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TIG arc starter inductor and X-ray tubes.


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From:  Malcolm Watts [SMTP:MALCOLM-at-directorate.wnp.ac.nz]
Sent:  Thursday, February 26, 1998 2:12 PM
To:  Tesla List
Subject:  Re: TIG arc starter inductor and X-ray tubes.

Hi Bill,

> From:  Bill the arcstarter [SMTP:arcstarter-at-hotmail-dot-com]
> Sent:  Wednesday, February 25, 1998 8:56 PM
> To:  tesla-at-pupman-dot-com
> Subject:  TIG arc starter inductor and X-ray tubes.
> 
> Coilers,
>  I measured the inductance of that TIG arc initiator coupling coil.
> This is 10 turns of #18 magnet wire, about 5/8 diameter, and about
> 4 inches long.  The inductance measures as 1.9 uH w/o the core.  All 
> inductance measurements were performed at a 1KHz test freq.
> 
>  With the core, it measures a whopping 6.9 uH.  So the core has a 
> net permeability of 3.5 or therabouts.  Remember-this was the core 
> which I x-rayed, and consisted of 5 small ferrite bars in series with 
> airgaps.  Someone on the list (forgot who) thought this bar would have 
> a low permeability because of the airgaps, and they were right!
> 
> In the Hobart welder from which it came, this core was operated in a 
> quenched-gap configuration with two paralleled 0.004 uf caps, which 
> works out to a resonant freq of about 600KHz...  Bypass caps across
> the primary power transformer routed the HV/HF around and into
> the metal being welded...
> 
> A very far-away picture of this coil (attached to my stick welder) is 
> on my page at: 
> http://www.geocities-dot-com/CapeCanaveral/hangar/6160/weld3.gif
> All you can see is the outer "power-coil" aluminum helix...
> 
> I can only assume the purpose of the core is to distort the flux 
> return path, so as to actually force it to link with the surrounding
> power-coil.  Perhaps w/o the core, the flux would hardly extend 
> beyond the outer envelope of this sparse solenoid and not link to the 
> power coil.  Any comments?  Am I deluding myself? :)

The main purpose of a core in any inductor is to confine the flux to 
the proximity of the windings (and airgap if any), the idea being to 
reduce unwanted coupling to other objects.

Malcolm
<snip>