Perspectives of Superconductivity Implementation in Propulsion Systems

Introduction

Superconductivity is a phenomenon where a conductor, once cooled below a certain temperature, loses all electrical resistance and ejects any magnetic fields within itself. When this state of superconductivity is achieved, the conductor will be able to transfer electrical energy without a loss in power.

However, this occurs only at very low temperatures. Without some form of cryogenics, achieving a superconductive state is impossible because of resistive losses that arise in the current-carrying conductor. Superconducting cables are proven able to raise the energy efficiency and power density of motors and generators and in turn, reduce the overall volume of the machine. Due to the lucrative benefits that superconducting machines bring, many have invested effort in this field to better refine the design of these machines, in the hopes of making its implementation widespread.

The temperature required for superconductivity to occur, dubbed as critical temperature, is dependent on the material used. Upon initial discovery, only liquid helium was capable of cooling certain materials below their critical temperature. These materials, termed as low-temperature superconductors (LTS), become superconductive at temperatures of around 4 K.

Due to the high cost of obtaining liquid helium, this extremely low temperature made superconductivity a relatively inaccessible area to study, let alone implement for industrial purposes. Fortunately, new materials that are able to achieve superconductivity at a higher temperature were discovered, using only liquid nitrogen as the refrigerant. These are generally termed as high-temperature superconductors (HTS). Current developments in superconducting motors and generators use HTS materials because of the lower cost of cooling with liquid nitrogen.

For HTS machines, the stator windings are usually made of special yttrium bismuth-based copper oxides (YBCO) in place of copper coils, cooled to temperatures ranging from 30 K to 40 K. Passing a direct current through the windings creates a region of very strong magnetic flux. This allows HTS motors to generate large amounts of torque. For naval propulsion systems, the need for such high-torque output motors favors the use of superconductors over traditional copper windings.

Operating principles of AC motors and generators

A motor is a machine that converts electrical energy into mechanical energy. The electrical energy in the form of electrical current is passed through the armature of a motor while inside the magnetic field of a pair permanent magnets, subsequently inducing an electromotive force (emf) in the armature which creates a force that rotates the rotor. The rotation of the rotor is the mechanical energy produced.

In a three-phase induction motor, the process is different. Alternating current (AC) is passed through the coils øA, øB and øC in the stator. The current in øA, øB and øC will be 120° out of phase from each other. When these coils are carrying the separate AC currents, a rotating magnetic field is formed. The speed of rotation of this magnetic field is called the synchronous speed, which is directly proportional to the speed of rotation of the rotor. The rotating magnetic field then causes a changing magnetic flux linkage in the rotor, and from Faraday’s Law of Electromagnetic Induction, a force is exerted from on the rotor to oppose the changes. This causes the rotor to start spinning. As the stator coils are using alternating current, the resultant magnetic field continues rotating and keeps the rotor moving at close to the synchronous speed.

The rotor will never spin faster or at synchronous speed because if that happens, the armature of the rotor will be stationary relative to the rotating magnetic field. This means that there will be no change in flux linkage within the rotor and as such, no force is exerted on the rotor, causing it to slow down. Slowing down would cause the change in magnetic flux to increase, exerting a larger force on the armature which spins the rotor faster, repeating the process.

A generator uses similar principles to a motor but converts mechanical energy into electrical energy instead. For a three-phase synchronous generator, the rotor is the component that creates the initial magnetic field, rather than the stator in an induction motor.

A direct current (DC) is passed through the wire coiled to the rotor, ‘exciting’ the rotor and forming a magnetic field around the rotor with the stator coils A, B and C within the region of the magnetic field. This ‘excitation’ current may be from an external source or from a small DC generator, attached to the same drive shaft. The rotor is then made to spin, and the magnetic field cuts the wire coils A, B and C, causing the change in magnetic flux within the coils to be non-zero. As a result, by Faraday’s Law, an emf is induced in the coils and a current is generated.

When the magnetic field lines first ‘cut’ the wires of the stator coil, the first lines to cut are in the downward (relative to the page) direction and by Lenz’s Law of Electromagnetic Induction, the resulting induced current will be flowing out of the page. As the field lines continue to cut the wire, the lines going upward will cut the wire and induce a current that flows into the page.

Due to the reversing direction of field lines cutting the stator coil, the three-phase generator outputs alternating current of three phases at 120° out of phase from each other as shown in Fig. 4. Additionally, the frequency of the output voltage is directly proportional to the synchronous speed of the magnetic field, thus, varying the rotation speed of the rotor results in a variation in the frequency of the output voltage. The reverse is true for AC motors – varying the input voltage changes the speed of rotation of the rotor.

Benefits of superconducting motors and generators

The idea of using superconducting machines for industrial applications has been gaining support recently as more experiments are conducted to prove their superiority over conventional motors and generators. The benefits to using superconductors obviously stem from the zero-resistance characteristic of superconductors for direct currents and very low losses from hysteresis for alternating currents, which has many implications in the design of superconducting machines.

The proposed benefits of superconducting machines over their conventional counterparts are its ability to carry greater electrical currents, lighter weight and lower volume of the machine. HTS materials are able to carry much higher currents for the same cross-sectional area as compared to copper cables, thus achieving a much higher current density in the wire windings for a smaller size.

In superconducting synchronous generators where the rotor windings are made of HTS materials, having a greater current-carrying capability will allow a much stronger magnetic field to be formed, increasing the change in magnetic flux linkage when the rotor spins. Thus, the output current of the overall machine is raised. A higher output would translate to better efficiency over a conventional synchronous generator of the same size, in terms of the machine’s output to volume ratio. Similarly, a superconducting motor would be able to generate higher torque than a conventional motor, due to the stronger magnetic field formed by the HTS stator windings.

For applications of induction motors and synchronous generators in areas where more torque or voltage is not necessarily better, using HTS machines downscaled to the required specifications will use lesser material in the construction of the machine. This results in a smaller size and weight of the said machine, which may save costs in installation and maintenance. In ship propulsion and power generation, the space and weight savings brought about by superconducting machines would disproportionally outweigh the use of conventional motors and generators because on board a ship, space is limited and freeing up some would be considered a great luxury.

Ignoring factors related to cost, in the conceptualization and design of superconducting machines some drawbacks of their implementation can be observed. Firstly, the possible complications of the cooling system required to keep superconductors functional poses a significant problem. Secondly, difficulties in synthesizing the superconducting compound and fabrication of the conductor continue to plague the field.

As with any superconductor, the problem of cooling them to temperatures below their critical value is not solved by simply refrigerating in liquid nitrogen. Consideration must be spared for the construction of the machine, knowing which part to be cooled and which parts to be insulated from the cold. Also, preventing thermal leakage is a difficult and expensive task, which further complicates things when designing a machine using superconductors.

The synthesizing of HTS compounds requires very accurate methodologies and precise instruments as the properties of HTS compounds can change drastically due to small imperfections in their molecular structure, affecting whether the final product can become a superconductor or not. As such, only several companies are able to supply high-quality HTS cables.

Conclusion

Adopting the use of superconductors in naval propulsion is not widespread as of now, as the technology is still not mature and requires further testing. However, it is important to note that despite the current challenges faced by superconductivity, their implementation in propulsion systems would be worthwhile in the future once these challenges are overcome. This is because preliminary results from initial tests have been promising, and the benefits of such technology for naval applications appear to greatly outweigh the cost. Examples of these benefits would be the space and weight savings on board a ship. In the case of warships, the extra space could be used for additional warfighting capabilities, and the lower weight usage would translate to lower fuel usage.