async rotary control

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

Original poster: "by way of Terry Fritz <twftesla-at-qwest-dot-net>" <A123X-at-aol-dot-com>

I have an async rotary made from a 12000rpm angle grinder. Even with only 2 
electrodes in the disk it's 400rpm and I wanted to try varying it's speed to 
something lower. I was going to use a dimmer switch but when I turn down the 
dimmer it actually sounds like it speeds up and also vibrates more. Any ideas 
of why that happens? Also I was wondering if a cheap variable transformer 
could be made by winding one or two layers of magnet wire on a normal EI core 
then sanding the windings on one side and making a device with brushes on it 
to slide across the sanded side to vary the voltage? I really don't feel like 
buying another variac to control the rotary but I have a nice core and some 
thick magnet wire to use and I couldn't think of any reason why a variac 
needs a O core except so that it allows for the use of a rotating contactor.

Mark