OH -- I see the problem. The homopolar motor must be spun backwards to generate electricity. This thing hasn't got a "backwards". That's the point, right?
Stephen A. Lawrence wrote: > > 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/ >> >> >> >> >
