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Re: Building your own HV transformer
Ed,
The book, and others like it, were a great source of inspiration.
The motorized winder worked extremely well. I use a variant of it to
wind Tesla coils up to 7" in diameter and 27" long.
The only real snag: (reading all those books on how it was, back when
it was fun) I developed an interest in technical history, now I'm two
octaves into a four octave pipe organ . . . that's a monumental task!
[snip?]
Powered by the 120V/60HZ line, the coil only outputs 8.2 KV. With a
spark, primary current is up around 14 amps, I try to hold it at ten
amps or less with the rotary gap. (the excitation current (no spark)
drops to less than 1/4 amp with TC cap across the HV side - suggesting
60 HZ resonance, or error in measurement). The secondary spark is very
hot. It will support an arc up to around 4" opening in a jacob's
ladder, and will melt the steel wire if the arc stops moving.
In an induction coil mode of operation - allow the magnetic field to
build, using DC, then interrupt the current suddenly, I can jump a spark
a little over four inches (no TC capacitor on the output). Mechanical
interrupters (10 amp relays) have to have their contact surfaces cleaned
after a few minutes of operation. The point condenser is a 660 VAC
type, and it will die occasionally.
If the spark gap is closed down (no cap on the output) the spark will
last for a perceptible time. It doesn't sound like a snap, but more
like a hissing sound. I assume the counter-emf, with the gap tightened,
is keeping the magnetic field from collapsing rapidly.
The only interrupter to work reliably is a transistor chopper using 3, 8
amp, 1500 V, TO3, transistors in the output (driven by another 8 amp
transistor, small power transistor and 555 astable). The transistors
are further protected by some small gas transient suppressers, a fuse,
and ferrite beads to trim some of the peaks.
The transformer survives with too large a spark gap on the secondary (no
internal arcing) - something I was really sweating. The layer
insulation is only paper from an adding machine roll (I designed the
width of the bobbins based on some 3-1/2" wide paper) with varnish.
There is a 1/4" winding margin on each layer. Worst-case, there is
probably over 500 volts between layers.
I've fooled around with another induction coil building technique that
looks promising. In the old days they wound thin "pies" using wax on
cotton covered wire for 1/8" thick bobbins, and lots of them. The wax
supported the coils. The beauty of it is that the secondary is easily
repairable, should it fail, and there is no requirement for insulating
between the layers, or winding without over-laps. The wire is "jumble
wound" onto the form.
I can cut the PVC tubes accurately enough (1/8" wide rings). Cut side
"plates" making compression cuts in heavy Mylar film with a glass cutter
against a 3/8" thick aluminum plate. A compass type holder, turns out
accurate mylar donuts. The pies, or bobbins are assembled with
"super-glue." The winding machine can be adapted to support the
end-plates.
I haven't got cotton covered wire, but it looks possible to run magnet
wire through a molten bath of hot glue and onto the pies. Wax seems too
thin. The trick seems to be controlling the temperature closely enough,
so the adhesive doesn't burn, and the thin film coating the wire, stays
molten long enough to make it onto the pie. It is not a kitchen stove
type project. It will take an accurate temperature controller. It
looks do-able . . .
Output should improve with pie type construction. With the winding
margins, on the current coil, and 1/8" thick end plates on the bobbins,
I have large gaps in the magnetic field (totaling over 2") that aren't
covered with secondary windings.
Building a large induction coil is fun. It wasn't 'work,' as I
understand the term. One unforeseen development . . . (I'm beginning to
need reading glasses) my eyesight improved!
I calculated 65,172 feet of wire in the coil. From measuring the
secondary resistance, and correcting for temperature, I was off by only
220'.
If word gets out, more folks will wind their own power transformers.
It's only a little more challenging than building a really good
capacitor, and a lot of 'fun.' Cost was around $150.
Take care
bob