Hi Bert, et al,
My newest gap is driven by a DC motor and has 8 equal spaced .5" tungsten
electrodes (one of several works in progress). The motor is very rugged but
will top out at 426 breaks calculated at full voltage rating DC (PWM).
I have an aircore choke that I use with my pig. It is thin wall 4" pvc wound
12 inches with milspec Teflon insulated 1000v silver coated 19 strand copper
held in place with a thin coat of epoxy. This was a recommendation from DC
Cox and he had one on top the Big Bruiser controller if I remember correctly
feeding out the "Big Red" HV line to the big gap and tank. I seem to
remember it was 100 turns.
I also have a stash of HV ceramic puck caps that screw together to send some
hash to ground.
** The charging inductor has always been the fly in the ointment on this
project ;-^)
Terry Fritz sold me a 20KV utility pole transient protector but it may it
may fire all the time though I think it is intended to handle lightning
transients. But I haven't looked at it in some time and never tested it.
I have six identical MOT's and others but didn't want to go down that road.
I will take the path of least resistance if it will be robust.
I did brake down that 10KVA dual C core transformer today, not easy! They
are big cores and I found a company that makes multi-sectional Bobbins in
selectable materials that fit it. I'll call their engineering department
tomorrow to see what they say.
Let's review the 3H charge inductor and .1uf tank combination: Using
Richie's standard F-res first equation formula and assuming a low loss
inductor I get 291 hertz. How do you arrive at 600hz? Since I will have a
lower break rate, is increasing the inductance slightly reasonable? This
brings down the frequency of resonance even lower, yes?
Bert I really want to extend my appreciation for your help on this long side
lined project. This Is turning out to be a "learning opportunity".
Viva Lichtenberg discharges, shrunken quarters, and long streamers!
Jim Mora
-----Original Message-----
From: Tesla [mailto:tesla-bounces@xxxxxxxxxx] On Behalf Of Bert Hickman
Sent: Monday, August 18, 2014 12:18 PM
To: Tesla Coil Mailing List
Subject: Re: [TCML] How I modified the 3 phase for dual wye 5KV 2xI
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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