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Transformer Rebuild?

Quoting Steve Crawshaw in reply to my question on his
transformer:

> This is from a spark testing machine used in cable manafacture,
> It is open frame with the laminated cores exposed, I can see no
> magnetic shunts so I assume this is not current limited? I do 
> not know the current rating, but judging from the size of the 
> unit (the core is about 9" square and 2" thick) I would say its
> about 1kW, which makes roughly 40mA at 25Kv output, and the 
> unit should draw 4 amps at 240v.

I have my doubts about using step-up transformers with no
internal current limiting on coils with simple static gaps. 
I have tried it using some heavy plate transformers in series, 
but I never was able to get the desired (or predicted) spark
without resorting to either a rotary or an air-blast gap. The
tendency is for the gap to short, the core saturates, and 
no power is switched into the tank circuit. Unless external
current limiting is employed on the step-up primary you can
also count on dimming the lights and possibly tripping the
breaker. The key here is the ability of the gap to effectivly
quench and switch power into the tank circuit where it is
processed. Ballast on the primary prevents dim lights, etc.

My friend Dave Hartwick (who builds "bargin basement" coils)
still does not have a rotary gap. He relies instead on an air-
blast gap, supplied with air from a shop-vac unit, to quench his
pole pig. He is using microwave oven xfmrs to ballast the LV
winding of the pig. Dave states the system works well and that
the coil runs smoothly. This is despite the fact that Dave
switches the pig "ON/OFF" with a deadman. Dave finally purchased
his first Powerstat which arrived yesterday... Also he may be
joining us here in the near future.

> I have a 240v - 110v transformer which I could use to reduce 
> the HV. However, this is about as big as the HV Trans, and it 
> seems like a lot of copper and iron to use when I could rewind
> my trannie. What I am really worried about is maximising power
> through put, and if I am limited to 20Kv max (with my new CP 
> Cap :) ), then I'd like to max out the current capability of 
> the HV transformer in order to use the cap to the fullest 
> extent.

To make this understandable for those who do not own a "CP cap"
purchased through the group (or don't recall the specs) this cap
is rated .025 microFarads at 20 KVAC RMS input - pulse discharge.

If you keep your input voltage at the capacitor rated maximum
of 20KVAC, you are going to need something like 190-200 miliamps
of current to effectively supply this capacitor. This is at the
point where I would go shopping for a pole pig to supply the
coil. On the other hand by reducing your input voltage to 10-11
kV you can supply the capacitor using your existing core,
assuming you rewind it with heavier wire capable of supplying
100 ma of current. The capacitor will be pretty well matched to
the power supply in both cases, but you see that by doubling the
working voltage you increase to power required by a factor of
four. I doubt that your existing core can be made to function in
the first scenario, while it should serve happily in the second.

The problem I have is with the time and energy required to rewind
a step-up transformer. These transformers are wound commercially
by machine. Try though I may; I can never get a winding that is
even close to commercial quality without spending hours and hours
of time on the project. Unless heavy kraft paper wraps are used
between layers I find that a single flaw in the base of the
winding magnifies to a disaster a few layers up. Even paper wraps 
are not sure cures to a quality project; unless the tensioning
of the wire is carefully controlled the windings tend to end up
with loose spots that vibrate; and I have never been able to get
as much inductance into the same small space as the commercial
manufacturer can. Maybe I am a poor winder, or maybe this point's
to the superiority of the commercial manufacturing process.

> Presumably, using a step down transformer will limit me to the
> existing 40mA figure, but if I rewind with 3/4 less windings 
> and 3/4 bigger wire (on the secondary) I will boost my current
> by 3/4 and reduce my voltage by 3/4. Please correct me if I am
> making elementary errors here.

I don't see any elementary errors. Your line of thinking is
absolutely correct. The only thing I have to ask is whether or
not this is the area where you want to focus so much time and
effort on the project. No matter how well this works out you are 
still only going to get ~1000 watts through this core.

> I've talked to scotty and he's doing the business bless him.
> Looks like the cap will arrive just after my exams finish!

He has my check. Scott has done us all a huge favor by taking the
time and making a tremendous effort to supply us with top quality
coiling components. The prices are fair, and everything he has
shipped to me has arrived in top flight condition. I have heard
zero complaints.

Richard Quick


... If all else fails... Throw another megavolt across it!
___ Blue Wave/QWK v2.12