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.
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.
Superconducting DC motors for marine propulsion represent a significant advancement in the field of maritime engineering. These motors offer numerous benefits, including higher efficiency, reduced size and weight, and lower operational costs compared to traditional marine propulsion systems. Here’s a detailed overview of their principles, advantages, and challenges:
Principles of Superconducting DC Motors
Superconductivity:
- Definition: Superconductivity is a phenomenon where a material exhibits zero electrical resistance below a certain critical temperature. Superconductors also expel magnetic fields (Meissner effect), which allows for efficient current flow and strong magnetic fields.
- Types: Superconductors can be either Type I (low-temperature superconductors, typically requiring cooling with liquid helium) or Type II (high-temperature superconductors, cooled with liquid nitrogen).
DC Motors:
- Operation: DC motors convert electrical energy into mechanical energy through the interaction of magnetic fields. In superconducting DC motors, superconducting coils create strong magnetic fields with minimal energy loss.
Advantages of Superconducting DC Motors for Marine Propulsion
Increased Efficiency:
- Superconductors offer zero electrical resistance, which significantly reduces energy losses due to heat and other inefficiencies in conventional motors.
Compact Size and Reduced Weight:
- Due to the high current densities possible in superconductors, these motors can be designed to be smaller and lighter, providing more space for cargo or passengers and reducing the overall weight of the vessel.
Higher Power Density:
- Superconducting motors can generate stronger magnetic fields, allowing for greater torque and power output from a smaller unit, which is particularly advantageous for large marine vessels.
Reduced Maintenance:
- Fewer moving parts and lower thermal stresses lead to reduced wear and tear, resulting in lower maintenance requirements and longer operational life.
Environmental Benefits:
- Higher efficiency translates to lower fuel consumption and reduced greenhouse gas emissions, contributing to more environmentally friendly marine operations.
Challenges and Considerations
Cryogenic Cooling:
- Superconducting motors require cooling to maintain the superconducting state. This involves complex cryogenic systems, which add to the initial cost and complexity of the system.
Material Costs:
- High-temperature superconductors (HTS) are still relatively expensive compared to conventional materials, although prices are expected to decrease with advancements in material science and production techniques.
Operational Challenges:
- Ensuring reliable operation of the cryogenic cooling systems in a marine environment, where conditions can be harsh and maintenance access is limited, poses significant engineering challenges.
Infrastructure and Integration:
- Existing marine vessels and infrastructure are designed for traditional propulsion systems. Adapting or retrofitting ships to accommodate superconducting motors requires significant investment and redesign.
Current Research and Development
Research in superconducting DC motors for marine propulsion is focused on:
Improving Superconductor Materials:
- Developing more cost-effective and higher performance superconducting materials to reduce overall system costs and improve efficiency.
Enhancing Cryogenic Technology:
- Innovating more reliable and efficient cryogenic cooling systems that are suitable for the maritime environment.
System Integration:
- Designing superconducting motors that can be seamlessly integrated into existing marine propulsion systems, minimizing the need for extensive retrofits.
Field Testing:
- Conducting real-world testing and pilot projects to validate the performance, reliability, and economic viability of superconducting propulsion systems in marine applications.
Superconducting DC motors hold great promise for the future of marine propulsion, offering a range of benefits from increased efficiency and reduced environmental impact to smaller and lighter motor designs. While there are challenges to be addressed, ongoing research and technological advancements are paving the way for their eventual widespread adoption in the maritime industry.
