Re: Blown Cap

On Fri, 31 May 1996 21:51:18 -0600, Tesla List
<tesla-at-poodle.pupman-dot-com>, you wrote:

>From: Richard Hull <hullr-at-whitlock-dot-com>

>> >The rolled amateur caps are for Neon Transformer systems and rather low
>> >power use (under 3KW).  Most of the problems that I have seen occur are
>> >when rotary gaps are used and the rep rate is taken up to levels beyond
>> >500 bps.
>> Oops,
>>         I took out the second 3 layer 0.030" dialectric cap with a
>> 15kV 60mA neon and a rotary gap at about 2500 bps (potentially).
>>         jim
>Good Grief!!!!!!!  2500 bps!!!!!  What were you using a .000006ufd cap! 

No, 0.016uF and ignorance! 


I built an eight point rotary and against rwstepnens's advice used a
1750 induction motor. That was a graphic visual demonstration of the
slippage of induction motors. The rotary fired with about 1 second on
and 3 seconds off. When it was firing, you could see the arc start
point precessing. (#8 welding goggle)

So I went down to my favorite surplus store looking for a fractional
HP universal motor. I found some 30VDC motors with a 5/8 shaft.
Probably take outs from a reel-to-reel tape drive. I had no info for
these, I just used one. The rotary fired very sporadically with my pig
and 5 ohms of resistive ballast. So I went back to using my 15kV 60mA
neon. Great arcs, for a few minutes, Then I blew my 1nF cap made from
3 layers of 0.030 poly, oil filled and vacuum outgassed.

I added a second set of stator points to get 16 breaks per rotation.
No improvement. I finally added 2 magnets to the rotor and used a
500uH drum inductor as a pickup. Imagine my supprise when I scoped
7000 RPM at 30volts. That makes 1870 BPS. I am only guessing that the
motor will spin to 10K rpm, I don't want to subject the rotor to the
added unbalanced of those magnets.

>That's about the only one you'd recharge at that rate!  The very fastest 
>we have ever dared to break at was 1000 BPS (NWL pulse caps) and that was 
>.012ufd!  We only realized about 800BPS at that usage , too!  Most of our 
>best work on two coil systems and even maggys are below 600 BPS.  
>Everyone I have heard of, who consistently blew caps, used rotaries 
>whizzing around at 5000 plus rpm with 10 or more points on the wheel.  If 
>you use more than 10 points on a wheel the RPM should be below 2400 RPM. 
> For high speed rotaries 4-6 points is the absolute limit or you are 
>probably missing 60% of your point presentations.  

Since then, I wound a 1000 turn inductor on a 12" by 3" coil form. It
improved my gap firing rate markedly from a few firings per second to
a smooth buzz all the way up to 7000+ RPM.

I just found my HV divider (I cleaned the garage last week;). I to
would guess that I am missing a good portion of the point
presentations and am curious to see how many firings per cycle I'm

I'm waiting for the current group buy CP cap to arrive and see what It
will do. I'll keep you RPM limitations in mind, I don't want to blow
this one!

>NWL engineers, (my cap maker of choice), have had long engineer to 
>engineer discussions with me and they say, unequivocally, that duty 
>cycles drop to near zero on even the most high end pulse caps once the 
>rep rate against AC power input exceeds 1000 BPS.  This is especially 
>true if real large impulse RF currents are involved (as they always are 
>in our kind of work).
Did they give any reason for this drop off? So many presentations at
low voltage portions of the 60Hz cycle?