# RE: [TCML] Questions about saturable reactor / magnetic amplifier

```Stefan,
If you want to know the core size needed for a particular power level, an
approximation for a normal transformer can be found by taking the square
root of the expected power and multiply that figure by 0.14.

Example:
A 25,000 watt transformer would have a core size of sqr(25000) * 0.14 = 22
sq inches. (from my site, but can't remember the original source)

Your 10.8 sq ins is about right for a 6000  watt unit based on that.
I am surmising that welding transformers may not follow that approximation
though, as they may operate in a slightly saturated mode.
Regards
Phil

www.hvtesla.com

-----Original Message-----
From: tesla-bounces@xxxxxxxxxx [mailto:tesla-bounces@xxxxxxxxxx] On Behalf
Of Teslalabor
Sent: 19 August 2010 07:22
To: Tesla Coil Mailing List
Subject: Re: [TCML] Questions about saturable reactor / magnetic amplifier

Hello Dave,

the cross sectional area of the core is 10,8 square inches! So I thought it
is big enough for at least arround 25kVA? It is a big welding transformer,
weights >100kg. Would this really give only 12A @ 400Volts? Now I am
disappointed ;-(

Stefan

----- Original Message -----
From: "David Speck" <Dave@xxxxxxxxxxxxxxxx>
To: "Tesla Coil Mailing List" <tesla@xxxxxxxxxx>
Sent: Wednesday, August 18, 2010 10:13 PM
Subject: Re: [TCML] Questions about saturable reactor / magnetic amplifier

>  Stephan,
>
> It sounds like you are saturating the core at only 10 amps of DC control
> voltage.  The 12 amps of 400 volts may be the limit that you can push
> through they system.
>
> What is the cross sectional area of the core that you are using?  There is

> an approximate formula that will tell you how many KVA you can push
> through a core based on its cross sectional area.
>
> Dave
>
> On 8/18/2010 1:07 PM, Teslalabor wrote:
>> Hello,
>> after several months searching for a suitable transformer for
>> modification into a saturable reactor, I now have found a big 3-phase
>> welding transformer, which was in use on 3-phase 400V here in Germany.
>> For my big teslacoil, which runs on 2-phase 400V, I want to modificate
>> this transformer into a magnetic amplifier. The goal is, to control the
>> current through the outer legs of the transformer @ 400V from 0-60
>> Amperes whith a DC current of 20A on the middle leg of the transformer.
>> Today I removed the heavy welding windings, and put the two outer legs of

>> the transformer in series and connected them on 400V. On the middle core
>> leg of the transformer, I wound a winding of several turns and applied 20

>> Amperes DC to it. I measured the 400V - current trough the outer
>> windings, and it arrised from 0,5A to 10A, when I regulated the DC from
>> 0 - 20A.
>>
>> There was something strange to me: The current in the 400V windings
>> firstly raised very fast when I raised the DC and then only very
>> smoothly.
>>
>> I then doubled the windings on the DC winding. I got the 10A in the 400V
>> windings at only 10A (instead of 20A before) but it only increased to 12A

>> when I pulled the DC to 20A. In the same way: firstly very rapid current
>> rising, than only 2A from 10A to 20A DC. Why this? How many thousend
>> turns do I need for reaching 60Amps in the outer 400V windings? Is it
>> even possible?
>>
>> Best Regards
>> Stefan
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