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Re: [TCML] Variable Inductive Ballast

This is very interesting stuff. The thing I notice is, does anyone make (or
have detailed instructions on building) a saturable reactor for 3-Phase
power? At 480VAC? I'm really interested in this as a solution. Given the
nature of what we do (and how impressive it is when it goes wrong.....which
happens a lot....) I would much rather use a heavy iron and copper system
than a nice tiny modern silicon system for this application.

Sometimes, low-tech is better. Especially when you make a living by using
products "in a manner inconsistent with its labeling" as our insurance
company likes to say. ;)

Thank you everyone for the replies!

On Sun, Oct 4, 2015 at 6:41 AM, Phil <pip@xxxxxxxxxxx> wrote:

> Thank you David for a comprehensive answer. You may think me rude not
> replying earlier but I missed your reply.
> I have some articles from the web 'somewhere' about constructing one
> myself,
> so was naturally interested in yours. I have always used a home-wound
> ballast, and the current ballast (
> https://www.youtube.com/watch?v=VTqRahADgds&feature=player_embedded  ) is
> wound on old radar transformer cores, and handles 8kw easily without
> getting
> warm. Drawback though is that sometimes you may need to contain the power
> level, and currently most of us can only reduce the main variac output
> volts. I favour the ability to run at full output volts on the variac to
> get
> the pig's output volts high, but also have a lower power level at the same
> time, short of swopping ballasts each time for different values, a SR is
> the
> way to go.
> Regards
> Phil Tuck
> www.hvtesla.com
> -----Original Message-----
> From: Tesla [mailto:tesla-bounces@xxxxxxxxxx] On Behalf Of David Speck
> Sent: 01 October 2015 00:56
> To: Tesla Coil Mailing List
> Subject: Re: [TCML] Variable Inductive Ballast
> Phil,
> Many years ago, I bought a medium sized TC from Harry Goldman.  He
> designed it with two esoteric unobtanium, epoxy potted, internally
> current limited 10 kVAC transformers in parallel that apparently came
> from some sort of radar installation.  One of them failed with the
> secondary shorted.
> As a replacement, I had a large Hammond 9 KV, 500 mA potential
> transformer, which had to be current limited to avoid overloading the
> TC.  On eBay, I found a 2 KVA saturable reactor (SR) for a decent price.
> The SR is wired in series with the primary of the potential
> transformer.  It has three windings on a figure "8" core, connected
> internally and 4 externally accessible terminals.  Two terminals are the
> DC control winding, and two are for the controlled AC line current.
> You feed the control winding with 0 - 75 volts of full wave rectified,
> but unfiltered DC, from a small Variac, at less than one amp.  The SR
> passes only a fraction of an amp through the high current winding when
> the DC control voltage is turned off, and the impedance of the SR
> decreases as the DC control voltage increases, up to the maximum current
> capacity of the unit.
> When driven with the SR / potential transformer combo, the TC runs
> better (much smoother firing, longer sparks), than it ever had with a
> Variac and the original power transformers.
> Interestingly, there is a little bit of delayed response of the SR. If
> you flip off the DC control voltage, the TC discharge tapers off
> gradually over about 1 - 1.5 seconds, rather than stopping instantly.
> There was a great deal of research in this field before WWII about
> magnetic amplifiers, and there are many web pages about them.  These
> were very effective in the days before solid state controls were
> available.  To me, the big advantage of them in TC use is that they are
> much less likely to release their "magic smoke" than today's IGBT circuits.
> I read somewhere a long time ago that you could make a home built SR by
> connecting two matched MOTs with their primaries in series, and their HV
> secondaries in opposing series.  You would apply the control DC to the
> nominal HV secondary windings.  If the MOTs are closely matched, the
> opposed secondary windings result in a net zero potential despite the
> voltages on each winding induced by the primary current.
> Only disadvantage is that the two MOTs in series are only good for
> controlling 120 VAC.  I've never had time to try this arrangement out,
> but the principle sounded right.  Aside from that, I've never tried to
> create my own SRs.  Wish I had the time to try, but that's a scarce
> commodity around here.
> Dave
> On 9/30/2015 6:27 PM, Phil wrote:
> > Dave,
> > Any tips, design details, or weblinks that you may have found useful when
> > building your saturable ballasts?
> >
> > Regards
> > Phil Tuck
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Chris Boden
The Geek Group National Science Center
We Build Awesome


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