Hi Gary,As Jim mentioned, the hoop stress is a result of outward-directed "magnetic pressure" in the winding stemming from Lorentz forces and scales as the square of peak current.
However, in coin shrinking there are additional radial forces stemming from Lenz Law repulsion between the shorted turn (the enclosed coin) and nearby coil windings. Analyzing these forces is considerably more complex, since it involves di/dt, skin depth, and magnetic diffusion - the solutions are typically not closed form. The first URL below is a "quick and dirty" analysis (not including Lenz repulsion). Much more in-depth analyses for pulsed coils and enclosed workpieces can be found in the next two URL's, "ELECTROMAGNETIC METAL FORMING HANDBOOK" and "Electrodynamics in Deformable Solids for Electromagnetic Forming"
http://www.niell.org/Crimping.pdf http://www.mse.eng.ohio-state.edu/%7Edaehn/metalforminghb/tabofcont/index.html http://oai.tudelft.nl/ir/asset/uuid:129d525c-2dd9-4f14-950e-08807e4c8bc0/ASSET1The combination of magnetic pressure and Lenz repulsion forces annihilate the work coil, but the highest combined forces and greatest fragmentation occur in the centermost turns (those closest to the coin).
Bert Lau, Gary wrote:
OK Jim, I'll bite. What's hoop stress? Maybe the thing that annihilates the work coil in quarter-shrinkers? Regars, Gary Lau MA, USA-----Original Message----- From: tesla-bounces@xxxxxxxxxx [mailto:tesla-bounces@xxxxxxxxxx] On Behalf Of jimlux Sent: Friday, May 01, 2009 12:51 AM To: Tesla Coil Mailing List Subject: [TCML] hoop stress in primary coilAnyone have a cookbook approximation kind of equation for the hoop stress in the primary? Or even, a single turn coil, given di/dt,etc. Tnx Jim_______________________________________________ Tesla mailing list Tesla@xxxxxxxxxxxxxx http://www.pupman.com/mailman/listinfo/tesla
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