Re: Variac blowing fuses (another newbie questin)

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: Esondrmn@xxxxxxx 

Ot,

It sounds like the excessive current draw is under full load and not 
an inrush current problem as the variac is powered up.  Do I recall 
you saying in an earlier post that you were using pfc caps equal to 
250 or 275 ufd?  I believe the normal value for that transformer 
would be 150 to 160 ufd.  I use a 15 kv 60 ma transformer on my 3" 
coil and have not had any breaker problems.  I do not have a fuse in 
the variac.  I also am only using a .015 ufd primary cap at this 
time.  I need to boost this up to .02 or .03 ufd to take advantage of 
the srsg but just haven't done it yet.  Then I might need to add pfc caps.

Ed Sonderman

In a message dated 11/29/2005 6:54:00 PM Pacific Standard Time, 
tesla@xxxxxxxxxx writes:
Original poster: otmaskin5@xxxxxxx

Thx for all the responses.  Several of you asked for more info in
order to diagnose.  Here's the info.

- The variac is supplying power to the NST only

- Original fuse was 20A - don't know whether it was fast or slow
blow.  Replacement fuse was 15A SLOW BLOW.  When it blew, the solder
breached the fuse's glass cylinder.

- The fuse blows after I've dialed the variac up to full power from
zero. It didn't blow immediately - rather, the coil ran for maybe
10-20 seconds.  I believe the variac was up to full power.  Both
times, the fuse blew during strong continuous arc to grounded
target.  Hasn't happened when streamers were not striking
anything.  I always power up starting the variac at zero.

- The variac provides 0-130 volts out.  Current rating is 20 amp.

- Fuse appears to be located in the input circuit of the variac (not
totally sure as I wasn't able to get the cover off far enough to
thoroughly examine the wiring).

- I don't know whether the NST is a high power factor type or a
normal.  I'll check with the
mfg to see if I can find out.  In the mean time, it's a Franceformer,
15000 volt, 60 ma, 890 VA, model # 15060.

- My tesla coil has a static spark gap.  9 gaps made of 3/4" copper
pipe 0.03" for each gap.  So far, its wired to use only 4 gaps (0.12"
total) and small fan for quenching.

- The value of my tank capacitor is 0.03 uF +10%, 35KV.  I'ts the
Maxwell cap you see on eBay quite often.

- The variac works (as indicated by the voltmeter) without blowing
fuses @ no load.  Also, I run my small coil (9/30 NST powered) using
the variac haven't blown fuses.

Hope this helps in figuring it out.  Thanks.