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If you were blowing a 20A fuse, then the variac was only seeing an
output of 20A fuse. The idea here is that you can increase the value of
the fuse beyond what it is recommended, provided you A) do not leave it
in, and B) you take into account the duty cycle.
Of course then you are going to run into the limit of what the breaker
on that branch circuit is rated for, and breakers tend to not behave
well after being tripped a lot of times.
That having been said, running two 15/60s should only be needing 1800VA,
or about 15 amps at 120VAC, so it's possible you just have a very
sensitive variac fuse. You might try a time-delay fuse, as they get
along better with inductive loads, or just get a higher value fuse (just
remember to take it out when you are not coiling).
Additionally, you might want to measure the input voltage and current of
your NST bank when the outputs are short-circuited to get a better
handle on what is going on.
Jon
On 04/01/2015 07:43 AM, Timothy Gilmore wrote:
> Wow Jon. So perhaps I was using much more than 20 Amps then being drawn
> from the NSTs.
>
> Interesting.
>
> On Wed, Apr 1, 2015 at 8:01 AM, Jon Danniken <danniken@xxxxxxxxxxx> wrote:
>
>> 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):
>>
>> http://i.imgur.com/2fOYa7N.jpg
>>
>> 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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