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Re: Primary field size

Subject: 
            Re: Primary field size
       Date: 
            Sat, 22 Mar 1997 12:27:15 -0800
       From: 
            Bert Hickman <bert.hickman-at-aquila-dot-com>
Organization: 
            Stoneridge Engineering
         To: 
            Tesla List <tesla-at-pupman-dot-com>
 References: 
            1


Tesla List wrote:
> 
> Subject:
>            Primary field size
>       Date:
>            Sat, 22 Mar 1997 09:48:13 -0500
>       From:
>            "Kevin M. Conkey" <teslacoil-at-mindspring-dot-com>
>         To:
>            Tesla List <tesla-at-pupman-dot-com>
> References:
>            1
> 
> How can you determine the useful field size that a given primary puts
> out?
> How did someone come up with the aspect ratios of pri/sec? Was it
> strictly through trial and error, or can you figure it out?
> --
> Kevin M. Conkey

Kevin,

For classic 2-coil systems, the optimal coefficient of coupling is in
the range of 0.15 - 0.28 (i.e., relatively "loose"). Since higher power
systems will reach streamer lengths of 2-3X secondary coil length, the
combination of the two factors tends to drive higher power coilers to
use flat primaries to maximize the point-to-point distance between the
top of the secondary and the primary. Flat primaries with larger number
of turns allow smaller tank caps and lower current power sources to be
used, accounting for the popularity of this configuration.

The bottom line: 
If you can simultaneously adjust the coupling coefficient to be within
the above range AND keep the primary and secondary circuits in tune, AND
provide sufficient insulation/clearance to prevent secondary:primary
flasovers,  the system should perform quite well irrespective of the
size, shape, and configuration of the primary vs secondary. The
trade-offs in actual primary size come not so much from the
primary:secondary aspect ratio, as from other effects with smallewr
primaries. These  include efficiently handling higher tank currents,
increased gap erosion and difficulty in quenching, and the need for a
beefier power source to supply larger capacitor recharge current. 

Fortunately, calculation of the actual primary field shape is not
necessary. Design your primary to match your planned tank cap and
secondary operating frequency. Then insure that you've got capability to
easily change K. Trial-and-error practical results by large number of
coilers has demonstrated that good performance can be obtained with
relatively few primary turns and large tank capacitance, with relatively
large number of primary turns and lower tank capacitance, and _anywhere_
in-between. However, primaries having a relatively large number of turns
(12-16) tend to place lower demands on the power source, gap, and cap
surge current levels. In this case, primary outer diameters tend to be
"roughly" the same as secondary winding length.

Hope this helps...

-- Bert H --