Re: NST resonance

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Original poster: "R.E.Burnett by way of Terry Fritz <twftesla-at-uswest-dot-net>" <R.E.Burnett-at-newcastle.ac.uk>


Hi Jan,

> Original poster: "Jan Ohlsson by way of Terry Fritz
<twftesla-at-uswest-dot-net>" <jan.ohlsson-at-mbox319.swipnet.se>
> 
> To investigate the resonance of a 8/60 NST I parallelled the secondary with
> MMC:s from 1.6 nF to 6.8 nF and made impedance sweeps with an audio sweep
> generator of the primary impedance. The impedance curve of the primary
> showed as expected a sharp impedance peak of high value and then a gentle
> rising caracteristic towards the upper end of the audio spectrum.

I thought maximum current was drawn at resonance because the capacitive
reactance cancels the inductive reactance of the NST shunts.  Therefore I
would expect you to see an impedance minima at resonance ???

> What surprised me was that the center frequency of the resonance peak did
> not change as expected if I, for example, quadrupled the value of the MMC.
> That should theoretically give half  the resonance frequency, but the
> actual value was higher than that.
> 
> I can only come to the conclusion that there are other sources of
> capacitance at work, probably within the NST, of the order of a couple of
> nanofarads. That would make simple calculations of NST/MMC resonances
> unpredictable.

nF at the secondary,  or maybe uF at the primary side.  Your NST may have
a PFC winding or a PFC capacitor at the primary side which will really
affect your impedance plots.

Interesting stuff.  Keep up the good work,

							Cheers,

							-Richie,