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Re: Ganging 1256D's (Variacs)
Original poster: "R.E.Burnett by way of Terry Fritz <twftesla-at-uswest-dot-net>" <R.E.Burnett-at-newcastle.ac.uk>
Hi all,
You could always use more than one ballast to share the load current
between several variacs. This is what Alan Sharp did last year to allow
his big coil to be run from three dissimilar variacs. The wiper positions
don't even need to be matched particularly well. This method also shared
the heavy running current between three standard 13A outlets.
Cheers,
-Richie,
On Sun, 18 Mar 2001, Tesla list wrote:
> 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
>
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