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Re: transformer design

In a message dated 5/14/99 5:02:22 AM Central Daylight Time, tesla-at-pupman-dot-com 
writes:


Bob

  There are several ways to rewind a three phase transformer.  If there is 
enough window area, you can wind the primary & secondary coils on the center 
leg.  This will have the best magnetic coupling with little leakage.

  The other way, if the core is made of different length "straight" 
laminations (not E shaped), is to restack them in a square or rectangle core 
form.  Then wind a primary winding on one leg and two other secondary 
windings with opposing phase on two other legs with one end of each winding 
grounded to the core.  This gives a center tapped secondary and insulation is 
easier with the total secondary voltage cut in half referenced to ground.
 This type of core design has a looser coupling, more reactance requiring 
less ballast and in some cases no ballast.
  I have rewound a 10 kW 3-phase 660/480 dry transformer of the latter design 
wound to 240 volt/13000 volt in oil and a steel enclosure.  When my large 
coil is properly tuned, I can run full power in with no ballast due to the 
leakage  of the magnetic path in this core.  

  If the core is rewound with a primary on the center leg, and secondaries on 
the outer legs, the secondary voltage will be half as much as if it was wound 
over the center leg primary assuming the same number of turns.  This is due 
to the magnetic flux being split in half through the outer legs.

 I have also rewound a 2 kW 3-phase reactor keeping the same coil form 
winding the pri/sec coils on the center leg.  This made a real hot little 
transformer housed in a 30 LB freon canister immersed in oil.

  You can also save cash if you reuse the primary wire.

Kevin E.



> Went to my favorite scrap yard today and hauled out a 3 phase choke. It is
>  marked "3 phase reactor 112 amps 415 volts" When I got it home I quickly
>  removed the windings,3 layers of 1/8"x1/4" section cotton covered copper
>  wire. This has left me with a core of the following dimensions 10" x10" 3"
>  with two windows 6" x 3" x 2". I have looked in the archives and think I
>  can work out a design. What worrys me is the permeability of the core as it
>  was a choke rather than a transformer,and whether I should interleave the
>  laminations or leave them as they are in an E and I configuration and
>  suffer the core losses, or try to design them out by increasing the turns
>  per volt. If any UK coilers are interested there are 4 or 5 of these in the
>  skip. I asked the man to put them on one side for me. They cost me 15 ukp
>  each and weigh 40 kilos each. heavy!!If anyone wants to run though the
>  numbers I would me grateful.
>  
>  many thanks 
>  
>  bob golding
>