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Re: Magnet Design for Tesla Coils



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Tesla List wrote:
> 
> Subject: Magnet Design for Tesla Coils
> Date: Sun, 9 Mar 1997 10:01:57 -0500 (EST)
> From: TimRaney-at-aol-dot-com
> To: tesla-at-pupman-dot-com
> 
> Folks:
> 
> Just some notes on electromagnet design and references.  Building a cyclotron
> is undoubtedly a very challenging project.  However, designing electromagnets
> to magnetically quench a spark gap in a disruptive Tesla Coil is not nearly
> as difficult.
> 
> Some folks will debate the utility of magnetic quenching applied to small
> Tesla Coils, but I think it has good possibilities.  Tesla used magnetically
> quenched gaps in some of his designs and those details are covered in "The
> Inventions, Researches and Writings of Nikola Tesla," by T.C. Martin (1894)
> and reprinted by Barnes & Noble Books. This book contains a lot of other
> great information and has been mentioned before on the Tesla List.
> 
> To put things in perspective, I built a "large" (everything's relative!)
> electromagnet and will relate some of the design features and problems.  This
> particular magnet was not used to quench spark gaps, but the design
> principles are the same.  This magnet used two coils mounted opposite each
> other on a mild steel frame and this configuration is known as a "Weiss
> Electromagnet' (Bleaney, et al, 1957).
> 
> Each coil consisted of 6 lbs. of #18 magnet wire, "scramble-wound" on 2 1/8"
> diameter cold rolled steel (mild steel) pole pieces.  The pole piece faces
> were machined at a 60 degree angle so the face was 1" across.  Magnet gap
> width was adjusted with steel washers as spacers.  Power supply was either a
> filament tranformer with rectifier or a Variac-rectifier combination. Magnet
> coil resistance was apprx. 15 ohms at 70 degrees F (resistance obviously will
> increase as coils warm up).
> 
> With the magnets' gap set at 2 cm, a 6 ampere current at approx. 100 VDC,
> produced a 10K Gauss magnetic flux in the center of the gap.  Measurements
> were taken with a Bell Gaussmeter (expensive item, but not if you can borrow
> one!).  This is not bad and would probably be higher if the coils were not
> scramble-wound.  However, at this current, the coils heat up rather quickly
> and that's not good (can use larger gauge wire).  And yes, you can calculate
> the magnetic flux:  that's easy for an air-core solenoid, but gets tedious
> with electromagnets.  At lower currents, this particular magnet serves its
> purpose of producing moderately strong fields from 1K to 5K Gauss without any
> dangerous rise in operating temperature.
> 
> So, in that regard the magnet was successful.  Again,  this is not really a
> big magnet by many standards, but similar designs can be used to quench spark
> gaps in disruptive Tesla Coils. An important difference in this application
> is insulating the spark gap from the magnet pole pieces (whether you use a
> permanent gap magnet or electromagnet).  This is not as easy as it sounds,
> especially when you don't have exactly what you need and must "improvise"
> (we're all kind of used to that!).
> 
> Applicable references follow and are certainly not all inclusive. Original
> publication dates included;  these references are still available, except for
> Bleaney (that's why it's a good idea to "cruise" used book shops). These are
> useful references that cover many facets of magnet design.  Other resources
> include most electrical engineering texts (especially the older ones) and
> physics text books.
> 
> Underhill, C.N.  Solenoids, Electromagnets and Electromagnetic Windings.
> Reprinted by Lindsey Pubs., 1921.
> 
> Lee, E.W. Magnetism - An Introductory Survey.  Dover Pubs. (and Lindsey Pubs,
> I believe), 1970.
> 
> Bleaney, B.I. Electricity and Magnetism. Oxford University Presss, London,
> 1957.
> 
> Montgomery, T.B. Design of Magnets and Electromagnets.  Reprinted by Lindsey
> Pubs., 1948.
> 
> Chabay, R. and Sherwood, B.  Electric and Magnetic Interactions.  John Wiley
> & Sons, Inc., New York, 1995.
> 
> Fretter, W.B.  Introduction to Experimental Physics.  Prentice-Hall, Inc.,
> New York, 1954.
> 
> Gingery, D.J. How to Build a Magneto Magnetizer.  Self-published, available
> through Lindsey Pubs., 1994.
> 
> Martin, T.C. The Inventions, Researches and Writings of Nikola Tesla.
>  Reprinted by Barnes & Noble,  1894.
> 
> Hope this helps!  Good luck experimenting!
> 
> //    S     //
> 
> TIMOTHY RANEY, TCBOR

Timothy,

Thanks for all the great references above. Did you see significant
improvement in coil performance when running at either the 5 or 10
kGauss level? Also, do you have any "feel" for comparative performance
between this approach and a straight high-velocity vacuum gap?  Also,
what'd you use to insulate the magnet poles from the arc - sounds like
it might have been something other than mica? 

Thanks in advance!

-- Bert --






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