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Transformer basics

Hi all 
      my intro post hasn't arrived here yet but some may remember me from
tesla-at-usa-dot-net. I posted there before about the problems i've been having  
getting suitable supply transformers. At the moment i dont have a working 
coil and the only transformers i could lay my hands on (10kv-at-30ma and
10kv-at-60ma) 
seem to be all that was available. I have all the local sign shops looking-
out as they dismantle old signs and have even contacted a company in london 
who used to wind a very large proportion of neon transformers in the uk but 
every one seems to be using solid state 600V H.F. supplys now, (probably 
another damn E.C. regulation). My two transformers give me about
1.1KVA after unpotting and removal of a couple of shunt plates, and are 
probably more than ample for my first attempts, although i'll probably soon
outgrow and /or destroy them in my first efforts. 

This has led me to look at alternatives and have considered microwave units
as a bit of a safty liability, lower power option. The local power board
is very reluctant to enter negotiations of ex-substation potential xfrmrs
and a pole pig is well over my horizons yet.

Currently i'm looking into the possibilitys of "rolling my own" and have
come across the following set of guide lines for designing transformers
which are as follows:-

First and foremost, determine the final required KVA rating of the xfrmr.
This determines the weight of the iron core to be used. The larger the KVA
the heavyer the core. The larger the core the less primary turns required
though thicker wire needed. Obviously the less primary turns the less 
secondary turns for a given voltage.

Once an initial core has been selected from available stampings ect. the
number of turns per volt of back emf in the primary is calculated, then 
multiplyed by the supply voltage, to give the total number of primary turns
required. 

The secondary is then simply calculated by volts ratio = turns ratio.

The wire gauge of of the conductors is chosen by the current required to 
flow in each winding, then as a final check, the resistance of the winding is 
calculated and used to determine the volt drop at full current and the next size
up selected if unacceptable.

That was the simple bit. Now the total size of the windings are calculated by
the Wire manufacturers Turns Per Inch data and adding the total thickness of
all insulation should be checked against the selected core to see if it will
in fact fit. If not select the next largest core dimensions and start again.

Phew!!

My questions are, has anybody wound their own before and if so could you offer
me any practical advice such as ball park core weight for a 2-3 KVA xfrmr, any
practical dimensions obtained and how the H.V. winding was wound for good 
insulation i.e. is it nessacery to varnish impregnate the windings ect.

I have some calculations for determining core weight ect. But were taken from
a book published in 1950's and has probably been far exceeded by modern 
silicone irons used in cores ect. In fact i checked  a couple of 440-240
power transformers and their weight was a little over two thirds the value 
calculated for the weight of the iron core alone(and this was the total
weight including copper and frames).

 
*******************************************
Ian Hopley ---->  i_hopley-at-wintermute.co.uk
Aberdeen
Scotland
*******************************************