Original poster: "Mike" <induction@xxxxxxxxxxx>
Hi Terry, List,
Well, if I am following this line of logic
about tight / high coupling or LERT right for the DRSSTC,
why are people not using the method that RF induction heating output
RF transformers, very well proven, use.
I can see where using an iron core is a problem on the RF units but
using a copper sheet secondary works great.
At starting frequencies of 450 kHz and down into the 150 kHz range,
this system runs hundreds of kW all 3 shifts in the CW mode. So, in
the DRSSTC, you could run it backwards. Your one turn copper sheet
with slot becomes the primary and of course potted between the
primary and secondary.
We use, build and fix these things (when a line breaks outside and
the water shorts it all out) so we sort of do what Finn is talking
about, only we usually make the primary from tubing 18 through 8
turns on the inside of copper shell /secondary and the copper sheet
with vertical slot, with connection block and water cooling, drives
the 1 or 2 turn work coil for pipe welding, etc.
Regardless if the machine is a tube or IGBT unit running in the 200
kHz range, the current is stepped up by the turns ratio and the
voltage of course stepped down by that same ratio.
The "secondary" can be from 4 inches high (two slightly spaced and
stacked for a N:2 ratio) or 8 through 18 or more inches high, again
of course with that slot to make two live ends. We even place a tab
halfway on the sheet for earth ground connection.
These are tightly coupled, 3/16 through 5/8 inch primary to
secondary and all in the vacuum pumped potting rubber material.
RF transformers work fine with the primary on the inside of the
shell and they are also done with the primary on the outside, though
this tends to make less room for the work coil connections, so it's
usually inside.
I've run them backwards before, works great. I've also dumped large
HV caps into the 15 turn primary (tubing) and got a 15 fold current
gain on the shell /copper sheet secondary.
Also done it backwards, dumped a large HV cap into the one turn
copper slotted "now backwards primary" and saw one impressive
voltage gain off the 15 turns.
Watching the discharge on a scope, even a cap dump had no rise time
issues in the typical TC frequency range.
I found the noise from the 25 kV 100 MFD (pulse rated) cap spark gap
less than desirable but the RF transformer, a stock commercial shell
in the 300 kW class, worked fine.
So, why can't you use this wide one or two turn sheet as your DRSSTC
tightly coupled primary?
Can't get much tighter than that.
Scoped ring time through the transformer was about the same as a
shorted to cap body spark gap shot though I think the ~ 31,225 J
blast harsh on the ears.
So why can't an IGBT in TC service pump such a wide and tight
coupled primary, after all, IGBT's do it all the time in induction
machines, just driven CW Vs pulse.
Normal or backwards, a transformer is a transformer.
Mike
----- Original Message ----- From: "Tesla list" <tesla@xxxxxxxxxx>
To: <tesla@xxxxxxxxxx>
Sent: Friday, December 16, 2005 9:39 PM
Subject: Re: A LUA file for BELA (a help to build electrostatic model of TC)
Original poster: Terry Fritz <vardin@xxxxxxxxxxxxxxxxxxxxxxx>
Hi DC,
They are trying to get high coupling with DRSSTCs to reduce the
number of cycles and thus time it takes to bring the secondary
voltage up. This helps to decrease the load on the IGBTs and gets
a better LERT.
LERT is Load Energy Rise Time. The DRSSTC folks have found that
there is an optimal time to energized the secondary to get the best
sparks. This time is fairly fast and demands a higher coupling.
ScanTesla (the computer program) says that coupling otherwise does
not matter too much and couplings like 0.13 might be just
fine. But that is before this fast rise time effect is taken into account.
ScanTesla can calculate LERT, but it does not really judge what is
good or bad. LERT is a little to unknown right now to make
"equations" but it seems to be a real important thing.
LERT might also be why coils with low surge impedance seem to do
well. We really don't know right now what it all means...
Cheers,
Terry
At 02:47 PM 12/16/2005, you wrote:
Has anyone tried purposefully reducing the coupling to see the
results? I'm not sure why you need the increase the coupling so
much more than for classic TCs. Some experiments along these
lines might produce interesting results.
Dr. Resonance
<snip
<You don't really need to do heavy electrostatic analysis or
differential equations to <get it right the first time!!
<snip
DC,
Your approach to Tesla Coil Design is fine for conventional
disruptive coils.
I have been here 8 years now, and have gained enough experience
to acnowledge that.
However, the electrostatic analysis i am playing with, is aimed
at the new breed of coils.
The DRSSTC`s.
Those of us that have built this type of coil can witness to the
fact, that flashovers from secondary to primary still is a
limiting factor with these coils.
In an attempt to solve this problem, I have tried to add
increasing amounts of dielectric in the space btwn. the 2 coils,
only to observe that the problem got worse.
.......