Re: Toroid = shorted turn ?

To: tesla-at-pupman-dot-com
Subject: Re: Toroid = shorted turn ?
From: bertpool-at-ticnet-dot-com (by way of Terry Fritz <terryf-at-verinet-dot-com>)
Date: Thu, 08 Oct 1998 21:07:20 -0600
Approved: terryf-at-verinet-dot-com
Date forwarded: 	Thu, 8 Oct 1998 17:37:00 -0600
Date sent:      	Thu, 08 Oct 1998 16:45:56 -0600
To:             	tesla-at-pupman-dot-com
Subject:        	Re: Toroid = shorted turn ?
Forwarded by:   	tesla-at-pupman-dot-com
From:           	Tesla List <tesla-at-pupman-dot-com>

> Original Poster: "R M Craven" <craven-at-globalnet.co.uk> 
> 
> <SNIP>
> 
> >>.  However, I would like to hear from anyone else who has investigated
> this.
> 
> 
> <SNIP>
> 
> >I have. If you measure the inductance of a coil with and without a
> >toroid on top the difference will be close to zilch. I once thought
> >the same as you. BTW, if the ring is a near perfect conductor, it
> >shouldn't absorb much energy at all.
> 
> <SNIP>
> 
> On a standard two-coil TC where the secondary has an aspect ratio of 3 or
> more, and coupling coefficient is low (a quarter or less,say),the average
> power coupled directly into the topload is indeed small. In this situation
> the "shorted-turn" losses of the toroid can be ignored.
> 
> If the coupling is increased between the primary and the secondary as might
> be the case in a valve powered TC, the proximity of the toroid to the
> primary is increased. A helical primary might couple relatively tightly to a
> secondary, but will also couple more tightly to the topload.
> 
> In this case, topload joule heating rises. However, another contributory
> factor is the rms power achieved in the primary. The peak power in a valve
> TC will generally be a lot lower than a spark-gap driven TC, but the average
> P will be relatively high for the size of secondary and the coupling. Thus
> the average P dissipated in the topload due to direct coupling to the
> primary will rise.
> 
> In short, it is not a problem to have a shorted-turn toroid in a typical
> two-coil system. It will become a likely loss mechanism in any TC where the
> distance from the primary to the toroid is small AND at the same time the
> average P is high.
> 
> In a magnifier where there is some field-shaping on the top of the secondary
> and there are high rep-rate primaries, it could become a problem. Has anyone
> with magnifier experience seen any toroid heating effects?
> 
> 
> Richard Craven, Malvern, England.
> 
> 

Yes, Wild Bill Emery and I had a problem with a small toroid on the bottom
of an 
"extra" coil which over-heated.  The toroid had no center aluminum disk, and 
formed an excellent shorted turn.  We removed the toroid and found that
output 
spark length increased about 10%.  It is very likely that if the toroid had
had a 
solid conductive disk in the center it would not have been so troublesome,
but we 
found we didn't need it for corona suppression after all.  I've never
noticed a top 
toroid having a heat problem, but our top loads have always had a center disk.