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Re: [TCML] Designing a Coil Around a SRSG
Hey---
After a couple of years of reading the wisdom in these posts, I still am not
sure what advantage the rotary gap has over a blown static gap. Could any
enlighten me? thanks much...
Carl
-----Original Message-----
From: Jim Lux
Sent: Saturday, March 23, 2013 6:57 AM
To: Tesla Coil Mailing List
Subject: Re: [TCML] Designing a Coil Around a SRSG
On 3/22/13 8:50 PM, Brandon Hendershot wrote:
Hi All,
Now that I've finally gotten something out of my long overdue 3.5" coil,
I'm already itching to start work on the next.
I've decided to make the switch to a rotary spark gap and have settled on
synchronous over asynchronous for the sake of performance and security of
my NSTs.
I want to be sure I do this coil right and plan everything out before I
buy
anything. Here are the parts and specs for what I know is going into the
new coil:
- 15/120 (4 x 15/30) NST Bank
- 7.25" x 26" Topload
- 0.25" x 0.25" Flat Primary Coil (Plenty long, should be enough
regardless)
*- ~6" x 24" 25** AWG Secondary Coil (*subject to change(**especially so))
I've read that 120 BPS is just about the best you can get when you're
using
NSTs, so I'm going to be building the rest around that. I haven't found a
motor yet, any specific RPM to look for? HP will vary since we don't know
what style we want yet.
120 BPS is going to give you the maximum "bang energy" since you can
adjust the gap to fire just at the peak voltage.
Typically, you have some number of electrodes on your rotary gap, so the
break rate is (motor RPM/60) * number of breaks/revolution.
An 1800 RPM motor spins at 30 rev/sec, so you'd need 4 sets of electrodes.
Note that synchronous motors come in speeds that are 3600/N RPM. speeds
like 1200, 1800, 3600 RPM (N=3,2,1 respectively). 1800 is very common
and is called a "4 pole" motor. Regular induction motors come in speeds
that are slightly less (1725 RPM, for instance).
There are instructions in the archives on how to take an induction motor
and turn it into a synchronous motor by grinding/filing flats onto the
armature. It's not all that tricky, but it is a lot of work, especially
if you do it by hand.
People have used DC motors and I think someone used a big stepper type
motor, but then you have the risk of destroying the controller if you
get a spark in the wrong place. the idea of a bulletproof DC servo,
though is very attractive, because it lets you vary the break rate AND
adjust the phasing. For an AC sync motor you have to mechanically move
the motor or adjust the AC phase.
Another attractive approach these days, I think Bert Hickman has tried
it, is to get an inexpensive Variable Frequency Drive to control a
induction motor. Basically you'll be feeding a bit higher than 60 Hz to
the motor, so that it runs at 1800 RPM, instead of 1725.
I'm assuming not (you know what they say about that), but does the
capacitance of the MMC vary depending on the BPS? Since it's synchronous
with the AC waveform and firing at a single frequency I imagine not...
Rather, isn't the Input watts the only determining factor? Silly me. Next
question:
Does the BPS have any influence on what the resonant frequency of the
secondary coil should be? <This would be my primary concern.
No. BPS controls the power input to the coil, really. There is also an
effect on streamer growth from the break rate.
One more quick irrelevant question, moving from a 15/30 to a 15/120, would
there need to be any modifications made to the Terry Filter?
Thanks a bunch,
Brandon H.
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