Yet again replying to my own post... Stephen A. Lawrence wrote: > 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?
If it's functioning as a homopolar motor, then spinning it faster "forward" just produces a bigger "back EMF"; it certainly doesn't result in a forward EMF, as I alleged two messages back! The only way to produce a "forward EMF" from a homopolar motor is to spin it "backwards". Exactly stopped, no work is being done and there's no back EMF. Spin it backwards, and mechanical work is going into the system, and the induced EMF is in the same direction as the current. "Forward" and "backward" are relative, of course, and in this case the spin determines their meaning, and so it's impossible to "spin it backwards". > > > > 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/ >>> >>> >>> >>> >
