Re: core saturation in variac

>>From richard.craven-at-mkbbs.co.ukMon Jun 17 07:29:37 1996
>Date: Sun, 16 Jun 96 23:12 +0000
>From: richard.craven-at-mkbbs.co.uk
>To: TESLA-at-poodle.pupman-dot-com
>Subject: core saturation in variac

>Hello Malcolm, Ed and all,

>I am struggling with the idea of using variacs as current 
>limiting reactors.

<big snip>

>As a final remark, I realise that the RWS approach avoids 
>this by using an air-core and lots of turns, so this is my 
>viable alternative. I might just go and buy a reel of 2.5mm 
>squared single core cable and measure its inductance on the 
>reel. This would be a nice lazy way of building up inductor 
>banks. Also you don't add to the magnetising current that 
>you already need for the pole transformer.
>Richard Craven


It sounds like you are on the right track in trying to adapt a 
variac for use as a series primary circuit control reactance.  I have 
no experience in the technique of splitting the core as you describe, 
but your thoughtful description does make sense to me.

My high value, air core RF chokes (Stephens' Mega-Henry brand) which 
go between the output H.V. side of the power transformer and the 
shunt break gap (fixed or rotary) are designed only to protect the 
secondary winding of the transformer from RF overvoltage which can 
heat tar and other dry insulations, reducing their breakdown strength 
and then leading to catastrophic breakdown and carbon tracking and 
sudden death syndrome to neon and other non pole-pig transformers.  They
are not intended as a substsiute for primary control reactance.
When using transformers without internal magnetic bypass shunts you should
still employ primary circuit series control reactance when used with my 
RF chokes.  Use of my RF chokes with these higher power, non-shunted 
transformers  may however explain why my rotary gaps quench so nicely
without the need for additional series H.V. quench gaps.

Happy Coiling!, rwstephens