Re: Modeling the transformer

["Finn Hammer" <f-hammer-at-xxxxxxxxxxxxx> (by way of Terry Fritz <twftesla-at-xxxxxxxxxx>)]

Herbert!

I think that what you have, is a potential transformer, which is
intended to deliver an accurate means of measuring the voltage on a
HV-line, but not deliver any great power doing so. The highish secondary
resistance points in that direction. I can only guess at the reason for
putting a large airgap in the core, the only reason that I can think of
is a desire to make the transformer immune to a DC-bias on the line.

The open circuit current is much to high for a power transformer this
size.

I would suggest trying to take that airgap out, by rearranging the core
elements, if possible. This would raise the inductances to more usefull
levels.

In the model, as in the real world, the voltage step up ratio is
determined by the inductive ratio, whereas the short circuit current is
determined by the coupling. (And I don`t think the coupling is
particularly dependant on an airgap, in fact it must be very high on
your transformer, due to the concentrig arrangement of the windings).

However, a transformer like yours is not in it self short circuit
current limited. The 130 mA rating has to do with the accuracy class
that the transformer is placed in. It will deliver much more current
into a short than that. You need to put an external inductor in the
primary feed to acheive current limiting, and it is this inductor that
should be in the 30-70mH range.

Anyway, I don`t think the inductances in the transformer need to be
modelled particularly accurate, with regard to the resonant situation.
The transformer is itself "transparent" , in the sence that it
transforms the  current limiting inductance into an equivalent higher
value on the secondary side, trough it`s turns ratio squared, and it is
this value that determines the resonant frequency in concord with the
tank capacitor.

To sum up:
If possibly, take the airgap out of the transformer, and get the open
circuit current down to a reasonable level.
Establish a model that provides the voltage step up level trough 
Lsec= Lpri^2 * V_stepup_ratio
Set the coupling to 0.9999
Introduce a current limiting inductor in the primary feed, and adjust
the size of it for appropriate short circuit current draw.

I am sending you the model that I use for my own needs, off list,
perhaps it will give you some new ideas. It is currently set up with 4
MOT`s.

Hope this helps.

Cheers, Finn Hammer 

Tesla list wrote:
> 
> Original poster: "Herbert Mehlhose" <herbert.mehlhose-at-kdt.de>
> 
> Hi,
> thank you Terry for your explanation to my question some time ago.
> I have now done my model as you told me.
> My measurements with open load and calculations lead to 68mH primary
> and 323H secondary inductivity. At 229V my transformer draws a current
> of 10.7A with open circuit.
> My transformer has a rating of 220V/15.4kV at 130mA. It has a 5mm
> gap in the core.
> I ran the model with closed output and adjusted coupling until the
> secondary current matched.
> With open load, it was not enough to change the secondary inductance
> just a bit, instead I had to change it to a much higher value. This again had
> an influence on the short current again..... So I ended up with several
> iterations until everything seemed to be fine.
> The model now looks like: L primary=0.0681H, L secondary=700H.
> R primary=0.25ohm, R secondary=1352ohm.
> Coupling is quite low: only 0.68 (due to the gap in the core...)
> What strikes me is the quite high difference in the secondary inductivity
> values: 323H vs. 700H.
> I now did a measurement, feeding voltage in the secondary and
> measuring the current with open primary. This leads to a
> value of 274H, much closer to 323H.
> I'm not sure about my 700H value now. This is especially important for me,
> as with 700H I will have about 40Hz resonance with my cap resulting
> in resonant charging effects. With 323H this would be 59Hz, so if I am
> somewhere in between, my resonance will be VERY close to
> 50Hz. So I will need a very good safety gap over my 100bps SRSG
> I'm currently building. I'm trying to add a model of the safety gap into
> my microsim schematic, but up to now only with strange results.
> Any ideas where this large difference comes from? Maybe the gap
> in the core makes this model inaccurate??
> Under
> http://home.wtal.de/herbs_teslapage/model2.html
> there is some information about the new coil I build, also a small
> picture of the transformer.
> 
> Thank you very much for any help.
> Herbert