Superconducting DC Motors for Marine Propulsion

Electric propulsion power transmission is usually economical only for special vessels. However, superconducting motors with their greatly reduced weights and sizes indicate that this type of propulsion may become more widespread in the future.

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/mm2 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 rexamination.

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°K.

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.

The torque of any motor is the product of the armature current and magnetic flux. In a superconducting motor the use of a pair of slip rings for each armature conductor permits a high armature current together with a high flux output, thus giving high torque.

With a constant field, speed varies linearly with voltage and torque varies linearly with current; thus for a propeller drive obeying the cube law, voltage varies with speed, current varies with speed squared and power varies with speed cubed.

The superconducting marine propulsion motors have the advantages of no practical upper power limit; very large torques; and excellent speed control.

A possible disadvantage is the need to provide a helium refrigeration system, but the main consequence of this is an economic cut-off below a certain rating, probably about 60 kW per rev/min.

An important feature of the motor design is the ability to operate the two halves of the field system independently so that, in the event of one half being inoperative, the motor can work at half voltage and full current, giving half power.

Other benefits are an integrated power plant with the use of more efficient propellers, manoeuvrability and better employment of space due to installation flexibility.

An integrated power plant means a "Central power station" concept with the minimum of auxiliary prime movers, which can lead to a significant reduction in the total installed power, together with general use of a single fuel-grade. It is also clear that a "Total energy" concept should be adopted, with heat recovery where possible.

The most efficient propeller combination can be used due to the freedom of choice of speed and torque.

Examples of ship types particularly suitable for application of superconductive drive motors are icebreakers, large tugs and tankers.