Re: OLTC II Initial calculations

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

Original poster: "Malcolm Watts by way of Terry Fritz <teslalist-at-qwest-dot-net>" <m.j.watts-at-massey.ac.nz>

Hi Stephen and other output voltage enquirers,
                                                It is well known that
you cannot judge output voltage by spark length in a disruptive TC.
As you up the breakrate from, say 1BPS to ??BPS, the sparks grow
progressively longer, at least to a point dictated by energy
availability per shot. For a unequivocal demonstration of this, a
simple expt is to use a single static gap and increase the breakrate
as suggested above, a feat made simple using a variac and NST. The
sparks grow as the breakrate increases but the primary energy per
shot doesn't and neither does the output voltage since it is a
function of energy per shot. It is the growth of sparks through
repetition using a pre-ionized or hot-air channel that allows TCs to
achieve the apparently magical effect of producing sparks far longer
than the secondary could theoretically withstand. I'd urge anyone who
wants to see this for themselves to do some single shot expts.

Malcolm

On 7 May 2003, at 7:46, Tesla list wrote:

 > Original poster: "Stephen Conner by way of Terry Fritz 
<teslalist-at-qwest-dot-net>" <steve-at-scopeboy-dot-com>
 >
 > OK, so I have John Freau telling me that 60" sparks from a 24" tall
 > secondary is fine, and Malcolm Watts telling me that a safe maximum voltage
 > gradient is 8ft per megavolt. I wonder what the voltage gradient associated
 > with 60" output from a 24" secondary is?
 >
 > 60" sparks at 120bps = 1250W
 >
 > 1250W -at- 120bps = 10.4J bang energy
 >
 > Capacitance of 6" x 26" toroid and secondary ~ 25pF
 >
 > Bang energy=0.5*Ctop*Vtop^2
 >
 > Vtop=sqrt(10.4/(0.5*25p))=900kV. Probably nearer 750-800 due to losses.
 >
 > Voltage gradient=800kV/24"= 30" per megavolt.
 >
 > I believe the Tesla coil secondary can stand a higher voltage gradient than
 > a plain insulator of the same length, because the winding forces the
 > electric field to be uniform. It's always "X" volts per turn.
 >
 > Anyway, I redid my design, I now have a 10" x 30" 2000 turn secondary that
 > resonates at 58kHz, a 17uf tank cap, and a theoretical output of 55". The
 > peak primary current is a whopping 3150A per brick. This is well within the
 > overdrive capabilities that our intrepid experimenters discovered for
 > discrete IGBTs. However we don't yet know if bricks behave the same.
 >
 > The main worry is that the inductance in the emitter wiring inside the
 > brick will put bad ringing onto the gate signal. Sure there is a "Kelvin
 > connection" for the emitter but it's not going to be perfect. If there are
 > several dies (Dice?), it can only be a true Kelvin connection for one of 
them.
 >
 > The manufacturer probably allowed for this but overdriving the gate voltage
 > will eat into the safety margin at the same time as the high emitter
 > current is making the ringing worse :( This could lead to uneven current
 > sharing as some of the dies turn partly off on current peaks, or Game Over
 > due to destruction of the gate insulation :6 The easiest way to find out is
 > probably to build the primary circuit and see- which is what I'm going 
to do.
 >
 > BTW I solved the charge circuit problem, using a voltage doubler and a
 > variac off 240V I can get 0-650V on my DC bus, and with a suitable meter
 > circuit (basically a small measuring capacitor that charges via a diode
 > from the main cap) I can read out the peak charging voltage. All nice and
 > simple and it still qualifies as Off-Line :D
 >
 > Steve C.
 >
 >
 >
 >
 >
 >