The Nobel Prize-winning discovery in 1986 of high-temperature superconductivity promised smaller, more efficient motors, generators, power cables, current limiters, and transformers. Since then, research on high-temperature superconductors (HTS) has focused on:
• Exploring the range of possible superconducting materials.
• Overcoming the challenges of turning a brittle ceramic material into long lengths of flexible wire with high filament current density, Jc — an essential step toward practical applications.
Most experts in the field now feel that commercial products will consist of present materials rather than new ones. No one foresees revolutionary breakthroughs for significantly higher flux pinning or a room-temperature superconductor. Thus, heeding to the needs of the marketplace, researchers are focusing attention on:
• Reducing wire cost.
• Creating sheath material that has adjustable resistivity.
• Developing wire with high tensile strength.
• Lowering ac losses.
• Determining the practical engineering, Je, and winding current densities, Jw.
Today, researchers can show their application prototypes as well as the improvements they have made in HTS wire, coil performance, and manufacturability. Two recent demonstrations, for example, displayed an application prototype 5-hp synchronous motor using superconductive coils, and a one-meter HTS wire functioning as a conductor for an underground transmission cable that carried over 4,200 A.
The U.S. Department of Energy (DOE) recently released funding for three additional development projects under the Superconductivity Partnership Initiative — a 100-hp HTS motor, design of a generator, and a current limiter.
HTS motor development
In 1987 the Electric Power Research Institute (EPRI) contracted with Reliance Electric Co. to investigate the use of HTS materials for large electric motors. They chose a synchronous motor with an HTS field winding in the rotor and traditional copper armature winding in the stator as the best topology for commercial application. The HTS-field winding in the rotor creates a higher magnetic field in the air gap than iron-core copper windings.
This design can reduce:
• Motor size.
• Motor losses.
• The amount of material needed in the armature.
• Iron losses.
Presently, researchers feel that HTS motors will be viable only for those applications above 1,000 hp because of the cost of the cooling system. However, in this size range, commercial HTS motors should have 30 to 50% fewer energy losses than conventional high-efficiency induction motors.
Working up to the 1,000-hp prototype, researchers demonstrated a series of dc motors with stationary HTS-field windings cooled in liquid nitrogen during 1990 to 1993. The motors showed progressively higher power outputs. Most importantly, the HTS coils did not show any degradation after dozens of thermal cycles.
In 1993, Reliance and EPRI demonstrated the first HTS synchronous motor. This ac motor had a stationary HTS-field winding cooled in a pool of liquid nitrogen. It generated 2 hp. The companies demonstrated a second 5-hp synchronous motor — the first one with rotating HTS coils — later that year, Figure 1. It generated 5 hp. The coils, Figure 2, resemble a race track configuration.
Each of the four coils is 14-in. long and uses approximately 330 meters of BSCCO wire per coil. Like the 2-hp motor, the synchronous motor has an iron core. However, it has a conventional rotating field winding and a stationary armature winding.
The next step in the HTS motor development program is to design and build a 100-hp HTS synchronous motor prototype. Development should finish in December 1995. For this 100-hp prototype, researchers plan to follow a design similar to the conceptual design of future commercial HTS motors.
The coils will be air-core design and cooled with cold helium gas to operate between 20 to 40 K. The DOE Superconductivity Partnership Initiative is funding this project in a cost sharing program.
HTS motor design and economics
Future HTS-based motors will be large (more than 1,000 hp) ac synchronous motors with an HTS-field winding. These coils will generate a magnetic field of 3 to 5 Tesla, (a Tesla is 1 weber/m2). The motor, Figure 3, will be air core, since iron saturates at less than 2 Tesla. At these stronger fields, the HTS coils will have to operate at 20 to 40 K. The reason for this colder temperature, relative to liquid nitrogen at 77 K, is that the maximum current density of the HTS material degrades at the higher temperatures, Figure 4. A mechanical refrigerator, often called a cryo-cooler, will provide the needed cold environment.
Superconducting motors have the potential to significantly reduce losses, Figure 5, thereby delivering high energy savings. The competing technology, iron-core induction motors, generally have losses of 231 kW for a 10,000-hp motor. Similar HTS motors have power losses of 118 kW. Reducing total motor losses from 231 kW to 118 kW, represents a 1-year energy saving of nearly 1 million kWh or $70,000 at 7¢/kWh based on a continuous- duty application. A present value for this energy saving over the lifetime of the motor is more than $500,000. The initial sale price of the HTS motor is approximately $400,000.
For motors over 2,000 hp in near continuous- duty applications, pay-back of the first cost premium over induction motors may be less than 2 years. Furthermore, this size HTS motor will be small when compared to an induction motor.
While progress in the development of HTS technology and motors has been steady, there are challenges to overcome including:
• Improving wire and coil performance.
• Reducing overall cost.
• Developing a less expensive cooling system.
• Proving system reliability.
However, none of these challenges requires a major breakthrough in technology. When will you see commercial HTS motors? Past progress and current findings indicate it will take an additional few years of development with a few more years of field testing before achieving customer acceptance.
Ned Schiff is manager of marketing and sales for American Superconductor Corp., in Westborough, Mass. Rich Schiferl is manager of HTS motors at Reliance Electric Co. Chad Josi, formerly manager of magnet systems at American Superconductor Corp., contributed to this article.