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Re: Pri-Sec Phasing





---------- Forwarded message ----------
Date: Sun, 05 Oct 1997 12:16:02 +0000
From: Greg Leyh <lod-at-pacbell-dot-net>
To: Tesla List <tesla-at-pupman-dot-com>
Subject: Re: Pri-Sec Phasing

Bert Hickman wrote:


> > When the pri and sec are physically close to each other, the relative
> > polarities of the pri and sec should be aligned, in order to minimize
> > the voltage difference between them.
> > Given all of the possible 'minus-sign' errors, and the fact that the
> > two coils are 90 deg out of phase, should the coils have the same or
> > opposite 'handedness' when wound?
> > -GL
> 
> Greg and all,
> 
> This is REALLY tough to visualize, especially if you attempt to
> simultaneously take into account coil "handedness", voltage polarities,
> and current directions.[snip]... some experiments were clearly in order!

Experimental verification?  Hey, that's cheating! :)

> I made a couple of small coils, each with about a 2" long single-layer
> winding of #21 AWG on 3.5" long PVC forms (about 65 turns each). The
> smaller diameter secondary winding was on a 1/2" Schedule 40 PVC (about
> 13/16" OD), and the primary on 1" Schedule 40 PVC (about 1 5/16" OD).
> Both coils were carefully wound in the same direction. The smaller
> secondary coil fit nicely inside the primary, and the degree of coupling
> could be easily adjusted by sliding the secondary to various depths. Two
> types of experiments were run - simple transformer and dual-tuned
> circuits under transient conditions.
> 
> Simple Transformer Case:
> ========================
> In the first case, the bottom of each coil was connected to ground. The
> primary was connected to a B&K 3011 function generator, with the signal
> coming in the top of the primary winding. The top primary was connected
> to channel 1, and the top of the secondary winding tied to channel 2 of
> a Tektronix 434 Storage Scope. A 150 Ohm loading resistor was also added
> across the secondary. In all cases, and with any degree of coupling, the
> primary and secondary voltages were in phase.

Thanks for setting up this experiment!  The 'in phase' result 
was unexpected, however, for the following reason:
In a xfmr where the windings are wound in the same direction,
(and most are so that the bobbin doesn't have to be removed)
the start of the primary winding has the same polarity as the
end of the secondary winding.  This is because both _currents_
must be in the same direction, but the primary is supplying 
power and the secondary is removing power.

So I am still confused -- isn't this how xfmrs work?  Or have I
missed another minus-sign somewhere?


-GL