Re: Variac vs. Fan speed control

Subject:  Re: Variac vs. Fan speed control
  Date:   Sun, 27 Apr 1997 15:48:14 +0500
  From:   "Alfred A. Skrocki" <alfred.skrocki-at-cybernetworking-dot-com>
    To:   Tesla List <tesla-at-pupman-dot-com>

On Fri, 25 Apr 1997 07:52:23 -0400 Thomas McGahee
<tom_mcgahee-at-sigmais-dot-com> wrote;

> Chip & Alfred,
> Just to throw in my two cents worth... Yes, the controller itself should
> be mounted in a metal enclosure used as a Faraday shield. It is important
> that the motor wiring be fully shielded, connected to the motor casing, and
> properly AC grounded, otherwise the wiring acts like a glorified antenna
> for the RF, and sneak into the Faraday shield. Not all AC controllers
> can take the wild transients that exist around a Tesla coil. A Faraday
> shield will always work as long as the shield completely encloses the entire
> circuit. Unfortunately, the power leads themselves can be bringing in
> nasty little spikes,

Tom, I always wind all power leads around a ferrite ring after they 
enter the Faraday shield, this tends to act as a choke and blocks the 
R.F. from entering through the leads. I guess one could also use 
shielded cable and connect the shield to the Faraday shield and this 
to ground.

> in which case many of the cheaper circuits go totally
> bonkers. The nice thing about a variac is that in Tesla use they are
> fairly bullet-proof and survive the environment. Well, most of the time :) 

Variac's are great BUT they are EXPENSIVE! This is why I use a 
home built saturable reactor to control the input power on my larger 
coils and triac phase shift controls with universal motors on my 
rotary spark gaps. BTW for any one monitoring this thread DON'T use a 
triac phase shift control on an induction motor it will burn out the 
windings as fast as you an through the switch. AC induction motors 
should be controlled with a variable frequency control and then they 
can only be controlled over a narrow range, because if they run too 
slow the centrifical switch that activates the starter windings will 
kick in and if the speed doesn't come up quickly again the starter 
windings will burn out. The best motors to use for variable speed is 
either D.C. or universal motors. Stepper motors will also work but 
tend to be expensive.


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