From: Peri Hartman > Is it possible to use multiple coils to focus the "beam"?
Magnetic fields are devilishly difficult to direct or focus. With electricity, we have great conductors (copper, silver, etc.) and great insulators (air, plastics, etc.) There are *many* orders of magnitude difference in their conductivity, so we can tightly control where the current flows. With magnetics, we have no good conductors, and no good insulators. It's as if our best electrical conductor was carbon (which we make resistors out of), and our best insulator was water (which conducts pretty well, especially if dirty). Imagine trying to make a circuit work where the conductors are all carbon, and it's submerged in water, which partially shorts everything to everything else! (Superconductors can give us good magnetic insulators; but they don't work except at cryogenic temperatures). > I don't know wave theory but I believe directional radio transmitters work > by having two or more antennas. Can something similar be done with inductive > coils? Yes; sort of. Every electric field inevitably has a magnetic field, and vice versa. That's why we call it "electromagnetics". However, for these fields to act like waves, which we can focus and direct like light, the frequencies need to be very high. The elements of a directional antenna need to have dimensions on the order of 1/4 wavelength or more. Wavelength (in meters) = 300 / Frequency (in MHz). At 100 MHz (the frequency of FM radio and the old VHF television), the wavelength is about 3 meters -- so a 1/4 wave antenna is about 0.75 meters or 30" long. It's not too hard to make antennas with multiple elements in parallel to focus and direct these frequencies (like the traditional TV antennas that look like giant metal combs). At 1 MHz (the AM radio broadcast band) the wavelength is about 300 meters; thus the tremendously high towers needed to effectively transmit it (the whole tower is the antenna). It's hopeless to make receiving antennas this big. We have to use far smaller antennas, that are far less efficient and require substantial amplification to work. The inductive chargers mentioned here are using 85 KHz. The wavelength is on the order of 3500 meters! It's impossible to direct such frequencies with the techniques used for radio antennas. Vicor makes switchmode converters that operate just over 1 MHz; about the highest practical frequency for state of the art switchmode converters. They had to go to heroic lengths to get their transformers to operate with reasonable efficiency (90%). Such frequencies are not yet practical for high power converters. Lower frequency transformers are more efficient. Conventional 60 Hz transformers can be over 99% efficient, if you use enough copper and iron. But to do so, they require *very* tight coupling between the primary and secondary -- minimal gap between them. This is the opposite of the requirement to have some separation between primary and secondary as imposed by the wireless charging proponents. I think the only way to make a practical "wireless" charger will be to use more or less ordinary frequencies, and mechanically position the primary and secondary coils as close as possible. This means either moving the car's secondary coil or the charging station's primary coil so they touch. -- Excellence does not require perfection. -- Henry James -- Lee A. Hart http://www.sunrise-ev.com/controllers.htm now includes the GE EV-1 _______________________________________________ UNSUBSCRIBE: http://www.evdl.org/help/index.html#usub http://lists.evdl.org/listinfo.cgi/ev-evdl.org For EV drag racing discussion, please use NEDRA (http://groups.yahoo.com/group/NEDRA)
