Re: Ballast question (fwd)

[<sroys@xxxxxxxxxxxxxxxxx>]

---------- Forwarded message ----------
Date: Thu, 18 Nov 2004 08:48:17 +1300
From: Malcolm Watts <m.j.watts@xxxxxxxxxxxx>
To: High Voltage list <hvlist@xxxxxxxxxx>
Subject: Re: Ballast question (fwd)

Hi Carl,

On 17 Nov 2004, at 8:25, High Voltage list wrote:

> Original poster: <sroys@xxxxxxxxxxxxxxxxx>
> 
> 
> 
> ---------- Forwarded message ----------
> Date: Tue, 16 Nov 2004 16:48:37 -0600
> From: Carl Litton <Carl_Litton@xxxxxxxxxx>
> To: High Voltage list <hvlist@xxxxxxxxxx>
> Subject: Ballast question
> 
> 
> 
> Carl Litton
> 
> Hello
> 
> Thanks to everyone who responded on the transformer stack question.
> Tremendous amount of useful knowledge gained that is much appreciated.
> 
> We have now gotten the use of a very nice inductance meter, measuring
> up to 500 Henries.  Of course, we immediately set about measuring
> every possible combination of ballasting devices with a wide variation
> in the results from a few microhenries to one that was 300 Henries. 
> One of the things we did with large frame transformers was to measure
> the primary winding with secondary short and open and the secondary
> winding with primary short and open.  
> 
> Without exception, the inductance of a transformer winding is several
> orders of magnitude higher when the other winding is open.  Shorting
> the other side drops inductance usually into the microhenries.
> 
> The question is:
> 
> Why is it that it is almost uniformly recommended that transformers
> used for ballasting (MOT's mostly) have the magnetically coupled
> winding shorted?  Is this related to changing the inductance or simply
> to prevent arcing?

It is related to the required inductance. If you run the numbers, 
you'll see that required ballasting on the primary side is in the mH 
range. What happens is that this "leakage inductance" is transformed 
by the turns ratio squared (the impedance ratio) to a value in H on 
the secondary side and it is this value which resonates (if chosen to 
so do) with the primary cap. As you've observed, you can get the 
inductance in the ballpark by shorting the other winding. What you 
are left with is the leakage inductance of the welder together with a 
suitably rated core and winding.

Malcolm