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Re: Pri-Sec Phasing
---------- Forwarded message ----------
Date: Mon, 6 Oct 1997 17:16:08 +1200
From: Malcolm Watts <MALCOLM-at-directorate.wnp.ac.nz>
To: tesla-at-pupman-dot-com
Subject: Re: Pri-Sec Phasing
Bert, Greg, all,
Here is what happens.....
> From: Bert Hickman <bert.hickman-at-aquila-dot-com>
> To: Tesla List <tesla-at-pupman-dot-com>
> Subject: Re: Pri-Sec Phasing
<snip>
> > 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 - |
> --------------*-----------------------
Iin in the inductor causes a magnetic field to build up such that it
is in opposition to the current (npi) state. That field generates an
emf which attempts to push current back out of the primary (or at
least to impede its flow). Since the flux lines cut both windings
without any change of sense, the secondary also has a current flow
out of itself. The situation is: V across both windings wound with
the same sense is identical in polarity. Current flow is the opposite
since one winding is having current fed in while the other is
attempting to source current out to the load.
Malcolm