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Re: Pri-Sec Phasing



---------- Forwarded message ----------
Date: Mon, 6 Oct 1997 12:34:48 +1200
From: Malcolm Watts <MALCOLM-at-directorate.wnp.ac.nz>
To: tesla-at-pupman-dot-com
Subject: Re: Pri-Sec Phasing

One way of solving the problem is to think about the implications of 
a bifilar winding when considering transformer action....

> 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?

The start of the primary should have the same polarity as the start 
of the secondary, always. Winding direction assumes great importance 
in various types of SMPS.

Malcolm