Re: MMC problems

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Gerry Reynolds" <gerryreynolds-at-earthlink-dot-net> 

Hi John,

I did a simulation of a 12KV/120ma system with your 66.7 nf Cp and got very
high rises (25KV if I remember right - check the archieves for that post).
The resonsant Cp for your power supply is 26.5 nf and 3.14 * Cres will be
83.3 nf.  This larger capacitance will be necessary to keep the voltage from
overvolting the system if the safety gaps are not set correctly.  I think on
your first time around, you said the safety gaps did not fire and this
suggest the 25KV may have occured.  If this was the case the peak current
would have been 25KV/17KV times greater than you think should have been the
case.

Good luck,
Gerry R.

 > Original poster: "John Richardson" <jprich-at-up-dot-net>
 >
 > Hi Gerry,
 >
 > I agree with the small topload train of thought.  4x24 is small for 1500
 > watts.  I am going to make an 6x32 or 8x32 soon.
 >
 > The capacitance shouldn't have been too low, as 4 strings of 9 comes out
to
 > about .0667 uF.  I do believe that the low number of tuning turns allowed
 > too much current per turn, and a large topload combined with an 8"ish
 > secondary will bring tuning to a greater number of turns, hence less
stress
 > per turn and greater inductance.
 >
 > As far as the safety gap, I think you are correct.  Terry gave me the
specs
 > for the spheres the gap is made up of, and I set accordingly.  MUCH better
 > than the set it with just the NST method.
 >
 > Still waiting for better weather to do some runs, though.  BTW, what kind
of
 > performance increase do you think the 8x32 topload and 8" secondary (1480
 > turns of #24 heavy build) will provide?  My gut tells me it should be
 > substantial.
 >
 > Thanks,
 > John Richardson
 >
 >  >
 >  > Hi John,
 >  >
 >  > I believe you had a multitude of problems that resulted in the failure.
 >  >
 >  > 1. Your top load was too small (maybe way too small) - resulted in the
 >  > number of turns for the primary being too low.
 >  >
 >  > 2. Your Cp was too low - allowed the voltage on the Cp to get too high.
 > This
 >  > coupled with low number of turns on primary allow peak currents to be
 >  > greater than expected.
 >  >
 >  > 3. First time around, I think your safety gap was not doing its job and
 > this
 >  > enabled the high voltages that #2 resulted in.
 >  >
 >  > With properly sized Cp (to keep the voltages reasonable in conjuntion
with
 > a
 >  > properly set safety gap),  you need to increase the topload capacitance
to
 >  > keep the number of primary turns reasonable to limit the peak current.
 >  >
 >  > Also, spreading the current between two strings (instead of one) will
 > help.
 >  >
 >  > Gerry R
 >  >
 >  >
 >  >  > Original poster: "John Richardson" <jprich-at-up-dot-net>
 >  >
 >  >  > I finally got my meters in place, and can give some more info into
what
 > my
 >  >  > coil is drawing.  Again, these are short runs, as I can't run things
 >  > outside
 >  >  > yet.  For anyone who may not remember, I had the caps blowing on my
 >  >  > 12000/120 system, and excessive primary heating.  I hope to run for
an
 >  >  > extended period in the next day or so, weather permitting, but for
now
 > all
 >  > I
 >  >  > can say that at exactly 119 volts in, with no PFC, I am pulling well
in
 >  >  > excess of 15 amps from the wall.  I can't be any more accurate than
 > that,
 >  > as
 >  >  > 15 amps is the max this meter will measure, and the needle is beyond
 >  > pegged.
 >  >  > With 300uF PFC, I am pulling between 12-13 amps from the wall.  This
 >  > sounds
 >  >  > right to me, but maybe others can shed more light.  I really
expected
 > the
 >  >  > trans to be pulling more, thinking that this may explain why caps
have
 >  > been
 >  >  > popping and things heating (ie trans outputs more than expected).
Will
 >  > have
 >  >  > more info soon.
 >  >  >
 >  >  > Thanks,
 >  >  > John Richardson
 >  >  >
 >
 >