Secondary Inductance (fwd)

To: tesla-at-pupman-dot-com
Subject: Secondary Inductance (fwd)
From: Tesla List <mod1-at-pupman-dot-com>
Date: Thu, 15 Apr 1999 21:13:12 -0600 (MDT)
Approved: mod1-at-pupman-dot-com


---------- Forwarded message ----------
Date: Thu, 15 Apr 1999 07:46:34 -0500
From: "Mark S. Rzeszotarski, Ph.D." <msr7-at-po.cwru.edu>
To: tesla-at-pupman-dot-com
Subject: Secondary Inductance

Steve Young said in part:
<snip>
>For example, suppose I close-wind an 8 inch diameter coil with a winding
>length of 30 inches with 1000 turns of #21 wire.  If I space wind another
>coil with 1000 turns of # 23 to the same 30 inch length, is the inductance
>approximately the same as the close-wound coil?
        There are formulas in Grover and the NBS publications which can
compute this for you.  Using the formula in Grover, the following results
are obtained:
21 gauge:  Ls=47.84 mH
23 gauge:  Ls=47.81 mH
        Obviously, the coil geometry determines the inductance, and the wire
diameter does not alter that geometry significantly.  However, that is not
the only thing to consider.  For example, you might want to know the A.C.
resistance of the coil, which is affected by operating frequency, wire D.C.
resistance and proximity and skin effects.  Taking these into account (using
empirical data from the literature and experimental data from a number of
sources), we obtain the following:
21 gauge:  Rac=127 ohms, Q=294
23 gauge:  Rac=155 ohms, Q=241
at an assumed operating frequency of 125 kHz (14 pF distributed capacitance
along the secondary plus a 20 pF toroid (5" x 20") on top).  This difference
gets smaller if a larger toroid is used.
        That is still not the whole story, since we must also consider the
possibility of voltage flashover (more of a possibility in the closewound
coil) and the actual operating Q, which is significantly lower when the
sparks start to break out.  These factors play a greater role than the
slight Q difference between coils.  If I had the choice, I would closewind
it with 23 AWG (more than 1000 turns) and insulate it well with epoxy or
multiple coats of polyurethane.  BTW, I have an 8" x 25" coil wound with 22
AWG and am pleased with its performance, albeit limited by my NST power
source at the present time.
        The H:D (height to diameter ratio) is a bit high for your proposed
coil unless you plan to use a huge toroid on top or have excellent
insulation on the coil former.  (The toroid serves to electrostatically
shape the field near the top of the coil but has a limited range of effect.)
Most of the larger diameter coils tend to be in the 2.5:1 to 3:1 range.
Since output voltage is directly proportional to the turns ratio even for
air core coils, pushing the H:D a bit buys you a little more voltage if you
can withstand the voltage rise along the secondary.

Regards,
Mark S. Rzeszotarski, Ph.D.
Mark S. Rzeszotarski, Ph.D., MetroHealth Medical Center,Radiology
Department, Cleveland OH 44109-1998