RE: transformer theory q

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

Original poster: "Loudner, Godfrey by way of Terry Fritz <twftesla-at-qwest-dot-net>" <gloudner-at-SINTE.EDU>

Hello Peter

The formulas posted on the internet for transformer design are for
approximate calculations, but they are close enough to actual operating
performance. The assumption is made that the coefficient of coupling between
the primary and secondary is close to 1. The formulas also account for
making maximum use of the core's ability to carry magnetic flux, and a
certain assumption is made about the magnetic permeability of the core. To
get a coefficient of coupling close to 1, one must pack as much copper as
possible close to the core and place the primary and secondary as close
together as possible. Without machine winding and professional insulating
materials, it is difficult to obtain such close coupling. Homebrew windings
tend to get very large. Even a small bump on a winding will become more
pronounced as the winding proceeds. Even if you do wind excellent coils,
there is the problem of heat sinking. If your transformer cannot dissipate
the heat generated at a fast enough rate, homebrew insulation is sure to
fail. This is why the homebrew transformer builders should get a large core
with a lot of window space. 

As you use less turns in the primary, it will draw more current. Then you
will have to use heavier wire to reduce the heat generated by the extra
current. There possibly goes your extra window space. Also using less turns
in the primary will increase the number of secondary turns necessary to get
one volt from the secondary. Now you will have to add more turns to the
secondary. So using less turns in the primary will actually cause you to
need more window space. A possible way out of this dilemma is to use a finer
wire in the secondary than what you have at hand. This will cause the
secondary to run hotter, but you might be able to get by on short runs. The
real way out is simply to find a larger core. But I know that big cores are
hard to find.

BTW a good insulating material is artist Kraft paper. Glue the paper
together only at the ends. When you submerge the transformer under oil, the
paper absorb oil and become an excellent insulator. It is not wise to paint
the paper with polyurethane, let the oil do the job.

Godfrey Loudner    

   

> -----Original Message-----
> From:	Tesla list [SMTP:tesla-at-pupman-dot-com]
> Sent:	Tuesday, November 13, 2001 1:40 PM
> To:	tesla-at-pupman-dot-com
> Subject:	transformer theory q
> 
> Original poster: "Peter Lawrence by way of Terry Fritz
> <twftesla-at-qwest-dot-net>" <Peter.Lawrence-at-Sun-dot-com>
> 
> 
> Tranformer gurus,
>                  I might actually make some progress on my mechanical
> xfrmr
> winding jig over Thanksgiving holiday, so I have a question about xfrmr
> design.
> 
> If I follow the design guide at for example 
> 	http://members.tripod-dot-com/~schematics/xform/xformer1.htm
> I get a certain number of turns in the primary to match the core cross
> section
> of the laminated core that I have obtained.
> 
> What if I use fewer turns in the primary to up the turns ratio (I've only
> got
> so much room for secondary turns in a fixed winding window size).
> 
> What happens with fewer primary turns than "recomended" for a given core.
> 
> thanks,
> Pete Lawrence.
> 
>