Back in October 2002 European Design Engineer carried an article about superconducting motors, reporting on developments from both American Superconductor in the USA and Siemens Automation and Drives in Europe. At that point American Superconductor had built and tested a high-temperature superconductor (HTS) motor with a power of around 3700kW (5000HP), while Siemens had been running trials on what was claimed to be Europe's first HTS motor, rated at 380 kW, which is order of magnitude smaller.

More recently, American Superconductor has taken a major step forward, constructing a prototype 5MW (6667HP) high torque, AC synchronous HTS motor that is designed to suit the needs of ships - both for in-hull and podded propulsion systems. This motor has been demonstrated successfully at full load, under steady-state operational conditions, at the Center for Advanced Power Systems (CAPS) at Florida State University in Tallahassee. The motor was developed under contract with the USA Navy's Office of Naval Research (ONR) to prove the viability of HTS technology for both military and commercial marine propulsion. After the 5MW motor completes load and ship mission profile simulation tests at CAPS, it will undergo additional testing at the Naval Surface Warfare Center, Carderock Division in Philadelphia. The USA Navy will then define further land-based and at-sea testing.
"We continue to be pleased with these new test results on the 5MW superconductor motor," said Rear Admiral Jay Cohen, Chief of Naval Research. "The HTS ship propulsion motors we have been developing continue to perform above our expectations and are providing an important new option for future Navy propulsion systems."
One of the major advantages of HTS motors is that they are extremely compact, measuring as little as one-third the weight and half the size of copper-based motors of the same power and torque rating, which means Navy ships can carry more fuel and munitions and have more room for the crew's quarters and weapons systems; similarly, commercial ship owners and operators can carry more passengers and cargo. In addition, HTS motors operate with higher fuel efficiency and are likely to have lower maintenance costs than their conventional counterparts.
Once HTS motors reach production, it is expected that their cost will be similar to that of conventional motors of the same power and torque rating.
David Paratore, American Superconductor's president and chief operating officer, believes that momentum is building in the commercialisation process for HTS ship propulsion motors: "In addition to continuing the demonstration of all technical aspects of our new HTS motors, we are continuing to strengthen and develop business relationships with ship builders, ship propulsion integrators and ship owners and operators to accelerate the adoption of HTS ship propulsion motors for both commercial and military applications."
The load testing was used to demonstrate how the HTS motor performs under the stresses and operating conditions it will experience when powering a vessel at sea. This final development stage has provided engineers and ship propulsion integrators with vital information regarding design options and the operating characteristics of the new motor. An important aspect of the results obtained is the validation of American Superconductor's electromagnetic, mechanical and thermal analytical models for HTS ship propulsion motors - a vital step in the development cycle for advanced electrical machines.

Motor fundamentals

Significantly, the HTS motors being developed by American Superconductor involve no major changes in fundamental motor technology. The machines operate in the same manner as conventional motors, gaining their substantial advantages by replacing copper rotor coils with HTS rotor coils. The rotors of HTS motors run 'cold', so they avoid the thermal stresses experienced by conventional machines during normal operation. The inability to achieve proper thermal management has been a major impediment in developing power-dense, high-torque motors for naval and commercial marine applications. Stresses caused by heat in other advanced, high-power motors often necessitate costly motor repair and refurbishment.
American Superconductor's 5MW HTS propulsion motor rotates at 230rpm and generates 200000Nm of torque at full power. This power and speed rating are typical for copper-based electric propulsion motors currently used in ferries and small cargo ships around the world, and the company expects this class of superconductor motor to become a standard power rating for certain military ships.
Nevertheless, the 5MW HTS motor is a subscale version of the 36.5MW (49000HP), 120rpm, 2.9MNm HTS motor now being built by American Superconductor and Northrop Grumman under a US$70million three-year contract from ONR.
Scheduled for delivery in the spring of 2006, the 36.5MW motor is being specifically designed to provide propulsion power for the next generation of warships. A motor of this scale also has direct commercial application in large cruise ships and merchant vessels. As an example, two 44MW conventional motors are used to propel the Queen ElizabethII cruise ship. These motors each weigh over 400 t; in contrast, the 36.5MW HTS motors will weigh approximately 75t each. Newer vessels, such as the QE2's sister ship the Queen MaryII which has a total propulsion requirement of 84MW, are excellent candidates for HTS motors.
Meanwhile, Siemens has also been continuing its development of HTS equipment. In 2004, the Siemens Industrial Solutions and Services Group (I&S) Marine Solutions in Hamburg started the development of a small, lightweight, high-efficiency synchronous generator with HTS rotor windings, suitable for power generation onboard marine vessels.
The 4MVA generator will be developed in co-operation with the Automation and Drives (A&D) and Corporate Technology (CT) business units within Siemens.
Benefits anticipated for the new generator are savings in terms of mass, volume and losses. In addition, the generator will be quieter, run more smoothly, be capable of sustaining multiple overloads and be insensitive to load changes.
The new generator is due to be tested later in 2005 during a test program scheduled to run at the A&D system test facility normally used for electric drives in Nuremberg. After this, the generator will be used onboard ships and offshore oil and gas platforms.

Current limiters

Another marine application for high-temperature superconductors being developed by Siemens is current limiters to protect against short-circuits. Siemens I&S has developed a current limiter based on HTS technology to protect ship power supply systems. Short-circuit currents are automatically limited to a non-critical value as soon as the current rises, owing to the sudden increase in the inherent resistance of the superconductor. After a current-limitation event, the operability of the HTS current limiter is self-restored in a very short time. The service life of the electrical system is consequently prolonged and its reliability is enhanced. This innovation is consistent with the trend towards all-electric ships.
In the past, switchgear and power supply systems on board ships have been protected from short-circuits by installing fused outgoing branches. Although tripping one of these fuses allows a short-circuit to be contained, the fuse is sacrificial. The full availability of the switchgear is therefore not restored until a new fuse is installed. However, the Siemens HTS protection device is effectively self-resetting.
The potential applications for HTS current limiters include ships where standalone systems need to be redesigned or where the existing electrical supply system is already stretched to maximum capacity. The use of a current limiter facilitates circuit-breakers with a much lower breaking capacity.
Moreover, their service life is prolonged as a result of the lower thermal and mechanical loads on the downstream system components. Substantial cost savings can therefore be achieved. By installing a current limiter at a power system tie bus, operation of the non-faulty power subsystem can be maintained if a short-circuit occurs. The availability of the complete ship is consequently enhanced.