On 01/20/2010 11:28 PM, Harry Veeder wrote: > > > > > ----- Original Message ---- >> From: Stephen A. Lawrence <[email protected]> To: >> [email protected] Sent: Wed, January 20, 2010 10:26:24 PM >> Subject: Re: [Vo]:steorn addendum video posted on youtube >> >> >> >> On 01/20/2010 08:51 PM, Harry Veeder wrote: >>> >>> >>> >>> >>> ----- Original Message ---- >>>> From: Stephen A. Lawrence To: [email protected] Sent: Wed, >>>> January 20, 2010 1:04:33 PM Subject: Re: [Vo]:steorn addendum >>>> video posted on youtube >>> >>>> >>>> As for the changing inductance in the changing external >>>> magnetic field, I don't think that happens while the power's >>>> on, because the core's being held in a saturated state by the >>>> field from the coil. >>> >>> Are you explaining how to electronically eliminate back EMF? >> >> No, not exactly. There's already no back EMF with a toroidal >> coil. (That's hard to show directly, but easy to show by >> conservation of momentum.) > > > >> What I'm talking about is something a little different. When a >> strong external magnetic field acts on a ferromagnetic core, it >> may, if I understand this correctly, actually put the core in a >> "saturated" state, such that an additional B field which is aligned >> with the external field won't result in any additional >> magnetization of the core. If the B field of the core doesn't >> increase with further increase in the applied field, what that says >> is the *permeability* of the core has, in effect, dropped. The >> reduced permeability of the core means that the inductance of the >> coil will be lower when the core is in this state than when it's >> unsaturated. >> >> So, changing the external field will change the inductance -- IF >> the coil has no current flowing through it. But, the inductance of >> this coil is already not a fixed value; it varies with the current. >> When the current increases to the point where the core saturates, >> the inductance drops, because the permeability of the core material >> has (in effect) dropped. In the case where the core is saturated >> by the effect of the current in the coil, adding (or removing) an >> additional external field may have no effect on the core, which may >> very well remain saturated throughout the operation. In that case, >> the permeability won't change due to a change in the external >> field, and the inductance of the coil won't change. > > Let me see if I have this right by re-expressing this in common > language. > > 1) You _can't_ induce a current in a core-less toroidal coil by > waving a magnet outside the coil. > > 2a)You _can_ induce a current in a toroidal coil with a core by > changing the magnetic strength of the core by waving a magnet outside > the coil... > > 2b)...but only if the core is in a state where it can absorb more > magnetism.
Yes, that's exactly how I understand it. Whether it's "really correct" or not, is another question ;-) > > Harry > >> Clear? (As mud?) >> >> My grasp of saturation in magnets is pretty weak, unfortunately, >> and I may be exposing my ignorance here. Anybody with a better >> understanding of magnets want to correct this? Terry? Bill? >> Horace? Anybody? > > > __________________________________________________________________ > Looking for the perfect gift? Give the gift of Flickr! > > http://www.flickr.com/gift/ >
