Re: OLTC update - primary IGBT loss

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Terry Fritz" <twftesla-at-qwest-dot-net>

Hi Paul,

At 01:58 PM 9/4/2002 +0100, you wrote:
>Terry wrote:
>> I wonder if coupling in three spread out rings hurts the waveform
>> on the secondary.
>
>I wrote:
>> No sign of a problem.  The FT of your beat envelope shows only the
>> normal amount of higher mode ringing.  No sign of HF modes from
>> the primary either, so no racing arcs from this cause!

Ok.  That's great news :-)

>
>On a closer look, there is quite a large initial excitation of higher
>modes, but these are decaying quite rapidly due to low Q.   So overall
>the energy is quite low on the FT, but it's concentrated in the first
>cycle or two of the bang.  The resulting initial high dI/dt near the
>coil base might eventually give an excessive vertical gradient across
>the lower turns.  
>

This coil likes to arc to the primary at about 8% up the secondary coil.
There is a copper edge there too that does not help.  A bundle of poly
sheet fixes this.

>Terry wrote:
>> I hooked up the 60kV Jennings probe to the OLTC tonight:
>> http://hot-streamer-dot-com/temp/OLTC09-03-02.jpg
>> http://hot-streamer-dot-com/temp/OLTC09-03-01.gif
>> http://hot-streamer-dot-com/temp/OLTC9-3v.CSV
>> http://hot-streamer-dot-com/temp/OLTC9-3i.CSV 
>
>Ok, I got the model lined up.  I've setup a single turn dummy primary
>to have the same k and Lp as your system exhibits. I've had to do
>three 'fiddles': An extra 7pF to the topload to allow for the Jennings,

It is 3pF plus the wire and the added geometry.  The total of 7pF sounds
right.  It lowered Fo from 38.3 to about 35.3.

>and some extra Rp and Rs to line up the model's Q factors with results
>of tcma analysis of your ringdowns.  The rest of the model comes 
>entirely from your geometry.
>
>The resulting time domain response of the model compared with your
>waveforms is in 
>
>  http://www.abelian.demon.co.uk/tssp/tmp/OLTC09-03a.gif

Nice!  Shows the mechanics are all basically working.

>
>The match of the higher mode components is quite poor, Q, F and phase
>errors are showing up here - thus the fine detail of the waveform is
>not properly modeled.  This may be due to the coating you have over
>the secondary which will modify the internal C in an unmodeled way,

I need to try wrapping another coil to see if that affects anything like
Antonio suggested.

>and may also be due to my 'virtual primary'. Still, it's not a
>problem for this exercise so I won't investigate further.
>
>The primary Q is very low at these small voltages.  That's going to be
>an embuggerance later on - if Q depends strongly on Vpri then it will
>be tricky to compare below/above breakout waveforms.  To get around
>this we'd have to characterise the primary Q/Vpri function, as we've
>already done, but more accurately.  Then we can make proper allowance
>for the intrinsic primary Q as the voltage goes above breakout.
>

The Vpri voltage could be held constant and we could control breakout with
the terminal.  A pin point will breakout very easily were the toroid does
not at a given voltage.

>Incidentally, the sec Q is up a little from 45 to 53, 

Really!!  I wonder if it is due to the added C of the probe.  The probe's
added C is very high Q.

>and the tuning
>seems a little funny because the Fsec has dropped due to the probe
>but Fpri remains the same.

I did not retune the coil for the added probe C since I didn't want to
fiddle too much.

>
>Returning to the issue of higher mode excitation, note your initial
>-ve going Ibase transient peaks at roughly half the peak Ibase.
>Compare that with say Marco's Thor system.

I think I have much higher coupling!  ~0.25

>
> http://www.abelian.demon.co.uk/tssp/md110701/
>
>or some of your earlier solid state gap tests,
>
> http://www.abelian.demon.co.uk/tssp/tfss270501/
>
>In these and other similar cases, the initial transient Ibase is
>only around 1/10th or less of the peak Ibase.
>
>So I'd say the higher modes are quite high, but just look weak on the
>FT because they decay quickly.  
>
>Note that this higher mode excitation is not a result of your
>switching, but is intrinsic to the geometry of the resonator - your
>OLTC dual resonator has a rich and bright tone with a quick decay - a
>bit of a tin can sound if you could hear it.
>
>The animation is in 
>
>  http://www.abelian.demon.co.uk/tssp/tmp/OLTC09-03a.anim.gif
>
>and you can clearly see the initial transient ripple.  It doesn't seem
>to excite much voltage along the secondary, but that's a little
>misleading, because it is the dV/dx that causes the breakdown. 
>The voltage gradient animation is
>
>  http://www.abelian.demon.co.uk/tssp/tmp/OLTC09-03a.grad.gif
>
>which shows the radial and vertical E-field strengths along the
>surface of the secondary.  Now you can see where the potential (excuse
>pun) for breakdown comes.  If your secondary is going to break down at
>some power level, it will be in the bottom 10% of turns, I'd say.
>
>All in all, I'd say the OLTC looks reasonably manageable for some
>breakout tests.
>
>> I am wondering if this is useful to you
>
>Oh yes,
>
>>  and what you would like to see?
>
>Breakout!  But from a well defined surface, eg a small sphere or
>toroid.  

Is a pin point OK?  Breaks out at very low voltage and easy to remove for
no breakout with little other effect.

>I have to model its surface field so it must be on the
>axis of the coil.  I guess it will take some experiment to find a 
>low voltage breakout from a suitable small ROC object, bearing in mind
>the Jennings ratings.  I'll bet you could push those ratings a bit
>though :)

If the probe breaks down, there are many layers of protection between it
and the scope.  But I fear the probe would be damaged if the coil hit it
hard.  They are "rare" if not antiques...

>
>Question is, can you obtain a suitable breakout in a single-shot mode?
>Because I'd like to look at the waveforms for a single shot, then for
>the second shot of a pair, then the 3rd shot of a triplet, etc. 

The primary can simply be charged with a DC power supply through a resistor
(isn't that neat!! ;-))  The arbitrary signal generator can be hooked right
to the controller to pulse the coil in just about any way imaginable.
Since the primary and all is running at conventional voltages.  It is
extremely easy to control things.  I bet firing frequency is really
interesting ;-))  I can also quench just about anywhere too.  In fact - I
can charge, fire, quench, recharge, and refire long before the secondary
rings down...  No other coil on earth can do that (maybe Ken's can?)!  Sort
of like the firing difference between a flint lock and a machine gun!
Almost a CW-disruptive coil...  Even though this coil operates in
conventional modes, it is all electronically controlled.  Without the high
voltage primary and conventional spark gap to worry about and with the all
conventual electronic controls, controlling the coil to one's whim is trivial.

Wonder what happens if one refires 180 degree off from the present ring...
That should send some shock waves up the secondary ! :o))

Cheers,

	Terry


>--
>Paul Nicholson
>--
>