Re: MOT-powered coil questions

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

Original poster: "Stephen Conner by way of Terry Fritz <teslalist-at-qwest-dot-net>" <steve-at-scopeboy-dot-com>

At 07:32 27/05/03 -0600, you wrote:
>Original poster: "Dwight Harm by way of Terry Fritz <teslalist-at-qwest-dot-net>" 
><dwighth-at-traxsoft-dot-com>
>
>Hi all,
>My son & I are close to starting our first TC.  It will be MOT-powered
>(these have been easy & free/cheap to scavenge).  We're thinking 2 MOTs to
>start with, then maybe a 4-MOT supply later.  We're planning on using the MO
>caps for secondary current limiting, and a static gap (maybe RQ-type) for
>now.  Some questions...
>
>1: Can we determine our approximate "coil wattage" by shorting the secondary
>(thru the caps) and measuring either primary or secondary current?

Possibly, there are a lot of variables, but it's unlikely that the true 
power would ever be more than half of what you measure on this 
short-circuit test. Secondary current is probably best to measure.


>2: I've seen 1000-1200 turns recommended on the secondary, at least for NST
>coils.  Is that true for MOT TCs, too?

It should be all right. Again there are lots of factors at work but the 
main thing is to make sure that you don't end up with too few primary 
turns. This will happen if you have a secondary with a high resonant 
frequency, and a large tank capacitor (which you'll probably want to get 
max performance out of MOTs) So best to err on the side of more secondary 
turns and/or larger diameter.


>3: How do we pick either the diameter of the secondary or the secondary wire
>gauge?  (Determining either will give an approximate value of the other,
>assuming a given number of turns, say 1000, and picking an aspect ratio of,
>say, 5:1.)  It'd be nice to wind a single secondary that'd work for either a
>2- or 4-MOT supply, if that's reasonable.

I'll probably attract a bit of controversy over this one but this is what I 
would recommend. First decide what length of sparks you want to produce. 
Check out some other MOT powered coils (there are some scary ones around 
but be aware they probably use oversized MOTs from commercial or 
old-fashioned ovens) or estimate the wattage of your power supply and use 
the spark length formula: length in inches=1.7*sqrt(power in watts)

Then pick the length of the secondary to be about two-thirds of your 
desired spark length. Now you can choose a wire gauge to give about 1200 
turns. Finally pick the diameter to get the resonant frequency you want. If 
you don't know what resonant frequency you want, just use one of the aspect 
ratios that people recommend, like 4:1, 5:1, etc.

Steve C.