Re: [TCML] Designing a Coil Around a SRSG

[Scott Bogard <sdbogard@xxxxxxxxx>]

Hi Brandon,
     it looks like this post got side tracked into triggered gap land, so
I'll answer your questions in case nobody else has.  Ok firstly looking
over your proposed specs (whether or not you were asking for it) my two
cents...
15/120...  that is some serious power for NSTs, 1800 watts worth, and that
number 1800 is the basis for lots of our calculations (if you want to do it
right, we must calculate.  -
7.25" x 26" Topload...  A bit under-gunned for that wattage, you could go
bigger for longer sparks, or leave it that size for multiple smaller ones
(the quick and dirty formula for top load size is (SQRT of watts) * (.5
though .9) = topload overall diameter in inches (.5 being multiple
streamers, .9 being just one.))  I would go bigger, but I like big top
loads only making one streamer.
*- ~6" x 24" 25** AWG Secondary Coil (*subject to change(**especially
so))...  This is a bit over-gunned for your wattage.  It does not hurt to
make a bigger coil, it is just unnecessary.  For reference I use a four
inch coil with about 2500 watts, but I like small coils with ridiculous
sparks.  The tend to look a bit silly, tiny coils dwarfed by their top
loads, but when you turn them on, this attracts more attention to the
sparks.
Ok, the capacitance of your coil is very specifically dependent on BPS.
Let me enlighten you to a newbie mistake, I made for several years after
starting coiling, there are two systems one must "tune" in a TC.  Firstly
is the well now resonate systems, getting frequencies fo tank and promary
to match secondary and topload, everyone knows this.  Secondly, and just as
importantly (and often overlooked) is the tuning of power draw, this is
done with ballast (not an option as you are using NST) tank cap
capacitance, and BPS.  a lower bps will have long to charge, and requires a
larger capacitor, up to where your ballast can no longer allow additional
power to be absorbed.  I don't like synchronous for this reason, you need a
HUGE capacitor for a given power draw, it is much easier to use a high BPS
and a small cap.  But, with NSTs, synch is the lowest stress mode of
operation, as such, it is good to use.  With that said, you have probably
heard of LTR caps?  LTR (larger than resonate) is the size you want to use
for a sync rotary, as it represents the size that will fill to around
60-75% capacity during operation (which if we consider how capacitors work
(namely how they charge fastest when empty) is the best place to have it as
it represents the most power transfer per second (charging higher will take
up more time than it is worth, and charging lower won't move enough juice
into the coil.))  You could use a "resonate" sized cap, this will give the
absolute best power transfer rate every time (the ultimate in power draw
tuning) BUT it will most likely fry your NSTs, as it will actually change
your wattage from 1800 to something higher, and require other design
changes as shown above.  As for how to calculate these, you can look it up,
there are resonate and LTR calculators everywhere.  Personally I don't use
nor like LTR caps, I like an external ballast, and a high BPS system, but
that is not an option for NSTs, as the ballast is built in.
Ok, onto srsg design, the easiest way is to get a surplus induction motor
and grind flats onto it, there is loads of info on this in the archives,
and others have done actual studies.  When I built my only sync gap, I got
lucky and the thing worked perfectly, but if you grind off too much the
motor is junk, and too little and it won't sync.  Then phase is adjusted
with a phase controller, or rotating the motor in its mount, you want it to
fire at the peak of the AC cycle.  To adjust mine, I taped a flourescent
lego to the rods and used a black light, and lined it up so the brightest
spot lined up with the rods (this will make more sense if you read into
them a bit.)  Ok, I feel like I've given you enough to chew on for a bit,
hopefully others will chime in as well!

Scott Bogard.


On Fri, Mar 22, 2013 at 11:50 PM, Brandon Hendershot <
brandonhendershot@xxxxxxxxx> 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.
> 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.
>
> 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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