An old road rack converted to a TC control panel. It has a 50 Amp magnetic input breaker. Volt meters, to monitor input and output voltage. A 220 volt SCR module described below.  Low voltage remote control. 20 amp 240 volt Variac. 50 Amp meter to monitor output current. 15 amp Magnetic breaker, line filter and 10 amp variac for fan or rotary spark gap.  It puts out 110 and 220 volts simutainously on different connectors. This gives flexibility to control any of my coils from small to large.

click to enlarge

Control Panel Schematic: I have maxed out the SCR module, so it has been replaced with a 60 AMP, MAGNETEK, 2 pole, mercury whetted relay.

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Click to enlarge photo at left.  Voltage control Variac's  Matching variac's are attached with original mounting bolts and spacers. A new 1" metal shaft was installed through both variacs.  The balancing choke is made from the core of a small neon transformer. It is wound with 10 turns per side of #6 THHN wire. See schematic on right.

Variacs in parallel with balancing choke

In the schematic on left the R1-C1-R2 network provides a degree of phase shift to the triac gate drive network, to ensure correct triac triggering. A snubber network , R3-C2, are included to prevent false triggering from high slew rate (di/dt)spikes.

The schematic at left shows a typical dual SCR switch. The important part is the zero crossing optoisolator. Without this type of optoisolator, random turn on is possible.

If the voltage difference between outputs of a pair of variacs is
defined as Vx = V1-V2, the balanced 1:1 transformer circuit (below) will
share this difference. The trick is to make the windings robust enough
to handle the current, and the core large enough to handle the maximum
expected voltage difference between pairs of wipers on linked variacs
(typically only a few volts).

                      Vout = [V1-(V1-V2)/2]

> Question 1, how many turns on what size core for a 30 amp system???

I'll give it a shot! Assuming you've got identical variacs, you only
need to handle a maximum of a few volts differential. The core should be
large enough to have at least a couple of square inches of core
cross-sectional area. In order to handle 30 amps, you'll need to use #8
or #10 AWG wire - regular THHN housewire will work. Standard silicon
steel transformer E-I material should be used for the core. A rule of
thumb for the number of turns for this type core material (from Lowden,
"Practical Transformer Design Handbook") follows:

   N = V*5/a  (assumes 60 Hz and 20 kilolines flux density)
Where:
   N = Number of turns required
   V = Volts across winding
   a = core cros-sectional area in square inches

So... Let's assume you had a core with a cross section of 2 square
inches, and wanted to be able to handle a worst-case wiper-to-wiper
voltage from the variacs of 4 volts. This implies that the total number
of turns would be 10. However, you can add more turns as long as your
winding window is large enough to permit. You don't need a very large
core since you're not developing much total voltage across the
winding.
> Question 2, How do you tell if it's working??? (I have all the
> stuff I need to start experimenting on this, I just have no clue
> as to how to tell if what I've made is doing it's job)

As long as you've wired it as though it was a single center-tapped
winding, it should work just fine. For testing purposes, you could use a
filament transformer to apply low AC voltage (2-3 volts) across the
winding to verify that you pull only a small amount of current.
>
> Question3, you mentioned silicon steel cores, I take it you don't
> mean a standard "E I" type laminated cores?

Regular E-I silicon steel transformer core is just ticket! Remember that
for this application the core doesn't need to be very large.

> Someone suggested another solution where you wind 3 coils on a single
> core, but there was no further information. Given the choice, I'd
> rather wind a single core, rather than 3, separate ones. Does anybody
> think this would work or have any idea of how many turns on what size
> core???

Usually, independent chokes are used and are the preferred
configuration. While I'm not aware of any simple way to totally
eliminate circulating currents with a single-core approach, the
following should control them. However, it will take a pretty large core
and winding window to handle the large wire guage and multiple windings
required.

Hook up one end of the chokes to the wipers of variacs 1, 2, and 3 as
shown and connect the other ends together. Center taps are not
necessary, but you should reverse the polarization of one ot the
windings. This arrangement should reduce circulating currents to
manageable levels in an inexpensive fashion.

            o
   W1  -----OOOOOO000000------
                             |
                       o     |
   W2  -----OOOOOO000000-----o--- Out
                             |
            o                |
   W3  -----OOOOOO000000------

>
> I have cleaned off a bunch of cores and have a matched set of 3
> that I could use to do the 3 center tapped method if I have to.
> It's just that I had planned to use these for current limiting
> reactors.
>
These may be larger than you really need for balancing chokes. You STILL
need to use an external ballast inductance for current limiting, though.

Safe coilin' to you!

-- Bert --
--
Bert Hickman
Stoneridge Engineering
http://www.teslamania.com