Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Path: utzoo!mnetor!uunet!husc6!rutgers!princeton!phoenix!neanders From: neanders@phoenix.Princeton.EDU (Nels Anderson) Newsgroups: comp.society.futures,rec.railroad Subject: Re: Warm superconductors (RAILWAYS and TRANSIT) Message-ID: <1062@phoenix.Princeton.EDU> Date: Fri, 6-Nov-87 01:21:44 EST Article-I.D.: phoenix.1062 Posted: Fri Nov 6 01:21:44 1987 Date-Received: Sun, 8-Nov-87 11:23:38 EST References: <2824@batcomputer.tn.cornell.edu> Reply-To: neanders@phoenix.UUCP (Nels Anderson) Organization: Princeton Univ. Dept. of Astrophysical Sciences Lines: 83 Keywords: superconductors trains Xref: mnetor comp.society.futures:173 rec.railroad:869 In connection with high-temperature superconductors, garry@oak.cadif.cornell.edu writes in article <2824@batcomputer.tn.cornell.edu>: >The most intriguing idea I've heard so far is to apply them to cheap magnetic >levitation for trains. > >Does anyone have technical knowledge? Well, I have a little. Just a bit of the physics. I don't know anything about the engineering problems. According to an article in the IEEE Spectrum about three years ago, there are two types of magnetic levitation: electromagnetic and electrodynamic. In the electromagnetic system both the train and the track contain electromagnets. I believe the train's magnets are actually below the track, in a monorail-type arrangement. The magnets are oriented so as to attract each other, with the result that the train is pulled up. A control system is required to maintain the proper separation between the two sets of magnets, reducing the field if they come to close and increasing it if they get too far apart. Superconducting electromagnets could be used to eliminate resistive losses. Electrodynamic suspension is somewhat more elegant. It is based on the principle that a changing magnetic field will tend to induce a current in a conductor (copper, aluminum or a superconductor, for example). The induced current will flow in such a way as to create a magnetic field counter to the original field. If, for example, the changing magnetic field is caused by a magnet moving over the surface of the conductor, the induced currents in the conductor will repel the magnet in the vertical direction while attracting it horizontally. So the magnet is levitated and at the same time held near the center of the conductor. The catch here is that in normal conductors the induced currents die out rapidly. Constant motion is required to maintain the induced magnetic field. In a superconductor, however, the currents continue to flow indefinitely. Thus, if you send a magnet skimming over a block of aluminum at sufficiently high speed, the magnet will be levitated. If it slows down, however, the levitation will disappear. If you do the same thing with a superconductor, you find that the levitation persists even when the magnet isn't moving. A New York Times article on superconductors a few months ago showed a magnet suspended over a superconductor (or it may have been a superconductor suspended over a magnet -- the principal is the same). Without superconducting technology, an electrodynamically suspended train would contain an ordinary electromagnet and might ride over an aluminum track. (The track just has to be a good conductor and must not be ferromagnetic. It would start out on wheels. After reaching sufficient speed it would lift off the track. The New York Times article mentioned the possibility of using superconducting technology for the electromagnet. If superconductors were really cheap, the track could be superconducting as well. Then the train would float as soon as its magnets were turned on -- there would be no need to get up to speed. Superconducting track would also save energy, since the induced currents in nonsuperconducting track will tend to dissipate. The energy they dissipate has to come from somewhere, and I believe it shows up as a retarding force on the train. This would be quite substantial; the induced currents holding up a fast-moving train would be comparable to currents in the train's own magnets. Magnetic trains have been demonstrated by the British, Germans and, of course, the Japanese. As I recall from the IEEE article, the British system was electromagnetic. I don't know about the other two, but I believe that either or both of the German and Japanese experiments rely on conventional cold (liquid helium) superconductors. Nothing has been said yet about propulsion, as opposed to levitation. I presume that it is done by magnetic repulsion or attraction between electromagnets in the track and the train. I could go on at greater length about these things, but I suspect that everybody is bored already. Unfortunately I know only about the physical principals involved and nothing about the practical engineering problems. (Besides, my thesis advisor would dispute my claim to understanding ANY physical principals.) neanders@phoenix.princeton.edu neanders@phoenix.UUCP 6070106@pucc.bitnet