XPost: sci.electronics.design   
   From: tiwill@seventransistorlabs.com   
      
   "Tom Del Rosso"  wrote in   
   message news:raf445$1vt$1@dont-email.me...   
   > Where do you measure l_e?   
      
   In general, you'd calculate it by integrating over space, in such a way that   
   you get the average of magnetic path lengths, weighted by their   
   contributions to total flux.  I guess that's a ratio between some Maxwell   
   equations but I can't think which ones at a glance.   
      
   When mu_r >> 1, the path is essentially all in the core (or gaps between   
   core pieces), so is the mean circumference of the core.  l_e is almost   
   exclusively used with cores, since it isn't very meaningful elsewhere...   
      
   Same for A_e, the effective area is the core cross section.  You can define   
   it easily enough for helical geometries (solenoid, toroid, whatever) as   
   well, but you'll always get an inductance greater than calculated because   
   there's leakage between turns as well as the main (intended?) field.   
      
   > What is V_s?   
      
   V.s is the product of volts and time, flux (webers).  (Notice I consistently   
   used underscore to denote subscript.)   
      
   > And you seem to be relating inductance H to mu, but isn't that a whole   
   > different H?  Inductance doesn't depend on current for one thing.   
      
   I bring up inductance because we're often concerned with circuit parameters   
   (volts, amps, winding flux, inductance), or what makes them up (inductivity   
   (inductance / turn^2), flux per turn, amp-turns), as well as the fields and   
   other bulk properties (flux density, magnetization, permeability).   
      
   I like to treat turns as their own unit, to keep track of whether I'm   
   talking about circuit values (turns cancel out), core values, or fields.   
      
   The thing about dimensional analysis is, you can always add dummy units and   
   track them through the operation -- a helpful tip just for hand-working   
   algebra -- but it's a lot harder to remove units, and doing so may invite   
   confusion (I would perhaps suggest avoiding the cgs system until one is very   
   comfortable with fields).   
      
   Yes, magnetization symbol is H (bold H if you're talking about vectors), and   
   the henry unit is H, one must be careful not to confuse the two.  I usually   
   use "==" to denote unit equivalence, and a regular "=" to denote   
   mathematical equivalence.   
      
   Also I tend to refer to H as magnetization, even though that's the built-in   
   magnetization M (i.e., a permanent magnet).  What I mean is "magnetic field   
   intensity" but ain't no one got time fo' dat.   
      
   Also also, inductance does vary with current, for practical ferromagnetic   
   cores -- that's one reason why we're interested in tracking the total flux   
   (circuit flux * turns / A_e = B), or sometimes magnetization (circuit   
   amperes * turns / l_e = H), in magnetic component design.   
      
   If you're more interested in fields in general, than component design, you   
   can ignore much of the circuit-oriented values.   
      
   Tim   
      
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   Seven Transistor Labs, LLC   
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