Horace Heffner wrote: > > On Sep 10, 2009, at 7:20 AM, Stephen A. Lawrence wrote: > >> >> A generator which powers its own field windings won't self start, >> right? You can spin it until you're blue in the face but nothing >> will happen, unless you apply some external power to it to energize >> the field windings and get it going. Once it's going you can take >> away your external power source, of course. (Of course if there's >> "residual magnetism" stuck in the iron core of the field windings, >> that may be enough to get it going -- but a brand new one, right >> out of the box, won't self start the first time it's used.) >> >> This may be the same deal. You spin a bearing, and nothing >> happens. Well, maybe if you apply external mechanical power /and/ >> you run a current through it to get it started, it will start >> generating electricity on its own. > > But it is not the same deal. If you apply power it works as a motor, > no matter which way it spins, or even if you apply AC.
I'm not following. Forget the AC for a moment; it may be possible to treat the AC case as a series of separate "short runs" of a DC motor. In any case let's just look at the DC case. If (big "if"!) the thing really is operating as a homopolar motor when it's running, then spinning it is what determines the direction of the B field, and consequently the direction in which the motor goes. Right so far? (Granted we don't know how this could be, since every analysis which assumes a net B field generated by the current in the races has fallen through, but let's assume it's true for the moment.) Then, if we spin it with no current going through it, nothing will happen. But if we spin it with a current already going through it, then it seems possible that it will produce a forward EMF as long as it's being spun rapidly by externally supplied mechanical power. Remove the external mechanical power, and it'll slow down and go back to producing a back EMF. Is that unreasonable? Am I yet again guilty of mushy thinking here? As a less speculative example of that sort of behavior, if we wrap one or two turns of wire around the outside of the bearing to provide a "field winding", then regardless of how it works when it's "really working", it should self start and form a "normal" homopolar motor. But it still won't generate electricity unless you "prime the pump" with an external current, and it very probably will never generate enough electricity to keep its "field winding" sufficiently energized to keep it working if you shut off the external power. **** BTW you posted an analysis a long time back which concluded that the "residual" field of the outer race could be responsible. (I think you subsequently abandoned that approach, but I'm not sure why; perhaps it was for the reason given here?) AFACS the "residual" field of a wire which has had current passed through it, but which no longer has any net current going through, is identically nil outside the wire as per Maxwell's equations (integral of B field around a loop is equal to integral of current over the enclosed surface). Inside the wire, it's another story, of course. > > > Best regards, > > Horace Heffner http://www.mtaonline.net/~hheffner/ > > > >
