Re: [TCML] MOT Measurements

[bartb <bartb@xxxxxxxxxxxxxxxx>]

Excellent Finn! Thanks for barging in ;-) .

What I've learned from shunted transformers is that it is necessary to 
account for the change in mutual inductance as the reactance changes 
throughout the range of source voltage. And what I mean by that is that 
it is not possible to say this xfmr = L1, L2 values without specifying 
the parameters.

For coupling, I measure the primary current in both short and open 
circuit modes in convenient 10V steps. To do this, I use a variac and 
take both readings one after the other so that I have the same 
operational value at both measurements. Then, k = sqrt[1-(Ioc/Isc)]. For 
the MOT case, the primary short circuit current can rise very high, so 
readings above 20A I am forced to use an analog rms meter after 
calibration against a true rms digital at center scale. NST's are 
however easy. I would not attempt such a feat with a pole transformer or 
anything really big with the equipment I have on hand.

In one of the archived threads you listed, a poster mentioned that the 
inductances may not be accurate due to a changing mutual inductance. 
This is true but not complete. The open circuit inductances are fine, 
but they are only true for the operating measured rms voltage. As soon 
as the voltage changes, those inductances will change due to current 
through the windings and shunted core and thus reactance. It should be 
noted the inductance ratio is then affected including all it's implications.

For simulations, coilers often like to change the source voltage to see 
what happens at peak, but the transformer in the simulation must "know" 
it's a peak value being inserted (and it doesn't). Coilers should insert 
the source rms value and adjust the measured voltage probe output to 
peak. If not, with shunted NST's, MOT's, etc., the transformer model 
changes and that affects the output.

I'm not sure how well Tero Ranta's equations will perform, but as I 
already have all the data, I will have some fun checking it against my 
own measurements. The simulation of his transformer model versus my own 
and what currents and voltages should be will help discriminate what is 
real and what is not. I expect some variation between the two, but I'm 
not sure how much. Both will be assessed against measurement data.

Take care,
Bart

Finn Hammer wrote:
> All,
>
> Pls. forgive me for barging in here, just trying to help.
>
> Of course, this work by Tero Ranta is included in the Hot-Streamer files:
>
> http://www.hammertone.com/hot-streamer/TeslaCoils/OtherPapers/TeroRanta/CurrentLimitedTransformers/NSTModel.htm 
>
>
> http://www.hammertone.com/hot-streamer/TeslaCoils/OtherPapers/TeroRanta/NSTCapMatching/ResonantCapacitorMatch.htm 
>
>
> Hope this helps,
>
> Finn Hammer
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