RE: [TCML] transformer theory questions

["Lau, Gary" <Gary.Lau@xxxxxx>]

Hi Thomas,

Comments interspersed:

> -----Original Message-----
> From: tesla-bounces@xxxxxxxxxx [mailto:tesla-bounces@xxxxxxxxxx] On
> Behalf Of Thomas Ryckmans
> Sent: Sunday, September 20, 2009 5:50 PM
> To: tesla@xxxxxxxxxx
> Subject: [TCML] transformer theory questions
> 
> 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.
> 
> 
> 
> How is the current output of a NST fixed?

An NST is designed to be current-limited, meaning that even if the secondary is shorted, the short circuit current is limited to the faceplate-value, like 30mA.  This allows us to use NST's without using external current limiting measures as one would need if using a pole pig or other non current-limited xfmr.  

The limiting is achieved by the design of the magnetic "circuit" of the core laminations.

However, while this mechanism will limit the short circuit or resistively-loaded current, it's very possible to draw far in excess of the faceplate or short circuit current, when using a capacitive load as we do.  This is why one commonly sees NST input current far exceeding the value expected based on the VA rating.

> 
> 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?

Yes indeed - very messy.  It's typically done only as a last resort, in an attempt to resurrect an NST that has a secondary short and would otherwise be trashed.  But in addition to reviving a broken NST, the bonus is that many (but NOT all) NST's can have some of their "shunts" removed.  Shunts are small, removable bits of steel laminations press-fit into the main portion of the core assembly.  This causes the current limiting threshold to be raised, effectively turbo-charging the NST.  One can easily double the current rating by removing shunts.  Just beware that the NST will get hot FAST and it would be unwise to attempt long runs.

Note that depotting and shunt removal are separate operations.  I depotted my Jefferson 15/60, only to find that this model did not have removable shunts.  But at least I got it working.

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

I wouldn't do that.  I expect that the consequences are largely mechanical - like you won't be able to get it back together again after the laminations are mangled.

> 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?
> 
> Many thanks
> 
> Thomas

Regards, Gary Lau
MA, USA
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