"Calling Nikola Tesla! Calling Nikola Tesla! Come back! All is forgiven!"
I bet *he'd* have something to say on this subject! MW On 21 Jul 2014, at 20:24, Lee Hart via EV wrote: > 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)
