The energy crisis of the early 1970s provided impetus for more sufficient energy conservation and for exploiting hitherto untapped resources.
The superconducting dc motor adds a potent new factor to this situation. It offers the best of these options in that it is lighter than the equivalent conventional ac motor and is a dc motor which may be operated from an ac generator via a simple diode-rectifier-convertor. It offers all the advantages of a dc propulsion system, together with high efficiency, and performance characteristics particularly suited to high efficiency propellers; with no practical power limit.
Superconductors are materials which, when cooled to below a critical temperature, exhibit zero electrical resistance and thus are able to sustain very high current densities. To qualify as an engineering material for electric machines, a superconductor must also tolerate a high magnetic field.
The superconductor is niobuim-titanium alloy which, when cooled to approximately 4.5°K, can carry a few thousand A/mm>2 in a field of approximately 6 tesla with no resistive losses. It is thus possible to construct a lossless winding with several million ampere-turns producing correspondingly high flux output.
In practice this means a very much smaller and lighter motor than conventional types of the same output so that the economics of electrical propulsion merit re-examination.
Two magnetically opposed superconducting field windings are in closely fitting liquid-helium vessels, formed by partitions in a single structure designed to contain the electro-magnetic separating forces between the coils. This assembly is suspended in a vacuum vessel on the support cylinder.
The purpose of this system of vessels is to minimize the heat-flow from ambient to the liquid-helium vessels and coils at 4.5°I\.
The complete assembly is located within the rotor on a fixed support that passes through the bore of the large bearing at the non-drive end and is connected to a steady bearing on a stub extension of the output shaft.
The armature rotor group assembly comprises a drum carrying the slip rings, and rotor conductors interconnecting pairs of slip rings; a non-drive-end section; a drive-end section; and the output shaft.
The armature stator group consists of brushgear and stator conductors, mounted on a structure able to withstand the motor torque reaction on the stator conductors. The stator also includes the bearing and the base frame. For a propulsion application the drive-end bearing could include the main thrust block.