Re: Xfrmr theory, was Re: I could cry......

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Malcolm Watts by way of Terry Fritz <twftesla-at-uswest-dot-net>" <m.j.watts-at-massey.ac.nz>

On 19 Apr 01, at 18:18, Tesla list wrote:

> Original poster: "Peter Lawrence by way of Terry Fritz
> <twftesla-at-uswest-dot-net>" <Peter.Lawrence-at-Sun-dot-com>
> 
> Guys,
>      neither your's nor John Freau's reply seem to make any sense to
>      me.
> >From a physisist's conservation of energy point of view (perhaps
> >incorrect,
> but I'ld like to understand why) I see energy comming in as
> electricity, being transformed into magnetic field energy, being
> transformed back into electric energy in the output winding. The
> maximum magnetic field that can be generated in the core (the
> saturation limit?) represents a limit on how much energy can be
> transfered through this mechanism.
> 
> The statement below that "[core area] in principle has nothing to do
> with the power handling capability of the transformer" flys in the
> face of common sense (which is not always correct, so explain if not
> so) and it flys in the face of the engineering data that says to
> select a core cross section based on Volt*Amps of the coil.
> 
> -Peter Lawrence.
<snip>

      I have to agree with Ed. Power handling relates mostly to the 
ability of the transformer to lose heat from both windings and core. 
Applying a load to the secondary actually reduces core flux because 
of flux opposition from the loaded secondary. So why the VA rating? 
It actually has to do with the available winding window for the core. 
How much thick copper can you squeeze into it? And the core is rated 
on a continuous delivery basis for the manufacturer set thermal 
running conditions (e.g. running in still air, running with oil and a 
radiator etc.). You can suck a huge amount of power from a 
transformer on an intermittent basis. Obviously, the windings are 
going to burn up sooner or later depending on the load and duration 
it is applied for. So it mostly a question of the transformer's 
ability to lose heat, mostly from the windings if I mag is set to a 
reasonably low value.
     One typically sets Imag to be a percentage of the reflected load 
current in the primary and of course Imag peak must not bring the 
core too close to saturation. The lower you set Imag, the more 
primary turns you must have for the applied voltage and the squeeze 
is on - more turns = greater length of wire in the primary and 
because of this, the copper size must increase if primary losses are 
to be kept low since the reflected load current will conspire to 
increase winding losses. A double whammy in fact. And of course, more 
primary turns = more secondary turns for the same secondary voltage 
and again you have a requirement to increase copper size in the 
secondary to meet loss requirements.
      If one sets Imag to be a high value (say = reflected load 
current), the primary turns will decrease accordingly and copper size 
in the primary must increase sharply to accomodate more current 
remembering that losses are dependent on I^2. Basically there is a 
happy medium for a given core.

Regards,
malcolm