Jon,
I bought 15 of the fast blow 250vac 20A fuses online as they were not found
locally in 30mm sizes. I like the idea of the slow blow and possibly going
up to 25 or 30 amp. I did also notice the blown fuses I had were pretty hot
when removed which makes sense. I would also assume that raising the fuse
rating might also blow the 15A (I'm assuming) outlet the variac is plugged
into. Don't know.
Sounds a bit scary to short together the 15 kv outputs of the NST to
measure the current of the 120vac input. Sounds like I should put a 10M ohm
resistor inline with the input to read the current from also.
Tim
On Wed, Apr 1, 2015 at 2:16 PM, Jon Danniken <danniken@xxxxxxxxxxx> wrote:
> 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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