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[TCML] More on balacing chokes from Bert Hickman
The three choke configuration looks like it is for a 4 stack. My bad.
<paste>
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 --
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