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Primary field strength



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From:  chris.swinson [SMTP:chris.swinson-at-zetnet.co.uk]
Sent:  Monday, August 17, 1998 2:53 PM
To:  Tesla List
Subject:  Re: Primary field strength




SNIP.....
>
>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.
>

I'd have to disagree here.  My secondary is within the primary field by
about 4 inches.  By your terms ( If I read it right ) I'd only need the
bottom of the coil actually in the field of the primary.  If this is the
case, Why bother to have the extra coil length, If its no in the flux surly
its not reciving energy ?


>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.
>


well , really.  So what do you think the maximum output is with the 8Kv 30ma
NST then?  I'll see if I can set the record then ;-)

>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.
>

My coil is 36" high, using 0.315mm wire.  Giving about 68 TI.  I'll sure let
you know of any results I have.  Only trouble really as I only have about
80" hight in my room, and the reels I buy the wire on are only 500g, so I'd
need to join on another roll half way though winding the coil .  Anyone know
the best way to do this ?


>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/
>
>
>