Re: Overrating a Variac

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Mike Harrison by way of Terry Fritz <twftesla-at-qwest-dot-net>" <mike-at-whitewing.co.uk>

On Mon, 29 Oct 2001 23:33:58 -0700, you wrote:

>Original poster: "by way of Terry Fritz <twftesla-at-qwest-dot-net>"
<Mddeming-at-aol-dot-com>
>
>In a message dated 10/29/01 6:20:01 PM Eastern Standard Time, tesla-at-pupman-dot-com
>writes: 
>
>
>>
>> Hi all, 
>>
>> I have a 10A variac and I have a 20A variac bruch that I picked up off a 
>> friend. Provided that it physically fits in the old variac, does this mean 
>> that I will be able to overrate the variac by more than usual?? 
>>
>> thanks 
>> Jason 
>
>
>
>Hi Jason, 
>Assuming that "bruch" is a typo for "brush", then probably not. The current
>limit is determined 99+% by the size of the wire and the size of the core. 
>Matt D.
>
>
A while ago someone dug up some info from a variac manufacturer
indicating that overrating up to 100% was possible for short periods. 

However the brush rating is also relevant - when running my big laser
(pulling about 50A peak), I grossly over-run a pair of 10A variacs,
and the first problem I saw was local brush heating - it got so hot it
discoloured the windings. A couple of big fans blasting the brushes
helped a lot.
Sparking while adjusting at high power was also an issue, causing
pitting of the windings - obviously this could get into a runaway
situation as a pitted winding will have more resistance.