Re: [TCML] 8" Secondary Woes

[Greg Peters <greg.j.peters@xxxxxxxxx>]

Hi all,
Thank you all so much for your replies. I can't recall ever seeing
such a widespread response on the list.

It occurred to me as I fell asleep that the logical solution is to
learn how to work with acrylic. Perhaps the effort that goes in to
cutting it would be offset by the lack of coil form preparation
needed.
Some of you have provided good advice for cutting it and today I'm
feeling optimistic. Thanks for the tips.

The other thing to do is to use the 9" with a reasonably short aspect
ratio (say 3.5:1), raise it up somewhat (if it proves to be necessary,
which I expect it will), and put a loop of 1.5mm LDPE sheet between
the primary and secondary to prevent flash overs.

I had considered using cardboard but that would have to be a final
option I think. Lots of preparation and the possibility of form
degradation over time.
Some have asked about my coil power etc. Well it's a long story.
Settling  on 8" kind of happened through a series of accidents and
coincidence. Secondary aside, the construction is already advanced, I
will have to post some photos somewhere. I have built so many coils of
500W to 15kW power that these days I don't even do basic calculations
to determine my coil parameters. I have a good sense of dimensions,
etc and there are very few design errors that can't be corrected by
untying an MMC string or changing the topload! I just start winding...
you can't go too wrong really...except when you assume that 8" pipe is
common!

I originally wanted to build a 6" coil running at about 3-4kW. That
could easily fill my available space with sparks. However, since I
just broke a variac bush and  and have 3 phase available  I thought I
would give a DC coil a go. It's smooth, BPS dependent power delivery
is appealing, as is the ability to not deplete a single phase and
annoy the neighbours.

 Sooo, in the search for a reasonable power supply I obtained 6 mots
with 2400v secondaries. These will be used to make a 3 phase six pulse
DC supply. I will run two mots per phase with the primaries in
parallel and the secondaries in series without the cores grounded (no
centre tap) (under oil). Using full bridge rectification and resonant
charging I should be able to get 24kv DC with minimal ripple and at
least 6kw power, probably more like 10kW. For low power testing I can
switch  a 3 pulse rectifier arrangement and/or a single mot per phase.
This gives me firing voltage options of 23.5kV (low ripple), 13.5kV
(high ripple) and 11.7kV (low ripple) or 6.8kV (high ripple). Then
theres the option of running the primaries in series across phases
(delta configuration) which opens up a whole other range of voltages.
The entire primary circuit (and mot secondaries) will be floating to
prevent any reference to ground and hopefully ensure no secondary to
core to primary arcs on the MOTs.

So suddenly, just because I wanted a DC supply I am looking at oodles
of power available by default. Suddenly it seemed that a bigger coil
might be good for when I can bother to set it up outside. Since 6kW
seems like a nice power level that won't present much of a load per
phase, 8" seemed "about right".

The other thing that happened was I went in search of copper tube for
the primary. Strangely enough, the cost sweet spot is 18m rolls of
1/2" tube. Smaller diameter tube is over double the price. Using 1/2"
tube for a 6" coil primary didn't please me aesthetically, and I
envisaged difficulties trying to bend it to such a small diameter.

Given the power and cheap copper tube available, 8" seemed to make sense!

Anyway, I'm going to order some acrylic tube and at least give it a go.

Thanks everyone for your replies and giving me a place to vent my frustration!

Greg.
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