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Re: dual MOT Tesla coil design is complete (fwd)
Original poster: "resonance" <resonance@xxxxxxxxxxxx>
The 'thumping" sound, usually in a variac, is caused by rapid rampup
of current leading to core saturation. This is one reason pole xmfr
powered systems work well for long sparks --- the peak current hits
allow super fast recharging of the capacitors --- usually to the
detriment of the variac. The usual solution is to use 10-15 more
turns on your series reactor.
Also, we use 1/2 to 3/4 Ohm resistors (7 kW) in series with the
pri. We use multiple elements of Nichrome wire in parallel to hit
the 1/2 to 3/4 Ohm rating. This completely stops the thumping
effect. It clips the current peaks. Abovbe 1.5 Ohms the spark
intensity diminishes. Below 1.5 Ohms the spark intensity remains
about the same.
Dr. Resonance
When I started coiling with a 10kva pig, I was using my 13.5"
diameter coil, the transformer thumped with fast current changes. I
came across a 10 ohm 3000W resistor (HUGE!!). The resistor was wire
wound on a very thick ceramic cylinder. I measured to the center and
snipped the winding. Thus, I had two 5 ohm resistors on the same
ceramic cylinder. I then paralleled these windings for 2.5 ohms and
used this in series with my variac ballast.
The thumping was gone. I used it for a couple years, but was never
satisfied with spark length. I eventually removed it. Spark length
was better and brighter. The thumping I had originally experienced
never did show up again, but then the coil itself had changed over
time (added 12" length to the secondary, new primary, new sparkgap, etc..).
I guess my experience is that it certainly did smooth out the change
in current curves, but it does dissipate power which robs coil
processing power. Not terribly though. I think there are systems
where it's appropriate and systems where it's not necessary. The
coil itself and the drive system powering it determine it's worth, in my mind.
Take care,
Bart