[TCML] transformer theory questions

[jimlux]

Thomas Ryckmans wrote:
> Hello,
> 
>  
> 
> I have general questions about transformer theory, and hopefully you'll be
> able to point me in the right direction. looking at textbooks, all
> transformers behave "ideally" so it is not of great use. 


Depends on the text book, eh?
I particularly like the one by Theodore Wildi...

> 
>  
> 
> How is the current output of a NST fixed?
> 

Lots of "leakage inductance"... essentially, a huge inductor in series 
with the output of an ideal transformer.  The impedance of the inductor 
(at line frequency) is such that the short circuit current is the 
nameplate current.


>  
> 
> What is depotting - I understand the removal of tar etc, but why do it
> (sounds messy) and does it allow to change the output current?
> 

Junked transformers might not have any output, due to an internal short 
or broken wire. Depotting allows you to find it.

The leakage inductance (see above) is done by adding a magnetic path 
(shunt) between the windings. That extra path can be removed, by hammer, 
chisel, or saw.


>  
> 
> What happens if you cut into the frame of a transformer, cutting the across
> the iron lamination?

Not much.. It doesn't help with the transformer action, of course.



> 
>  
> 
> I also had a play with an old MOT, cutting the secondary winding and
> replacing it with a few turns of heavy jumper cable to make a spot welder.
> Works alrightish. It is an EI configuration with both the primary and the
> secondary coiled around the central bar of the "E". Is it possible to
> improve performance by changing the "EI" e.g. cutting a gap in it?

Not really.. your transformer has both windings on the center bar, 
right?.. Think of it as a magnetic circuit, with "magnetism" flowing 
through the center bar and returning through the outsides. I'll bet 
you'll find that the total cross sectional area of the two outsides is 
about the same as the cross section of the central bar.

>