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Re: Pri-Sec Phasing





---------- Forwarded message ----------
Date: Sun, 05 Oct 1997 20:22:05 -0700
From: Bert Hickman <bert.hickman-at-aquila-dot-com>
To: Tesla List <tesla-at-pupman-dot-com>
Subject: Re: Pri-Sec Phasing

Tesla List wrote:
> 
> ---------- 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! :)
> 
<Experiment snipped...>
> 
> 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

Greg and all,

Interestingly enough, when I first looked at this from strictly a
hand-waving basis, I initially drew the same conclusion. However, I then
began thinking about how an autotransformer works (see sketch below).
Both portions of the winding definately have the same winding sense, and
yet the induced voltage, Vx, must be in phase and additive to Vin. I
suspect there's either a polarity reversal due to Lenz's law, or perhaps
in the definition of the directions of primary and secondary current
flow. It sure can get confusing, can't it...! :^) 

Anyway, "same winding sense, same polarity", is at least easy to
remember!


                 O
                 O<----------------------
                 O  +          +        |
                 O  Vx      ------>     |
                 O  -        Iout       |
    -------------*                      | 
    +  ----->    O                      |
         Iin     O            Vout     LOAD   
                 O                      |
    Vin          O                      | 
                 O                      |
                 O                      |
    -            O             -        |
   --------------*----------------------- 

Safe mega-coilin' to you, Greg!


-- Bert --