Motor/ generator action: Super-strong magnetic fields from air-core solenoids

..The magnetic field developed by the iron-core windings, or electromagnets, of rotating electrical machines is limited to a strength of about 15,000 gauss (lines per square centimeter). Above this value, the iron goes into saturation, and greater field strengths are dearly paid for in terms of current and temperature rise. Although much has been accomplished in materials technology, a ferromagnetic material that has significantly higher saturation levels and is also economical and workable for manufacturing purposes has not yet appeared on the horizon. Because higher magnetic-field strengths lead directly to more powerful motors and to generators with greater output capabilities, any technique for attaining superior magnetic-field strength is of prime interest to the electrical-machinery industry.

After many years of quiet resignation to the saturation behavior of ferromagnetic materials in general, and iron in particular, a new and sophisticated method of generating tremendously powerful magnetic fields has come into being. This new technique involves the use of the interesting, and somewhat mysterious, phenomenon of superconductivity. In principle, the ohmic resistance of many metals vanishes at extremely low temperatures. That being the case, solenoids can be wound with little regard for cross-sectional area of the current-carrying conductor. Because current flow encounters negligible resistance at these extremely low temperatures, gigantic currents can be carried by strips or films of appropriate material. The most significant aspect of the new art is that iron or other ferromagnetic substances are not used—the super-magnetic fields are produced in air. And the air neither saturates nor becomes involved in any breakdown mechanism.

The brittle intermetallic niobium-tin compounds have been used a lot in super conducting magnets. By means of vapor deposition or by a diffusion process, the compound is formed on a supporting strip of a more ductile material. By similar techniques, alumina or an organic coating is applied as an insulating film. A solenoid wound with this composite material is then immersed in a liquid-helium bath. Superconductivity commences quite abruptly in the vicinity of —260°C.

Successful motors and generators have been constructed using superconducting fields in the neighborhood of 60,000 gauss. Such machines have included 150-kW generators and motors with output capacities exceeding 3000 horsepower. Moreover, it appears practical to make superconducting electromagnets with field strengths of over 140,000 gauss. Such magnets need be only about 15 cm in diameter and operate continuously from a dc source of a few hundred watts. Cryogenic techniques, material technology, and experimental discoveries are advancing at a hectic pace; it would be difficult to impose either upper or lower limits on the parameters associated with superconductivity. A question not yet answered is: Can a material be produced that will superconduct at “ordinary” temperatures?