There may be another cause for a net B field through the ring: the Earth's. (apologies if this was mentioned before, haven't followed the discussion closely)
Michel 2009/8/24, Stephen A. Lawrence <[email protected]>: > I think I may see how this works. Unfortunately I'm going to be swamped > this week, I think, and I won't have time to write it up sensibly with > pictures 'n' such any time soon, but here's a quick sketch. > > The key is the outer race. We've got 100 amps flowing through that > race. IF the current were going unidirectionally, it would produce a B > field inside the bearing which would, given the radial current through > the balls, make the bearing spin. (In fact this would be exactly like a > unipolar motor.) > > But the current is going to split, going in one direction on one side > of the race, and the other direction on the other side, and the overall > effects cancel; the average current direction in the outer ring is null, > which is why it doesn't self-start. > > But that "null average" ignores the INDUCTION of the race, and the fact > that the balls are moving and the configuration is changing. That's the > solution; the rest is details. > > Herewith some details. > > Now, assume there's just one ball, for simplicity, and just one point of > contact with the race. Assume the contact point is positive, and is at > 12 o'clock, and the ball is at 3 o' clock. Then there will be something > like 3x as much current going down the ring to the right (from 12 to 3) > as there is going down the ring to the left (from 12 to 3 'the long > way'), due to the resistance of the race material. > > Keep the contact point at 12 o'clock. > > Move the ball down to 6 o'clock. Now the current splits evenly. > > Move the ball around to 9 o'clock. Now 3x as much current is going down > the left side as the right side. > > If the bearing is spinning clockwise, then the current going to the > right must be DECREASING as the ball goes down around the bottom of the > ring, and the current to the left must be INCREASING. > > But the ring has inductance, and with 100 amps total going through it we > can't neglect that. The inductance will tend to resist current change, > which means the current pattern is going to "lag" what a static analysis > would lead us to expect. In short, with the bearing spinning clockwise, > and a positive contact on the outer race, there will be more current > going CLOCKWISE than we expect. In other words, the average current > direction in the outer ring won't be zero, it will be clockwise, and as > a result there will be a net B field in the ring, pointing INTO the ring > as we look at it. > > With this configuration, with a positive contact on the outer ring, the > current in the balls is going from the outside to the inside, and the > force on the balls due to the (net, average) B field from the ring will > also drive them CLOCKWISE. > > And so the ring will continue to spin. > > QED (I think!). > > Could this be tested, using multiple contact points to the bearing, > maybe? Not sure. > > OH, yeah -- and what about the need for magnetic material? Well, if I'm > not mistaken, a ferromagnetic race (and adjacent balls) will increase > the inductance of the race versus nonmagnetic material. That in turn > should make the motor work a lot better, if this explanation is correct. > (And didn't somebody cite a source indicating that the motor might > work, *some*, with non-magnetic materials?) > > I hope this is clear enough that folks can follow what I'm trying to > say. In any case I have to go back to bed at this point. > > 'Till later.... > >
