[Amps] HV transformer and Variac

[Ron Youvan]

On 07/24/2015 12:33 PM, Gerald Williamson via Amps wrote:

> Hi Larry, thanks for adding that analysis.

> Your approach clears up something that has bothered me about Variacs for a
> long time: When you examine the tap/brush part of the device it is obvious
> the  more then one turn of the autotransformer is being connected to the
> sliding  tap at the same time. That means that one or more turns are shorted.

> A shorted turn in a conventional transformer will result in a large current
>   flow, copious amounts of smoke and likely some sparks as well.

   Your analysis omitted the two resistors in the circuit, one
contacting each turn, they are made of a carbon block, if you
substitute copper or brass for the carbon brush you *will* blow
the fuse.
   Powerstats (or Variacs) are a compromise that are more economical 
that using a rotary transformer, (that uses no brushes) but cost big
and are (as rule) very noisy.  (audible HUM!)
  Powerstats (or Variacs) do the job, give little trouble and are not
terribly inefficient.
-- 
   Ron  KA4INM - Youvan's corollary:
                 Every action results in unwanted side effects.