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Primary field strength
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From: Bert Hickman [SMTP:bert.hickman-at-aquila-dot-com]
Sent: Saturday, August 15, 1998 12:10 PM
To: Tesla List
Subject: Re: Primary field strength
Chris,
I'm not exactly clear what field you're describing as being 38" high
with a 11.45nF cap), filling a room (with a 6.4nF cap), and how you
measured it. Can you be a bit more specific?
The length of a secondary is not very critical as long as it's at least
long enough to withstand the voltage stress it'll see during operation.
However, even with a shorter secondary, most of the magnetic coupling
between the primary and secondary takes place at the bottom-most portion
of the secondary. In fact, 80% or more of the primary's magnetic field
is NOT coupled to the secondary coil... by design. And, increasing the
length of your secondary coil while leaving your primary unchanged will
actually DECREASE the amount of coupling. Paradoxically, in a system
with a poorly quenching sparkgap, this can actually increase spark
length.. and it fixes a symptom of coil inefficiency but not the cause.
The ratio of the length versus height is also not especially critical,
and Tesla Coils can be made to operate with long "candlestick"
secondaries, although not as efficiently. In order to get the best
secondary efficiency, the ratio of the secondary's inductance (actually
it's inductive reactance) versus its effective resistance should be kept
high in order to minimize losses. Tesla used large diameter coils to
help prevent corona breakdown and to provide as much inductance as
possible while keeping coil resistance and distributed capacitance to a
minimum. It's a case of trying to balance conflicting objectives in
order to maximize overall performance. Long, thin, coils tend to have
lower Q, and thus lower operating efficiencies.
Ultimately, spark length is primarily a function input power and coil
efficiency. An 8 KV 30 MA power source (240 VA) will not create 72"
streamers irrespective of coilform length - it simply takes more power
to achieve this level of performance. You'll need 1-2 KVA to reach this,
even in a well-designed system.
You also didn't indicate the diameter and wiregauge of your secondary,
but it sounds like it's about 3" and 21 AWG based upon the inductances
you've quoted. As voltage levels rise, you also need to increase the
coilform diameter to prevent premature breakout from the coilform
itself. Since you have the rest of the system constructed, try
constructing a couple of experimental coils using say 60" or 80"
lengths, see if the improvement you observed continues.
And, please let us know the results.
And BTW, BTW = By The Way... :^)
Safe coilin' to you!
-- Bert --
Tesla List wrote:
>
> ----------
> From: chris.swinson [SMTP:chris.swinson-at-zetnet.co.uk]
> Sent: Friday, August 14, 1998 3:20 PM
> To: tesla list
> Subject: Primary field strength
>
> Hi all,
>
> I discovered today that my coil's field strength drops off at about 38". I
> used a 8Kv 30ma NST. My secondary ( 38" ) and cap ( 11.45nf ) were not used
> at first. This gave me a clear indication that the field rose to a hight of
> 38". I could gain about another 2" spark if a increased the coil's length
> but thats not the issue here.
>
> When I tried a 6.4nf cap, with only 1mm spark gap, The field was so big that
> it filled the whole room. If I had used a 5mm gap ( about max ) Wouldn't it
> be possible to have a secondary say 200" in hight and still power it off the
> 8Kv 30ma source.
>
> The secondary would still be in the field of the primary as the cap give's a
> high current boost each cycle.
>
> But even so, why am I told not to go over 24" for my secondary height ?
>
> The only problem I can see is the resistance of the secondary would be huge
> due to the extra wire, but I can double wind my coil to get around this. So
> in fact I could get about a 72" spark off a 200" coil, with the power still
> being only 8Kv -at- 30ma.
>
> I know Tesla used large inductive coils, perhaps he did this to use bigger
> caps and get better effieciency.
>
> The only other drawback being the frequency would drop due to the high
> inductance, but my specualted coil at 200" high, would only give 44mH
> inducatnce anyway. My 36" coil is only about 8mH anyway. So the way I see
> it, its a win-win situation.
>
> Can anyone confirm this or have anything to comment on ?
>
> By the way, I'm a bit think, so whats BTW stand for ? Its driving me mad !
>
> All the beST,
> Chris.
>
> chris.swinson-at-zetnet.co.uk
> www.users.globalnet.co.uk/~mega01/ <--- moved site to...
> www.users.zetnet.co.uk/mega.disczine/