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Re: [TCML] Mid sized Tesla Coil using a 20A variac

On 04/01/2015 05:02 AM, Timothy Gilmore wrote:
> When using my 20A variac, it is listed as 0-130vac but when I measure with
> a meter it is 0-145vac. I can never bring the voltage past about 120 vac on
> the dial (which is really about 130vac measured or so) because then the
> variac 20A fuse blows when connected to my 2 15kv 60mA NSTs for my mid
> sized Tesla Coil. During a tesla coil show at my home this week, I was
> running it for about 1 minute increments and blew the fuse 3 times.
> Do I have to re-wire my garage outlet and/or circuit breaker for 30A via
> hiring an electrician? or I'm assuming its at 15A now and live with it?

Hi Timothy, one thing to keep in mind about variacs is that most
domestic versions are rated for continuous duty. This means that a 20A
variac is rated to control 20A 24/7.

If you want to run them at a higher current than they are rated for, you
can do this, provided that you limit the duty cycle of the variac (the
limiting factor is the amount of heat that builds up). The manufacturers
actually plan for this, and provide a duty cycle chart for the units.

As an example, here is the duty cycle chart for GE Volt-Pac variacs
(other manufacturer use a similar curve):


As you can see, you can run a variac at double the rated current,
provided that you limit the duration to less than ten minutes. In Tesla
coiling, this usually isn't a problem, because we generally limit the
run time to less than a minute or two.


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