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Re: homemade transformer

Original poster: Jim Lux <jimlux@xxxxxxxxxxxxx>

At 04:51 PM 2/2/2005, you wrote: 
Original poster: Bobby Amaya <dimon20042004@xxxxxxxxx>

Hi everyone,

Sorry about the confusion. I think i'll try to re
write
my e-mail:

Hi everyone,
I am perferably looking for plans to build a
transformer with an input of 120v 5-20Amp and an
output of around 15kv and at least 50mA. Sorry for
the confusion ;-)

Bobby,

===== 
Here's some design guidelines...
It's an iterative process...


Start with picking wire sizes.. 750 circular mil/amp for the windings
The HV winding will be mostly limited by winding strength (say, AWG30 to AWG40)
Figuring for 20Amps.. you want 15,000 circular mil cross section (15kcmil).. get out the wire table: AWG8 is about 16kcmil and AWG10 is about 10kcmil.

That's pretty big wire (and a royal pain to wind!)...
You might want to go with a bit bolder design, say 500 cmil/amp... that would get you down in the AWG 12 or AWG14 territory. A bit more loss, a bit more heating, but you're probably not going to be running 24/7 at rated load, either. Sink the beast in oil for better cooling and you can be a LOT more bold with the wire size.



Figure for a max flux (Bmax) of 1 Tesla (10kGauss)

volts/turn = 1/(4.44 *freq* Bmax * area)
(area in square meters!)
Most transformers wind up being a few volts/turn.. Say you want 2 volts/turn

You want to solve for core area:

area = 1/(4.44 * freq* Bmax *Volts/turn)
     = 1 (4.44 * 60 * 1 * 2)

= 18 sq cm, which is not too huge... (about 3 square inches)

You already picked 2 volts/turn, so you know you're going to need about 60 turns for the primary, and some 7500 turns on the secondary. A motorized winder for the secondary will make life a LOT easier (I can't imagine winding that many turns by hand!). You can modify a bicycle odometer to count turns. Wind the coils on bobbins that fit over the core. You're probably not looking for the world's most compact transformer.

The hard thing to find is going to be the transformer core. The best way to go about this is to find a transformer that is already built for about the right power level (2-3 kW in your case), use the existing 120V winding, and wind a new HV winding. If you choose a transformer that is somewhat bigger than what you need, it will have bigger spaces for your homemade secondary, which will never be as dense as that nice machine wound commercial product.

Measure the core, make some guesses...and start figuring. You can put a single turn through the core (using any old fairly small wire), put 120V on the primary, and measure the voltage on your new secondary turn with a voltmeter to get the volts/turn ratio.