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Re: [TCML] Variable Inductive Ballast

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.

Phil Tuck


-----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


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.


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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