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Here some good info re "fractional turns"

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Tony Matt

Wed Sept 24 2014, 01:45AM

Registered Member #3700 Joined: Sat Feb 19 2011, 12:59PM
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Posts: 107

Hi !
I have followed the paper to construct fractional turns trafos:

Here a most recent publication

See figure 11 b, two cores as close as possible. There are two primaries each one coiled in each core, connected in parallel. The secondary, just one secondary, coiled over the two cores and embracing the primaries.

If the dispersion reactance is not big deal, the fractional turn is fine.

EDIT: fixing the addresses

Patrick

Wed Sept 24 2014, 01:59AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

Tony Matt wrote ...

cache
Similares

am i the only one who cant see these links?

kimbomba

Wed Sept 24 2014, 03:00AM

Registered Member #3854 Joined: Fri Apr 29 2011, 03:45AM
Location: Mexico
Posts: 95

[/quote1411523962]am i the only one who cant see these links?

[/quote1411527596]

Seems so, i can.

Dr. Slack

Wed Sept 24 2014, 06:10AM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

I can see them. The first iis correct in that balancing the flux beyween the outer limbs is the way to go. But it's very wordy and not as clear as my diagrams in the other thread.

The second link doesn't seem to say anything about fractional turns, at least not on a quick skim through, seems to be more concerned with fast risetimes, which means minimising residualC * leakage L. This would have the side effect of trying to improve leakage L, which if you'd cocked it up with trying to make the wrong sort of half turn would be relevant, but it's not the thrust of the paper.

Patrick

Wed Sept 24 2014, 06:19AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

so in the first link, the winding labeled 1A is 1.5 turns as drawn right? 1 full turn around the center leg, plus more wire around the half flux-containing outer limb. Right?

Dr. Slack

Thu Sept 25 2014, 08:58AM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

I've been doing some more thinking about fractional turns in transformers. While balancing the number of turns round all sub-cores will work, and tends to be sufficient, it is not necessary. However, the conditions for a transformer to work without meeting that condition needs an unusual winding for the secondary, one that will probably defeat the object of the transformer, like obtaining high insulation distances.

The conditions for a transformer to work could be restated like this

a) Any winding scheme should not rely on finite permeability or resistance to work, it must also work in an ideal transformer

b) An ideal transformer has infinite core permeability

c) With infinite ur, any holes in the core that are threaded by current carrying wire *must be capable of being fully cancelled*. If they are not, it will result in infinite leakage inductance, and so the transformer will not work

d) In a transformer with one primary and one secondary threading a single magnetic loop, we can always find a secondary current that cancels the primary current, to meet (c). The turns ratio defines the current ratio for which cancellation occurs (equal amprere.turns product on primary and secondary windings)

e) Topologically, fractional turns do not exist. But we can split the core into portions, and put complete turns round parts of the core, whether those sub-cores are the outer limbs of an E core, or multiple cores assembled into a larger core. These turns behave as far as flux linkage and magnetising force as if they were partial turns on the whole core.

f) In a transformer with more than one magnetic loop, (c) has to be met in all loops simultaneously. This means that all holes in the core must be threaded by a bundle of wires that have the same turns ratio, in order that a primary and secondary current of a specific ratio can meet (c) in all holes at the same time.

g) The consequence of (f) for a transformer that is wound with a single secondary round all sub-cores in a multi-magnetic-loop transformer, then the primary must have exactly the same number of turns round each sub-core, otherwise the turns ratio for each magnetic loop is different. In this case, meeting (c) balances the number of turns round each sub-core.

h) If the primary puts different numbers of turns round each sub-core, then the secondary can only meet (c) by also using different numbers of turns on each sub-core, to maintain the turns ratio. This will result in a non-homogeneous secondary, almost certainly not suitable for an HV transformer. A non-homogenous primary is generally more tolerable, being a few turns of low voltage wire.

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