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Fuzzy Phazing
---------- Forwarded message ----------
Date: Thu, 9 Oct 1997 19:16:31 -0400
From: Thomas McGahee <tom_mcgahee-at-sigmais-dot-com>
To: Tesla List <tesla-at-pupman-dot-com>
Subject: Fuzzy Phazing
----------
> From: Tesla List <tesla-at-pupman-dot-com>
> To: tesla-at-pupman-dot-com
> Subject: Re: Pri-Sec Phasing
> Date: Tuesday, October 07, 1997 11:02 PM
>
>
>
> ---------- Forwarded message ----------
> Date: Tue, 07 Oct 1997 18:54:31 +0000
> From: Greg Leyh <lod-at-pacbell-dot-net>
> To: Tesla List <tesla-at-pupman-dot-com>
> Subject: Re: Pri-Sec Phasing
>
> Thomas McGahee wrote:
>
>
> > Greg,
> > I agree with Malcolm. If what the book said was true, then if you
> > bifilar wound two primary coils and then connected the start of
the
> > windings together and then the endings together you would have a
> > current flow between them due to their supposedly opposite
> > polarities. But that just ain't so.
>
> That is true, assuming both parallel filaments make up a primary
> winding. But what if one filament was the primary(sourcing power),
> and the other was the secondary(removing power)?
>
>
> -GL
The "mental problem" that afflicts us as we try to unravel
the polarity and current direction issue is due to the fact that
there are two different sets of things going on at the same time.
Imagine a Perfect transformer. The primary has an AC voltage applied.
In a perfect transformer with an open-circuit secondary there would
be no current in the primary (in the real world there is always some
imperfections such as a finite core, etc...) This is so because any
changing magnetic field produced will result in a self-induction that
will exactly oppose the source current direction (and magnitude). In
the case of the
secondary, the induction has nothing to oppose, so you get a voltage
polarity out that is opposite that of the source (but no current if
it is open-circuit).
Now, if you put a load on the secondary, this will cause actual
primary "source" current to flow. The resulting magnetic field from
the primary becomes the "source" of the current that flows in the
secondary. The direction of the producing and produced current flows
is opposite (for the sake of discussion, assume primary and secondary
are bifilar wound, so windings are identical). Assuming no loss in
the core, the primary and secondary currents (in our bifilar example)
would be equal and opposite in direction. In the primary, current
flow produces a changing magnetic field. In the secondary, the
changing magnetic field produces current. (But only if there is a
load)
One of the things that trips people up is that there is no primary
current when there is no secondary load. When there is no secondary
load, primary inducing current is equal and opposite to primary
self-induced current. So the sum of the two is zero/zip/zilch. Add a
secondary load, and current is being created/removed/used/consumed at
the
secondary. So there must be a corresponding primary current "loss" so
to speak, since the primary is ultimately the source of the current
created/removed/used/consumed in the secondary.
I'm writing this in between a class period change, so it is somewhat
rushed. It is not meant to be an absolutely complete exposition on
transformers and sense of winding. It is meant only to perhaps help
some members of the list to visualize what is being discussed.
Hope I haven't made things any fuzzier for anyone.
Fr. Tom McGahee