On 12/30/2009 07:00 PM, Terry Blanton wrote: > Gnorts, Vorts, > > Some of us are confusing the issues above. Energy cannot be stored in > an inductor because there are no magnetic charge carriers. Hence, > when trying to open the circuit on an inductor, the magnetic field > WILL COLLAPSE. This forces the potential of the two port device to > approach infinity until the field collapses. This means arcing fer > sure because the current must leave the inductor. > > Back EMF, while related, is not exactly the same. Take a NdFeB magnet > in your hand and rapidly move it across a sheet of copper. You will > feel Lorentz reach up and grab your wrist clutching tighter the faster > you try to move. This is an example of Lenz Law, a changing magnetic > field in a conductor generates a force in opposition to the change. > > There is a great demonstration of the Lenz law on the web: > > http://www.youtube.com/watch?v=fxC-AEC0ROk > > When Steorn claims no BEMF, they are not saying there is no arcing > when switching the inductor. Indeed, this is why their reed switches > were failing. > What they are claiming is that there is no drag when > the magnet moves near the conductor. And they are right due to the > asymmetry of the toroid. In an external changing field, half the > toroid is creating a current in one direction and the other half > creates a current in opposition. >
This is approximately correct but, in this case, it is not *exactly* correct, and I think this is a lot of the problem in understanding this thing. If the external field is *uniform* it's true that the EMF in the two sides of the torus will cancel. But real magnetic fields are typically not entirely uniform, and the non-uniformity is not just an artifact; it's how one magnet "knows" where another magnet is. In a perfectly uniform field a permanent magnet would rotate to align itself with the field, but would not be drawn in any direction -- when something is attracted to a magnet, it's moving up the field gradient. In particular, the force on a magnetic dipole in a field aligned with the dipole is the gradient of the field strength times the strength of the dipole. No field gradient implies there's no force. The fact that there is attraction between two magnets tells you right away that their fields are non-uniform. If I understand the geometry of this motor (which is, admittedly, debatable!) then, in fact, the fields are oriented such that the field of the receding magnet is going to be stronger on one side of the torus than the other. Consequently the induced voltages won't exactly cancel and there will be a BEMF. Stronger core magnets will require more current (or more turns) to cancel their fields. The more current you put through the coil (or the more turns it has with fixed current), the more work the induced voltage is going to be doing. Similarly, stronger moving magnets will result in stronger forces driving the motor, but will also result in larger induced voltages during the "quenched" motion. > Finally note that they are applying voltage to the toroid continuously > except when the magnet approaches. They then briefly turn the voltage > off and the magnet is attracted to the core. When they re-energize > the toroid, they force realignment of the domains of the core and the > rotor with the magnet doesn't stick because it's attraction to the > core is diminished. > > There is nothing here which is not known. They are actually showing > an INVERSE pulse motor which must be remarkably inefficient since the > pulse motors I have tested are only about 20% efficient. They > energize the coil through most of the cycle whereas a pulse motor only > energizes the coil during the approach of the magnet. > > I think. > > Terry > >
