> it is very > easy to make a serious mistake in regard to the above, and which is > not related to leakage flux at all. That mistake is to not ensure > that an even number of winding layers is used for each torus, with > winding direction reversed at layer termination boundaries, so as to > avoid major axis net current loops.
Indeed Horace you must be right that each winding layer not compensated by a reverse wound layer must act as a single turn current loop around the major axis, with the same current as in the winding. This effect, resulting from a turn of wire not being flat in reality, had never occurred to me, many thanks for the enlightening post! Michel ----- Original Message ----- From: "Horace Heffner" <[EMAIL PROTECTED]> To: <[email protected]> Sent: Saturday, March 08, 2008 9:48 AM Subject: Re: [Vo]:Toroidal inductors > > On Mar 7, 2008, at 2:25 PM, Robin van Spaandonk wrote: > >> Hi, >> >> If one has two separate toroidally wound inductors, and one passes >> a DC current >> through each coil, do they experience any force from one another, >> particularly >> when sharing a common major axis? >> >> I'm interested in both theoretical and experimental responses. > > > It is typically assumed that, with the exception of leakage flux, > that all of the flux is held within the tori minor radii, thus there > is no interaction between current carrying tori when pure DC is > used. There indeed is always, both theoretically and experimentally, > small leakage fields from the windings, which can readily be detected > when AC current is used. However, when experimenting, it is very > easy to make a serious mistake in regard to the above, and which is > not related to leakage flux at all. That mistake is to not ensure > that an even number of winding layers is used for each torus, with > winding direction reversed at layer termination boundaries, so as to > avoid major axis net current loops. For example, if a single winding > layer comprised of 100 turns is used, then the torus winding is > equivalent to 100 ideal current hoops about the minor axis carrying > i, plus the equivalent of a single conductor hoop coil centered on > the major axis and having the major radius and carrying current i > (this is equivalent to a current hoop running through the center of > the "cake of the doughnut" carrying current i). If both tori have an > odd number of winding layers, or even if multiple winding layers are > used but all or most proceed in the same major axis direction, or > some combination of the above resulting in a net major axis current > hoop, then they both carry a significant external magnetic field > equivalent to hoop coils about their major axes. A pair of tori with > such equivalent hoop coils will exhibit significant mutual forces and/ > or torques depending on location and orientation. Note that such > forces can be larger than just the force between the major axis hoop > currents, because flux from one hoop coil can enter the "cake of the > doughnut" volume of the adjacent torus, and thus interact with the > flux there (or be viewed as interacting with the small radius > windings) to produce much larger forces than might otherwise be > anticipated. This also means unexpected force interactions can arise > between a major axis hoop current carrying torus and a torus not > having such a hoop equivalent current, including a permanent magnet > torus in which all flux is internal. Flux repels (or attracts) > parallel flux via magnetic pressure. > > A very interesting and surprising experiment (for me anyway) was the > investigation of the vicinity of an iron core toroid coil by means of > an approximation to a magnetic monopole probe I made by taping > together a long (about 6" long) stack of 3/8" thick circular ceramic > magnets (3/4" dia. if I recall). When the toroid coil is driven by > AC current it is very easy to sense magnetic field strength manually > from the vibration of the probe when it is hand held (at least with > the coils I used, which were #10 or #12 wire carrying 20 amps or > so). By far, the strongest vibrations are obtained when a probe tip > is in the center of the torus. I put a little plywood platform in > the center of the torus and placed a single disc ceramic magnet > there. It danced about in a lively fashion and slowly rotated as well. > > A much better approximation to a monopole probe could be made using > smaller diameter magnets joined into a longer probe. This probe > technique seemed much more sensitive, but provided similar results to > a FET probe I made, with regard to determining field envelope shape. > The FET probe required AC, but the simple monopole approximating > magnetic probe should work with DC. > > Horace Heffner > http://www.mtaonline.net/~hheffner/ > > >
