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Hi Jim,
Winding and insulating a single HV inductor that can reliably withstand
25 kV swings from scratch will be a challenging task. At the very least,
the winding should consist of a number of identical "pies", each
robustly insulated from their neighbors and from the core. The winding
construction would be similar to that used in large induction coils or
higher-voltage transformers.
Another approach is the inductor equivalent of an MMC: break the
inductor into a number of smaller identical isolated inductors and
connect these in series to provide the desired bulk inductance and share
the total voltage stress. Designing an inductor to handle 25 kV is much
tougher than designing (or buying) a number of smaller chokes that can
withstand 3-5 kV apiece. I would suggest constructing or buying six to
eight smaller independently-insulated inductors and then either
immersing them in oil or vacuum impregnating them with a dielectric resin.
For your system, let's assume your tank cap is 0.1 uF and your target
break rate is 400 BPS with an input of 208V 3-phase, to deliver 10 kW of
output power. Your 6-pulse DC Supply voltage (optionally using a 1-2 uF
DC storage cap and your rewired transformer) is about 12.25 kV. To
provide your stated maximum break rate of 600 BPS (and ~15 kW of power),
the charging inductor should be ~3 Henries. The peak voltage across
the charging inductor assembly will be ~25 kV whenever the spark gap fires.
Assuming you use eight isolated inductors, each inductor would be ~375
mH. Each inductor should be designed to operate at an RMS current of 1.5
amp without saturation in order to handle maximum break rate of 600 BPS.
Each choke will need to withstand ~3 kV from end-to-end, insulation for
each inductor is not very stringent. All cores should be floating or
connected to the inner-most winding and mounted on standoffs. To reduce
corona (especially for those inductors nearest the output side),
immersion in oil or other means of E-field control and corona
suppression may be necessary.
Remember that the core material in your chokes will be operating at the
resonant frequency of your DC charging system - for your system, about
600 Hz. You may want to use thinner laminations (scavenged from 400 Hz
or audio transformers) or iron-powder or similar E-I material to reduce
core losses during operation. Standard 60Hz core material will work, but
it will get quite hot in this high ripple current application.
You may also want to insert a well-insulated single-layer RF choke
between the charging inductor module that's nearest the 2X output and
the spark gap. This inductor must also be capable of withstanding 25 kV.
The RFC helps to reduce excessive turn-to-turn voltage stress that will
otherwise appear across the last few winding layers of the charging
inductor created by high dv/dt transients when the spark gap fires. A
small bypass capacitor or a reverse-biased HV rectifier from the
charging inductor/RFC node to the negative DC rail can also help
suppress the VHF transients from backing up into your charging
inductors. See Greg Leyh's recently posted comments about his Medium
Power DC Tesla Coil in earlier posts.
Good luck and best wishes,
Bert
--
Bert Hickman
Stoneridge Engineering
http://www.capturedlightning.com
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Jim Mora wrote:
Hello Group,
I posted few more pictures on my blog spot. The tap that goes down to the
standoff goes under the other two legs and joins the star point at the back.
It is held off of ground to reduce HV stress ( of coarse the primary Wye is
grounded for safety from the factory) and phase fault interruption.
There is also a couple of pictures of the recent door stop transformer I
acquired. The tag does indeed say 10KVA. It was a fortuitous find. I have
found some bobbins that will fit it as well. I have a choice of sectional
(good for HV I would think) or two bobbins to fill the window. Otherwise,
the sectional would be 4 winding windows per leg. Either way, I can do both
legs with these for series or parallel. Two cores may be better. Saturation
may be an issue since it is pulsed DC. I'm not really clear on that except
clearly there is always a voltage change going on and resonance with the
tank cap between breaks.
There is a fair amount of threads in the Pupman achieves, but I did not land
on one that actually is a walk through on construction of a charge inductor
in this power range, other than MOT's. I have a fair amount of magnet wire
remnants on spools so I will proceed with a sectioned bobbin and measure one
section inductance and resistance starting with #22 AWG. I would like to end
up with two 4 or 5 Henry chokes that can hold off 22KV min resonant rise.
Again these will gapped and be under oil and well isolated at the core. I
may be able to produce 10KVA with the supply. If there is sag, I will resort
to the dreadful Cprimary cap of 10x or 20x my tank cap as stated by Bert
Hickman. The whole idea of the 6 pulse is to avoid this deadly cap which
dictates a stiff transformer supply. After the Raytheon is back together,
I'm in for some bench experimentation over some time!
Check out the couple of new pics.
Jim Mora
http://teslawavetuner.blogspot.com/
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