Variac problem

• To: tesla-at-pupman-dot-com
• Subject: Variac problem
• From: "R.E.Burnett" <R.E.Burnett-at-newcastle.ac.uk> (by way of Terry Fritz <twftesla-at-uswest-dot-net>)
• Date: Thu, 07 Oct 1999 08:01:07 -0600
• Approved: twftesla-at-uswest-dot-net
• Delivered-To: fixup-tesla-at-pupman-dot-com-at-fixme

Hi Variac Experts,

I have a quick questions about a Variac I have been given.  It is
a 240V 15A unit which I plan to use for voltage control duty in my
Tesla Coil power supply. Windings, brushes and insulation all appear
in excellent condition.  However I found a problem when checking
resistance from wiper to windings.

At the ends of the wiper travel there are patches where the contact
resistance increases to several Kilo-ohms.  The brush pressure is
good, and more pressure does not reduce the contact resistance.  I
cleaned the swept area of the windings with a rag but it seems to
have got slightly worse if anything. (The winding is not broken,
it measures OK end to end.)

Will this problem dissapear when the variac is supplying some
current to a load,  or do I need to add some kind of graphite or
something to the winding surface to reduce contact resistance first.

I have never reconditioned a variac before and would appreciate
any advice.  I have applied power but am reluctant to draw off any
load current because I don't want to overheat the brushes due to
high contact resistance.  Please help.

TerryF - Do you still have the Variac OL graph,  I couldn't find it
on your site.  Could you Email it to me if you get a spare minute ?

				Cheers,

					- Richie,
					- In Sunny Newcastle.

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