Re: [Vo]:Back EMF vs Inductor Energy Storage
On Wed, Dec 30, 2009 at 10:13 PM, Stephen A. Lawrence sa...@pobox.com wrote: Consequently the induced voltages won't exactly cancel and there will be a BEMF. Thanks, Stephen. Agreed; and, indeed you can see some of that BEMF in the scope trace ripple in some of the replications on youtube. Terry
[Vo]:Back EMF vs Inductor Energy Storage
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. 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
Re: [Vo]:Back EMF vs Inductor Energy Storage
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
RE: [Vo]:Back EMF vs Inductor Energy Storage
The energy is stored in the mag fld, not the inductor. Also, I've seen orbos that seemed to have a core with the toroid, and some that didn't, or at least it certainly didn't look like there was a core. I also was under the impression that the stator cores were NOT PMs, but simply iron cores. A few more thoughts, and I'll spare you the useless speculations... - Work is definitely being done as the permanent magnets on the rotor are being attracted to (accelerating towards) the stator cores. ABD seemed to imply, or state, that the only time any work was being done was after the rotor PMs passed the toroids... He also stated that the toroids must be fed constant current for a significant period after that point... I believe this is not the case. - From the oscilloscope screen shots, and contrary to ABD's comment, the toroids are only being pulsed for a short time as the rotor magnets pass TDC to overcome the cogging effect. From other PM motors like Sprain and Butch LaFonte, the single electromagnet (at the end of the PMs making up the stator) only needs a very short pulse to allow the rotor to pass the cogging point. The time for the mag-fld to build up and then collapse is considerably longer than the electric pulse. - When I mentioned some of the things that Thane Heins had learned over his time at Ottowa U, this forum pretty much dismissed it as nothing new. One of the keys to Orbo is something that Thane also discovered -- namely, that there is a time lag in the response of the magnetic material (magnetic permeability). It was this asymmetry that allowed enough of a lag in the collapse of the mag-fld of the coil, that generated a PUSH against the PMs after they passed TDC, thus causing acceleration; and in some cases, going from 2200rpms to 2300, 2400, 2500, 2600 in one or two second intervals. Accelerating his large rotor 100rpms/sec is no small force. - One more thing that Thane discovered, and my explanation might be a bit off, was that one could 'reroute' the energy that would cause the BEMF by using the proper core material (low or hi permeability??? Can't remember), thus keeping that energy out of the air-gap (btwn PMs in rotor and stator coil/core) and off of the rotor. It is kept within the core material of the stator (and he had VERY large cores), routing it to the opposite pole of the adjacent PM; i.e., he provided a closed 'magnetic circuit'. In some work that I'm involved in right now, we are using permanent magnets and we have them mounted to a soft iron housing, which basically acts like a wire to route the magnetic flux/fld from the south pole of one set of magnets to the north side of the other set. If you don't do this, you've got mag-fld squishing out all over the place... Not a pretty site! :-) - Thane eventually discovered that his system required a tuning of the coils resistance and inductance to optimize the acceleration effect. At first he was using rather modest coil windings, but he ended up using a very high resistance coil (lots of turns of very fine wire) to take advantage of the time lag. He also ended up with some very hefty 'U' shaped cores whose open end width was the same as the spacing of the PMs on the rotor in order to provide a magnetic circuit in which to route the BEMF energy. As for the time lag of magnetic materials (domains), I don't know if Thane ever went as far as to explain it from a physics point of view, but I'll take a stab -- Electrons (elec currents) are much lighter than nuclei (magnetic moments), so electric currents can respond much faster than magnetic domains. Thus, if one designs the PM/EM/COIL systems properly, they can take advantage of that time lag and put it to good use. -Mark No virus found in this outgoing message. Checked by AVG - www.avg.com Version: 9.0.722 / Virus Database: 270.14.123/2592 - Release Date: 12/28/09 23:47:00
