"flux cancelling windings" ?

Question

Hi all, I noted the following (below) on a website. What is Nc? Number of turns compared to what? I want to improve a transformer with very low capacitance between the pri and sec, this might help improve the coupling while maintaning the low capacitance. Do you have any more information on this technique? Thanks Mike K.

It is also possible to reduce leakage using a pair of "flux cancelling"

windings, each with Nc turns. Wind Nc turns first on bobbin. Then wind transformer. Then wind Nc turns (so one winding is "inside" the other "outside"). Connect the windings in parallel. Because the inner and outer windings have different flux linkages due to leakage, current will flow thereby creating a flux which counters the imbalance.

this trick gets used in HV transformers, where the secondary is a
>triangular profile to minimise capacitance (and keep HV the hell away)
>and a split primary is generally not used.

The Phantom · Accepted Answer

Can you provide a link so we can see it for ourselves?

amdxjunk · Answer

The website is just a repeat of threads from newsgroups.

formatting link

Third letter down, I think. I sent a letter to the author of the info but have not received a reply. Mike

The Phantom · Answer

He posts here regularly.  Maybe we can get his attention.

Tony · Answer

Nc doesn't matter, as the "Nc" windings are just shorted out anyway.Could be a single turn of foil for each (as long as the external linkswere of similar low resistance.But bear in mind, this is really a "flux shorting ring"-type ofmagnetic shielding, not a flux-canceling technique (it requires animbalance to work), and of course it generates losses, and it stilladds capacitance. If you have the space for this method, you might bebetter off putting it to more effective use by sandwiching one of yourexisting windings between a split version of the other. Maybe somejudicious electrostatic shielding will reduce the effect of the extracapacitance?Tony (remove the "_" to reply by email)

Terry Given · Answer

ya have now :)probably not. He's notoriously unobservant :)"Rise Time Reduction in High-Voltage Pulse Transformers Using Auxiliary Windings, L.M. Redondo, E. Margato, J.F. Silva, IEEE Trans. Power Electronics Vol. 17 No. 2 March 2002I have not tried this myself, but the paper is a good read. They used a transformer with:Np = 50Ns = 506Nc  = 25T (they call it Naux)The xfmr had double P-S insulation and "staircase" secondary construction to reduce capacitance. They play around with various resistances connecting the two Nc windings.CheersTerry

"flux cancelling windings" ?

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