I think the problem is the size of your primary capacitor. With my 15/60
NST and 120BPS SRSG, my cap is .04uF. If you have twice the NST current
you need roughly twice the capacitor value. Having a too-small capacitor
will cause voltage reversals (high mains current) and dangerous
over-voltage excursions (toasted MOV's in Terry filter). Think about it -
in a proper setup, the current from the NST is just enough to charge the
cap from zero to bang-voltage in one mains half cycle. If you have a cap
that's half as large, it's going to charge much faster. It's not going to
plateau as soon as it arrives at bang-voltage; it's going to continue to
draw current and start charging in the opposite direction, and this time
swing WAY lower than minus-bang voltage. It might sound like it's somehow
wasting the energy spent charging the cap on the way up, but that energy
isn't discarded. It's transferred from voltage in the cap to current in
the NST secondary inductance as the voltage heads back towards zero. Very
stressful on the NST!
Terry filter resistors WILL get hot, and resistors are built to deal with
that. But MOV's getting hot is a red flag that must not be ignored, that
should never happen unless something is seriously wrong. I suspect that
you were unable to stop the safety gap from firing so you opened it up to
where it stopped?
In a well-designed TC, the power drawn by an NST will GREATLY exceed the
faceplate power rating, even with PFC caps, and that's a good thing. We
want to squeeze as much juice out of that NST as possible without
destroying it. It's not well understood but the the built-in current
limiting of NST's is somewhat defeated when core saturation occurs, thanks
to energy stored in that secondary inductance. My single 15/60 pulls well
in excess of 20A when humming at 140VAC. The power drawn with a
short-circuit on the NST is MUCH less. Thank you core saturation!
Regards, Gary Lau
MA, USA
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On Sun, Nov 18, 2018 at 9:54 PM Daniel Kunkel <dankunkel@xxxxxxxxx> wrote:
> I finally got my SRSG online tonight. To my dismay I started blowing 30 amp
> fuses in my supply variac. I am hoping the list can help me figure out why
> this is happening...we have guests coming over for Thanksgiving and they
> are expecting a show!!! LOL
>
> When I was running the previous setup using a vacuum quenched single gap,
> my NST bank (15kv @ 120ma) was only pulling 10 amps using a total of 302 uF
> in PFC caps. I was expecting similar performance with the rotary gap.
>
> Basic specs are:
> 6.5 x 26.5" secondary with 1,057 turns or 22 awg
> .033uF MMC
> 1.8 kva NST's
> Terry Filter
> 1/2 HP, 3,600 RPM motor
> 2 flying electrodes (1/8" tungsten, through disc design), for 120 BPS
>
> Thanks for any insight,
> ~Dan
> Kansas City area
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