Re: Transformer Rewind?

["Malcolm Watts" <M.J.Watts-at-massey.ac.nz> (by way of Terry Fritz <twftesla-at-uswest-dot-net>)]

Hi Garth,
          Nice score.

On 27 Jul 00, at 16:58, Tesla list wrote:

> Original poster: "Garth van Sittert" <garth-at-mediasupplies.co.za> 
> 
> Hi
>  
> I have recently laid my hands on a resonably large transformer that I
> intend to rewind to my coil specs.  It originally weighed in at about 20kg
> with the copper windings.  How do I calculate the VA rating from the core
> size and ampere turns/windings for the primary.  The core measures as
follows:

Your easiest bet for TC use is to run with the original 
primary on one leg and a new secondary on the other. You will 
have to find out how many primary turns there are in order to 
design a new winding for a specified output voltage. You can 
do this by first measuring the existing turns ratio at some 
low voltage then counting secondary turns as you unwind them, 
or alternatively winding a new temporary secondary of say 50 
turns over the existing secondary and again measuring the 
turns ratio between primary and this new winding at low 
voltage.  
  
>        ==========
>        ==========
>        *==*  *==*           height = 10 inches
>        *==*  *==*           width = 7 inches
>        *==*  *==*           sectional area = 4  sq inches (2" x 2")
>        *==*  *==*           bobbin height = 6 inches
>        ==========
>        ==========
>  
> There are two bobbins (*  *)on the square shaped core.  I am hoping to get
> at least 3kVA out of this baby.  Is this wishful thiking.   What size/guage
> wire do I use for x current.

I think you could do 3kVA on an intermittent basis based on a 
similar but smaller transformer I wound for one of my coils. 
Once you know how many turns you need you should use the 
largest wire which allows you to insulate internal windings 
from each other and the bobbin inside and cheeks from the core 
and any other objects including the primary  in its vicinity. 
If you use good quality materials and some ingenuity you can 
make very good use of the available winding window. For 
example, suspending the bobbin so that neither cheek sits hard 
agains a leg enables you to pack some insulating material 
between the cheek and core. Using an air drying varnish on 
each layer together with a thin layer of transformer paper 
wetted with varnish between layers provides plenty of 
interlayer insulation. I wound mine so that each layer went 
hard up against each cheek with no creepage gaps in between. 
If you do this well, each layer should screen its successor 
from the layer below it. You then have to ensure the bobbin 
cheek is clear of the core to discourage a spark from punching 
through it. Cover the entire winding with a thick impervious 
coating of synthetic rubber or plastic or epoxy - anything 
which won't melt at elevated temperatures. I was also able to 
get two layers of teflon tape between my bobbin and the core 
as I didn't wish to ground the winding. Look for any weak spot 
of any sort - put yourself in a spark's place to see where a 
weakness may exist.
    Another precaution I took was to put a complete wrap of 
teflon tape in every 15 layers or so making sure the tape was 
somewhat wider than the core so that it partially climbed up 
the inside of both cheeks as it was pulled tight.
    BTW - a transformer wound like this will have plenty of 
leakage inductance so shouldn't need an external inductor for 
ballasting.

Good Luck!
Malcolm