Thanks for clearing that up, Eric. I have been saturated with work,
and my personal life has been in flux, so I was reluctant to pull out
my electromagnetics textbook in order to get to the core of the
issue :):)
73 de Mike, W4EF...............................
----- Original Message -----
From: <EVonvaltie@aol.com>
To: <Amps@contesting.com>
Sent: Tuesday, May 07, 2002 9:05 AM
Subject: [Amps] Basic transformer theory
> Sorry - none of you has it right. Here is how it goes:
>
> At no load, the primary sits there drawing a current which is V/X, where X
is
> the inductive reactance of the primary. This current is commonly referred
to
> as the "magnetizing current". This current will indeed produce a flux
density
> and total flux given by the well-known transformer equation. If you
analyze
> this formula in detail (which I have done numerous times for my students)
you
> will see that the voltage, current and time-integral of the flux are all
> inner related. That is, a given voltage will produce a specific value of
> current with the inductance being the "scale factor". It (inductance)
> contains all of the geometric data for the studied device.
>
> Now, when a load is applied, the secondary current that flows now begins
to
> also make an ampere-turn contribution to the total flux, because it shares
a
> common core with the primary. This flux then produces a reaction back on
the
> primary in the form of an induced voltage that produces an equal and
opposite
> flux in the core. With total cancellation, the only remaining core flux
and
> primary current is the values that were present with no load.
>
> This analysis is much easier to visualize if you actually do it formally
with
> the correct equations. What you get is a pair of coupled equations for the
> core flux, which have to be solved simultaneously. The result is simply
the
> following, as the astute contributors to this discussion have already
stated:
>
> 1. The total flux and flux density is invariant with load.
> 2. The primary current contains two components: I(magentizing) and
> I(load)
> where the first is simply V/X(L) and the second is
> V(pri)xN/R(load)
>
> Of course, this is the zero-th order of approximation. In a real power
supply
> the resistances and additional core data become significant, and then the
> plot really thickens!
>
> Eric von Valtier K8LV
>
>
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