From: Bert Hickman [SMTP:bert.hickman-at-aquila-dot-com]
Sent: Wednesday, February 11, 1998 8:42 AM
To: Tesla List
Subject: Re: rotary problem
Tesla List wrote:
> From: dwight duncan [SMTP:duncand-at-ccsalpha2.nrl.navy.mil]
> Sent: Tuesday, February 10, 1998 6:46 AM
> To: tesla-at-pupman-dot-com
> Subject: rotary problem
> Hi all,
> Mr John Freau made the suggestion of making my stationary electrodes
> flat wrt. the rotor disk. I gave this a try and there was a slight
> improvement. I also took out every other electrode on my disc to a new
> total of 8. The motor is running markedly faster and is rated at 2400 rpm
> and is of the brushed variety. Still my safety gaps are still firing
> almost continually.
> My is two 5 Kv 300 ma transformers put in series with the center or
> commons on the HV side grounded. This gives me 10 Kv at 300 ma. Just
> after the transformers are 4 nf caps in parallel to ground on each side of
> the output. next is a laminated iron core common mode choke with each side
> at 4 mH. Finally is the safety gap with a ground post in the center and it
> was set so it would not fire when the feeds were disconnected from the tank.
> John mentioned the problem may be in the toroid. Using the statement
> from Richard Hull I went for as large of a toroid that I could make. I is
> a 16" dia. disc with a 4" aluminium dryer hose wrapped around the OD. I
> did an order of magnitude calculation and came up with 25 pf. This
> calculation made some assumptions that may not be valid.
> Anyway this is my whole system and I am running out of ideas for the
> cause of this problem and to John or anyone that might have some insight
> please reply and any or all comments are greatly appreciated.
> PS. The secondaries resonance is measured at 220 KHz and the primary is
> tapped out at almost five turns.
It IS possible to have too small a tank capacitor! In the case of your
system, it sounds as though you don't have enough tank capacitance for
the current delivery capability of your transformers. Once the gap
fires, and the tank cap has discharged, you'd like the cap to recharge
slowly enough (i.e. "take" enough current) so that the voltage across
the gap doesn't rise so quickly that the gap is reignited. A 10 KV 300
mA source will optimally drive a 0.08 uF (80 nF) tank cap. You're using
one that's only 1/20th this size! Once the gap fires, the transformers
rapidly recharge the tank cap, allowing the voltage across the rotary to
rise quickly enough that the gap reignites again (re-igniting at perhaps
a few hundred volts). Once this re-ignition occurs, you've got a power
arc until the gap finally breaks for good. You've got a couple of
options, both of which involve more closely matching current delivery
capability with the recharge current demanded by the tank cap.
The first option is to add significant ballast inductance in series with
your power transformer's primary circuit to further limit the
short-circuit current capability of the transformers. You could use a
couple of microwave transformers, or a couple of neons, with their
outputs short-circuited, in series with the primaries of your
The second option is to migrate to a physically larger coil and a
correspondingly larger tank capacitor in the 40 - 80 nF range.
Hope this helps, and safe coilin' to you, Dwight!
-- Bert --