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Re: Power Transformers
Tesla List wrote:
>
> >From rwstephens-at-ptbo.igs-dot-netTue Nov 12 22:39:37 1996
> Date: Wed, 13 Nov 1996 10:13:24 -0500
> From: "Robert W. Stephens" <rwstephens-at-ptbo.igs-dot-net>
> To: tesla-at-pupman-dot-com
> Subject: Re: Power Transformers
>
> >>From bert.hickman-at-aquila-dot-comMon Nov 11 22:11:06 1996
> >Date: Sun, 10 Nov 1996 22:45:37 -0800
> >From: Bert Hickman <bert.hickman-at-aquila-dot-com>
> >To: tesla-at-pupman-dot-com
> >Subject: Re: Power Transformers
>
> Bert wrote:
>
> <snip>
>
> >Arc-lighting ballast
> >inductors can often be found at scrapyards for $10-20 (since they have a
> >fair amount of copper and iron). I just purchased one at the local yard
> >that handles 6.5 KVA and weighs 52 pounds for about $20.
>
> I think I'd be looking for these too. Good score Bert!
>
> >Note that high-power shunting resistors are essential to keep voltage
> >transients down to a manageble level when the gap is quenched. These can
> >be space-heaters, hot-water heater elements, or oven elements, but must
> >be capable of safely dissipating up to several kilowatts of heat. Not
> >using these can mean that inductive kickbacks will get back into your
> >power mains and wipe out your computer, VCR, answering machine, etc...
>
> >Safe coilin' to ya!
>
> >-- Bert --
>
> Bert, All,
>
> My only limited experience with installing shunt resistance across
> the primary control/limiting reactance has been that when the
> resistance was added, the power drawn from the mains was reduced, and
> the system performance went for a crap.
>
> I use a good commercial line RFI filter between my coil primary power
> circuit and the rest of the planet. Since I have employed this filter, and a serious
> grounding and screening system in the firing room at my old lab
> location, I stopped blowing up 'stuff' upstream on the mains. Don't
> misunderstand me. I have blown up lotsa 'stuff' upstream on the mains
> during my 'learning curve'.
>
> I haven't had a chance to 'scope' the goings on in the primary
> circuit except briefly once. I agree with you that some extreme spikes can be
> created. I wouldn't be suprised if these surges aren't working for me in a
> positive way in my very successful MTC system where I employ 10
> millihenries of primary control reactance without resistive bypass damping,
> and have silky smooth variac control with no power bumping
> whatsoever. Note that I also employ 240 mH series inductance out of
> the secondary of the power transformer on this unit.
>
> I do know that the extra, series secondary inductance causes huge decrement
> waves every time the break closes, which are impressed on the transformer
> secondary. This circuit is a wild place. I hope to scope the total
> goings-on soon but in the meantime, all I know for sure is that my
> synergistic arrangement works like gangbusters, even though I cannot
> tell you why at this time.
>
> My point is that your advice to employ damping resistance across the
> control reactor does not ALWAYS work best.
>
> rwstephens
R*obert,
Well I should know better than to say Always or Never - there's Always
an exception! :^>
Although I haven't done much simulations in this area as yet, I suspect
that you're absolutely right about the benefit coming from the inductive
surges from the ballast inductor. I would also suspect that you have
some fairly robust caps in your EMI filters, and don't have any
especially sensitive electronic equipment tied to the same branch. If
you've synergistically balanced the system, the increased current drawn
by the pig, when shunted by the gap, pumps up the energy stored in the
ballast inductor. This stored energy assists in rapidly recharging the
tank cap once the gap opens.
By properly balancing gap dwell times, ballast inductance, and tank
capacitance, transients seen at the 240 Volt mains _should_ be
manageable. Under this condition, adding shunting resistance across the
ballast would only detract from optimal performance. This sounds like
the situation you've arrived at in your system - congratulations!! If
this was by design, are there some rules of thumb you've discovered
along the way? If not, it'd really be nice to nail down some of these
relationships for new designs. Sounds like an area for fruitful coiling
work in the future!
Could I get some more info on your system? What voltage pole pig are you
running, and what's the size of your tank cap(s)? Any idea what dwell
time you're running, and PPS rate? Finally, if you bypass your
secondary's 240 mH inductance do you notice any performance change?
In the final analysis, adding shunting resistance should help damp
excessive inductive transients for non-synergistically balanced systems
- an example would be longer dwell times coupled with smaller tank
capacitances. Heavy-duty PFC's across the mains may be an effective,
non-lossy, alternative for keeping 240V primary transients down.
However, damping resistance may _still_ be required to prevent too much
ballast energy from over-stressing the tank cap once the gap opens...
Safe coilin' to ya', Robert!
-- Bert --