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Re: [TCML] MOT Measurements

Hi Ted,

Good measurements.
I understand why you arrive at 4.5H. If your looking at Cres from a standard transformer impedance model (of which is commonly stated), then yes, you would calculate down to this low Henry. But with forced leakage inductance, I see this as changing. For a given frequency, there is an LC equal and opposite reactance. The reactance however changes throughout the range of power and so does Cres in these transformers. Try throwing different voltages to the MOT with the variac and redo the measurement to find out what Cres value is found. I'm not sure if 120V is too high for your MOT at Cres without possible breakdown, but if possible, it would be nice to double the voltage. My MOT should be resonant at 1.47uF with 60V input and again resonant at 2.84uF at 120V input. Should I use a resonant LC impedance formula, I would calculate a low Henry. And if I were to throw a standard model with that C, that is exactly the inductance I should need. But I no longer agree with this as it relates to shunted transformers.
Unfortunately, we don't do well at accounting for this complex case and we typically hand out Cres based on rated Vout/Iout as a constant impedance. I am guilty of this myself. If I were to use that impedance to find L, I would end up with a low inductance, and regardless of actual input, I would assume it as a constant throughout the range. We also see increased currents in shunted transformers at times resulting in higher bps that should not occur with constant impedance. We then come up with possible reasons why such as ferro-resonance or similar. It may be that the transformer itself behaves a differently than we would assume under loaded LC situations. 

I will also perform a similar measurement and see what I come up with.

Take care,
Bart


tesla wrote:

Greetings again team, thanks all for comments on this interesting thread
I had a practical look tonight into the resonance effects of a MOT . I had been assuming that the thread was concerned with finding out what resonant rise would occur and at what capacitance of a single MOT without any primary ballasting in that cct. 

METHOD
I used a normal 600 watt MOT with magnetic shunts in place.
I incremented the secondary load by 150nF steps up to 2.475uF and measured the voltage across the secondary. The source driving the MOT was a large Variac i.e. a voltage source and a very low source impedance. 

The results were interesting.
With sufficient primary excitation to cause over well 10 amps of primary current at resonance the resonance was very slippery changing with excitation as saturation effects altered the inductance. It was possible to find excitation values where you could watch the whole system "pull" into resonance slowly At lower excitation resonance was stable and occurred at 2.25uF above that capacitance the voltage magnification dropped off very quickly. I did not measure the primary voltage but it was quite low of the order of 60 volts. The voltmeter was a digital panel meter version of Peter Terren's "High Voltage Meter" on his Tesla down-under site (a great site) 

Some of the data points were

Load 0.6uF Secondary voltage out 398
Load 2.25uF Secondary voltage out 1352 (at or close to resonance)
Load 2.4uF Secondary  voltage out 513
The inductance seen by these capacitors is thus 4.5Hy in this configuration. Whether this is leakage L or transformed L from the primary side by N^2 or all of the above it seems to me that is the inductance the practical external load has to deal with when deciding what capacitive load will actually cause resonance.
Hope this is useful data, full data can be sent to anybody who would like it 

Best
Ted L in NZ
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