[Amps] HV transformer and Variac

[donroden at hiwaay.net]

Quoting John Lyles <jtml at losalamos.com>:

> Interesting discussion about 'Variacs', one brand known as Powerstats.
> I wondered about the shorted turn where the wiper contacted, the  
> carbon theory makes most sense.

On a TV transmitter, we used a linear 480vac three phase variac that  
wasn't circular, but rather linear....... Three tall wound cores that  
were potted in a hard epoxy and it used square wire that was embedded  
in the epoxy.  From the factory, the working side of the core was  
machined down to just below the surface of the square wire... making a  
very smooth surface.
Small bicycle chain top to bottom moved the brushes up and down.

The carbon brushes on each core were three across and each connected  
through high current diodes that were paralleled front-to-back / back  
to front  before they were connected together.  There must have been a  
volt or so difference in the potential where the three brushes touched  
the core.  The diodes probably prevented carbon to carbon conduction  
and equalized the voltage between the brushes.

When the chain jumped the sprockets ( not IF ) sparks could fly if it  
fell onto the core.


We always think of carbon brushes as lossy, but these brushes were a  
composite of graphite and bronze, so maybe not a lot of heating loss.

Don R