke to
> > put
> > > > between the 2 pairs of powerstats that are already ganged in parallel
> > with
> > > > paralleling chokes?
> > >
> > > Snipperzzz...
> > >
> > > yes you will need another choke to balance out the voltage "differences"
> > > between the 2 sets of
> > > variacs.
> > >
> > > Scot D
> > >
> > >
> > >
> > >
> >
> >
> >
> >
> ___________________________
> Tesla Technology Research