It seems to me that maybe you don't really want to minimize L.
If I understand correctly, you're not really interested in the rise time, L/R, per se; rather, you're interested in the time it takes the field to get to some particular value, H_0. Is that correct? As I understand it, the field strength depends linearly on the inductance and the current, with some constant of proportionality which depends on the core material and shape. Right? Something like this: H_0 = k * L * I_0 I'm totally in the dark when it comes to electromagnet core materials, performance, and geometry so I won't say anything more about "k". Change in current in an inductor goes linearly in the applied voltage (minus the back EMF due to resistance), and inversely as the inductance; like this: dI/dt = (V - RI)/L Cross multiply: L*dI/(V-RI) = dt and then the rise time you're interested in is presumably: T_0 = integral(0->(H_0/kL)) [L/(V-RI)] dI Exercising my feeble integration skills, if I didn't blow it, that's T_0 = (L/R) ln[kVL/(kVL - H_0*R)] Decreasing L and increasing R makes the L/R term smaller but makes the ln[] term larger. If we make the (probably awful) assumption that R has been reduced and/or V and L have been boosted so that the ln[] term is close to zero, then we can replace it with a first order approximation, and get T_0 ~ (L/R) [H_0*R/(kVL - H_0*R)] or, replacing (L/R) with (1/R)/(1/L) and multiplying through, T_0 ~ H_0 * 1/(kV - (H_0*R/L)) This suggests you want to _boost_ L and V as far as you can, boost "k" (which you already knew), and reduce R as much as possible. So, silver wire, for sure. I still don't think skin effect is going to be a major issue here, though, so plating on the wire is probably irrelevant. Terry Blanton wrote: > On 12/20/06, Stephen A. Lawrence <[EMAIL PROTECTED]> wrote: > >> Do you mean reduce resistance? > > For a given voltage the time for the current to rise to 67% of its > steady state value is given by L/R. Since I must achieve a certain > current to achieve a certain magnetic field strength of the > electromagnet, the rise time is essentially wasted duty cycle. If I > am duty cycle limited, I want to minimize the rise time. > > The obvious way is to raise the voltage; but, I don't want to exceed > 0.5 times the break down voltage of the insulation. As Jones points > out, I could use a different core; but, the super permalloy (aka > spermalloy :-) being used is probably the best choice. > > I think we've decided that the least expensive approach for now is to > use solid CO2 to increase the duty cycle of the EM by pulling out any > heat generated by the coil. (BTW, Jones, virtually every local > supermarket sells dry ice these days.) Besides, the condensing water > vapor will give the whole test rig that mad scientist look. :-) > > Ironic that one of the goals of a magnetic motor is to reduce > greenhouse gas emissions, however. > > Terry >
