Reliance Electric and EPRI started a joint program in 1987 to evaluate the use of HTS materials in electric motors. This initial program, which included American Superconductor Corporation (ASC) as the HTS conductor supplier and coil manufacturer, eventually was transitioned in 1994 into a DOE SPI program. The SPI program also included Centerior Energy, representing the end user, and Sandia National Laboratory, which assisted Reliance on the cryogenic cooling system analysis. The focus for the SPI program was demonstration of a "model" air core ac synchronous motor using HTS windings that would be scaleable to larger ratings suitable for commercialization.
In February 1996, Reliance Electric successfully tested a four-pole, 1,800 rpm synchronous motor using HTS windings operating at 27 K, at a continuous output of 200 hp (see Fig. 1.2, p. 5). The HTS coils, manufactured by ASC using Bi-2223 tape, achieved currents of 100 A at 27 K, which is 25% over the initial goal of 80 A. The current density at 100 A corresponds to 7,500 A/cm2 in the SC wire. The peak field achieved by the four coils during the testing was nearly 1 T. Rotating tests at different speeds (600 rpm and 1,800 rpm) did not show any change in performance for the coil current, indicating good mechanical capability and acceptable cryogenic cooling for the HTS coils (Schiferl, Zhang, Shoykhet et al. 1996; Schiferl, Zhang, Driscoll et al. 1996).
Fig. 2.3 shows a cross-section of the rotor, illustrating the placement of the HTS coils and cryogenic cooling scheme. This rotor design, which is similar to designs used for SC ac generators, has a number of key features (Schiferl, Zhang, Shoykhet et al. 1996; Schiferl, Zhang, Driscoll et al. 1996):
Fig. 2.3.: Cross-section of Reliance Motor showing HTS coils and cryogenic system.
In August 1996, the Reliance-led program was extended by DOE into a Phase II SPI effort to develop a precommercial prototype of an HTS motor at a 5,000 hp rating. This motor, combined with an adjustable speed drive (ASD), will offer marked efficiency improvement and more operational flexibility than conventional induction motors; specifically, the motor speed will be able to be properly matched to changing load requirements. Superior performance and energy efficiency compared to conventional induction motors should make the HTS motor extremely attractive to customers and highly competitive in the marketplace for large motors above 1,000 hp. The 5,000 hp motor with ASD should show an efficiency exceeding 98%, which is ~2% higher than that of a conventional motor (Schiferl, Zhang, Shoykhet et al. 1996; Schiferl, Zhang, Driscoll et al. 1996). The impact of refrigerator power on the efficiency for a motor operating at 77 K should result in a ~0.1% decrease, giving an efficiency improvement of 1.9%. Reducing the operating temperature from 77 K to 30 K would decrease efficiency by an additional ~0.2%, yielding a net efficiency improvement of 1.7%. This estimate is based on an assumption of ~100 W heat removal at 30 K and a refrigerator coefficient of performance of 0.10, which would require approximately 10 kW of refrigerator power. In contrast to ac generators at ratings of 200-400 MW, which are much larger than the 5,000 hp motor, which corresponds to 3.73 MW, the refrigerator power has a much more severe impact on motor efficiency, giving an incentive to operate at higher temperatures near 77 K. If operation at 30 K is necessary due to HTS characteristics, the net efficiency improvement of ~1.7 % is still likely to be attractive for the 5,000 hp application.
Another DOE SPI program on SC ac synchronous generators was carried out by a General Electric Company (GE) team made up of engineers from its research laboratory and its Power Generation Engineering and Power Systems Engineering departments. The GE program, initiated in 1994, was directed at the conceptual design and assessment of a 100 MVA generator and the development of an HTS racetrack coil suitable for use in a full-scale generator. A more complete description of the GE program is presented in an article in Advances in Cryogenic Engineering (Lay, Herd, and King 1996), which discusses the program objectives; the early coil development; related HTS conductor activities on Bi-2223 tape; and the alternate HTS conductor, Tl-1223 tape, which offers improved temperature and field capability over Bi-2223. The complete details on the development, fabrication, and final testing of the racetrack coil is reported in two companion papers presented at the August 1996 Applied Superconductivity Conference (Herd et al. 1996; Salasso et al. 1996). The Bi-2223 tape for this racetrack was supplied by Intermagnetics General Corporation (IGC), which was the HTS wire and tape manufacturer and key partner on the GE/SPI program. The Argonne National Laboratory also supported IGC on the Bi-2223 development. Additional national laboratory support to GE and IGC was provided by Oak Ridge National Laboratory and Los Alamos National Laboratory.
The racetrack coil, shown in Fig. 2.4, is cooled by a heat exchanger wound external to the surface of the epoxy-impregnated coil. Helium gas, from a closed-cycle helium refrigerator, is circulated through the heat exchanger to maintain steady state temperature control at test temperatures of 20 K and 25 K. The racetrack coil achieved 34 A at 25 K, corresponding to ~40,000 ampere-turns, which is sufficient for consideration in a generator application (Herd et al. 1996). Additional tests on the ac loss and transient capability were also extremely positive. GE researchers concluded, "The capability of 20 K (and higher temperature) HTS coils to deal with current overdrive transients of many times Ic is remarkable. Much higher transient heating can be tolerated than for the previous generation of LTS generator designs" (Salasso et al. 1996). This conclusion supports the earlier comments in the section Advantages of HTS Coils (p. 12) on the use and limitations of liquid-helium-cooled LTS coils for generator applications.
Fig. 2.4. General Electric prototype Bi-2223 racetrack coil for generator application.