[Amps] Variac repairs

[Ron Youvan]

> Steve, I have seen that smoking variac problem a couple of times and both  
> times were due to my wiring error.

> If you somehow connect 120vac between the wiper and one end of  the 
> winding, as you rotate the shaft the 120vac gets connected to fewer and  fewer 
> turns until smoke is released.

> Others have mentioned how to repair the unit.

   I had one "powerstat" go bad at one spot (and seen many more over the years) out of 10 in use for 
over 20 years.  The normal variable transformer has a carbon brush that constantly touches more than 
  one winding at the same time, this creates a few shorted turns that get quite warm constantly.
If the brush (resistor) was replaced with metal brush the transformer would smoke.  The brush's 
resistance prevents this.  Staying warm for years eventually burns the insulation and dipping.
If the brush only touches one winding at a time and not bridge two when in between turn this 
wouldn't occur, but there would be a flick off of power between winding turns.
If the "variac" is changed from time to time they generally last for ever, if never changed they 
cook and darken the insulation/dipping where the brush sits.

   There is a patented (held by Libert) kind of variable auto-transformer that has several brushes 
and diodes that have brushes that do not span more than one winding at any time and no power 
dropouts between windings.  (sold as part of CVTs)
-- 
    Ron  KA4INM - There is no time like the pleasant.