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Re: [TCML] MOT Measurements



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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