The present invention relates generally to applying a super-conducting coil to a high power density synchronous rotating machine. More particularly, the present invention relates to a synchronous machine with a conventional stator, and a magnetically saturated solid iron core rotor having a super-conducting coil.
Synchronous electrical machines having field coil windings include, but are not limited to, rotary generators, rotary motors, and linear motors. These machines generally comprise a stator and rotor that are electromagnetically coupled. The rotor may include a multi-pole rotor core and one or more coil windings mounted on the rotor core. The rotor cores may include a magnetically-permeable solid material, such as an iron-core rotor.
Conventional copper windings are commonly used in the rotors of synchronous electrical machines. However, the electrical resistance of copper windings (although low by conventional measures) is sufficient to contribute to substantial heating of the rotor and to diminish the power efficiency of the machine. Recently, super-conducting (SC) coil windings have been developed for rotors. SC windings have effectively no resistance and are highly advantageous rotor coil windings.
Iron-core rotors saturate at an air-gap magnetic field strength of about 2 Tesla. Known super-conducting rotors employ air-core designs, with no iron in the rotor, to achieve air-gap magnetic fields of 3 Tesla or higher. These high air-gap magnetic fields yield increased power densities of the electrical machine, and result in significant reduction in weight and size of the machine. Air-core super-conducting rotors require large amounts of super-conducting wire. The large amounts of SC wire add to the number of coils required, the complexity of the coil supports, and the cost of the SC coil windings and rotor.
While iron core super-conducting rotors have been largely ignored by industry, iron core rotors offer certain advantages over air-core rotors, when operated at magnetic field saturation to increase the air-gap magnetic field and power density of the machine. The advantage is that it takes considerably less super-conductor material in a magnetically saturated iron-core rotor to attain the same benefits of high machine power density as compared to an air-core rotor.
High temperature SC coil field windings are formed of super-conducting materials that are brittle, and must be cooled to a temperature at or below a critical temperature, e.g., 27xc2x0 K, to achieve and maintain super-conductivity. The SC windings may be formed of a high temperature super-conducting material, such as a BSCCO (BixSrxCaxCuxOx) based conductor.
Super-conducting coils have not been adapted for commercial use in the rotors of synchronous machines. Attempts have been made to incorporate SC coils into high power density generators and other such synchronous machines. The potential benefits of adding SC coils to high power density machines include light weight and compact machines. These high power density machines typically include an air-core rotor and an air-gap stator with no stator iron teeth. However, high power density machines tend to be expensive and have been commercially impractical.
SC coils, their coil supports and the associated refrigeration systems have been expensive and complex. SC coils are expensive materials, such as BSCCO. These materials are also brittle. The coil support systems needed for SC coils must withstand the tremendous forces encountered in the rotor of a large synchronous machine and protect the brittle coils. Moreover, these support systems must not transfer substantial heat into the cryogenically cooled coils.
Further the refrigeration systems that provide cryogenic cooling fluids, such as helium, are complex and expensive. Accordingly, the cost and complexity of incorporating SC coils into a synchronous machine have been high. For SC coils to become commercially viable, their associated costs should be reduced to well below the advantages gained by substituting SC coils for conventional copper coils in the rotor.
The cost of using SC coils has become more affordable with the development of high temperature super-conducting (HTS) materials. Because they maintain super-conducting conditions (including no resistance) at relative high temperatures, e.g. 27xc2x0 K, the cost to cool a HTS coil is substantially reduced as compared to cooling costs for prior SC that had to be cooled to lower temperatures. There is still a need for lower cost SC coils and coil support systems.
Super-conducting coils have been cooled by liquid helium. After passing through the windings of the rotor, the hot, used helium is returned as room-temperature gaseous helium. Using liquid helium for cryogenic cooling requires continuous reliquefaction of the returned, room-temperature gaseous helium, and such reliquefaction poses significant reliability problems and requires significant auxiliary power.
Prior SC coil cooling techniques include cooling an epoxy-impregnated SC coil through a solid conduction path from a cryocooler. Alternatively, cooling tubes in the rotor may convey a liquid and/or gaseous cryogen to a porous SC coil winding that is immersed in the flow of the liquid and/or gaseous cryogen. However, immersion cooling requires the entire field winding and rotor structure to be at cryogenic temperature, as a result no iron can be used in the rotor magnetic circuit because of the brittle nature of iron at cryogenic temperatures.
What is needed is a HTS electrical machine that is substantially less expensive that prior HTS machines, and is competitively priced with existing conventional copper coil machines. To become commercially successful, HTS machines need to become cost competitive with conventional copper machines. Potential technical areas for reducing costs further include the coil support system, the rotor design and retrofitting existing machines with HTS rotors. Further, there is a need for an improved rotor field winding assemblage for an electrical machine that does not have the disadvantages of the air-core and liquid-cooled super-conducting field winding assemblages of, for example, known super-conducting rotors.
Developing support systems for HTS coil has been a difficult challenge in adapting SC coils to HTS rotors. Examples of coil support systems for HTS rotors that have previously been proposed are disclosed in U.S. Pat. Nos. 5,548,168; 5,532,663; 5,672,921; 5,777,420; 6,169,353, and 6,066,906. However, these coil support systems suffer various problems, such as being expensive, complex and requiring an excessive number of components. There is a long-felt need for a HTS rotor having a coil support system for a SC coil. The need also exists for a coil support system made with low cost and easy to fabricate components.
A high power density super-conducting machine with a rotor having a SC coil field winding has been developed that appears to be cost competitive with existing copper coil, low power density machines. Costs may be reduced by employing a magnetically saturated solid core rotor, a conventional stator and a minimal coil support structure. Using these technologies, an efficient HTS machine having the advantages of SC coil has been developed. Moreover, the cost to build such a HTS machine can be sufficiently reduced so that the machine is economical.
The HTS machine includes a conventional stator and a HTS rotor. The conventional stator is designed for high air-gap magnetic fields that are provided by the HTS rotor. The rotor includes a two-pole core body formed of a solid magnetic material, such as iron. The rotor core body is generally cylindrical and has flat surfaces machined longitudinally along its length. The HTS coil is assembled around these flat surfaces and the coil has a race-track shape that extends around the core. The rotor coil ampere-turns are sufficiently high to magnetically saturate the rotor core and operate the machine at high air-gap magnetic fields.
The race-track coil is supported by tension coil support members that extend through the iron core rotor body. Drive and collector shafts are mechanically fastened to the rotor core. A cylindrical shell electromagnetic shield surrounds the HTS coil and iron core rotor body.
The iron core rotor significantly reduces the field winding ampere-turns, super-conductor utilization and cost with respect to air-core rotors. The single race-track shaped HTS coil replaces typical complex saddle-shaped coil windings. The tension coil support provides direct support to the HTS coil so as to reduce the strains on the coil during cool-down and centrifugal loading. Moreover, the coil support system is at cryogenic temperatures with the coil.
The HTS rotor may be implemented in a machine originally designed to include a SC coil(s). The rotor and its SC coil are described in the context of a generator, but the HTS coil rotor and coil support disclosed here are also suitable for use in other synchronous machines.
In a first embodiment the invention is a high power density synchronous machine comprising: a stator having conventional stator coils arranged in an annulus around a vacuum cylindrical cavity; a cylindrical magnetically saturated solid rotor core; a race-track super-conducting coil winding extending around the rotor core, and a coil support extending through the core and attaching to opposite long sides of the coil winding.
In a second embodiment of the invention is a high power density synchronous machine having a rotate capacity of at least 100 MVA comprising: a conventional stator having stator coils arranged in an annulus forming a vacuum rotor cavity; a cylindrical magnetically saturated rotor core having a pair of planer sections on opposite sides of the core and extending longitudinally along the core, and a super-conducting coil winding extending around at least a portion of the rotor core, the coil winding having a pair of side sections adjacent the planer sections of the core.