Superconductor technology: apparatus – material – process – High temperature devices – systems – apparatus – com- ponents,... – Dynamoelectric machine – or components thereof
Reexamination Certificate
2001-09-17
2004-03-23
Tamai, Karl (Department: 2834)
Superconductor technology: apparatus, material, process
High temperature devices, systems, apparatus, com- ponents,...
Dynamoelectric machine , or components thereof
C505S120000, C310S052000, C310S054000, C310S261100
Reexamination Certificate
active
06711422
ABSTRACT:
BACKGROUND OF THE INVENTION
The field of the invention is superconducting motors and specifically squirrel cage motors that use superconductors. Superconductors are used for the purpose of increasing the power rating per unit weight of the motor and of reducing the electric Ohmic losses in the motor windings. Both of the latter improvements contribute to increasing the commercial value of the motor.
The early generation of superconducting rotating machines was restricted mostly to synchronous motors and alternators. Both of these machines require for their operation a fixed magnetic field generated by circulating a direct current in coil windings in the rotor which is connected to an external current source. The advantage of using superconductors in the rotor coil is to produce very high magnetic fields, considerably higher than can be generated by coil windings set in magnetic laminations. As a result, the synchronous motors and alternators are more compact and extremely efficient.
Synchronous electric motors employ a fixed magnetic field generated by the rotor acted upon by a rotating magnetic field created by the stator coils. The stationary magnetic field on the rotor constantly tries to align itself with the rotating magnetic field produced by the stator, causing the rotor to rotate and produce mechanical torque. Unfortunately, synchronous motors cannot operate at variable speeds and they are not self-starting. To obviate the two aforementioned drawbacks of the synchronous motor, direct current motors with superconducting field windings in the stator and conventional rotor coils with commutators were constructed and operated successfully.
The reason superconducting synchronous and direct current motors were the only superconducting electric motors to have been developed, so far, is due mainly to the limitations imposed by the properties of the commercially produced superconducting wires. There are two types of superconducting wires: the low temperature superconducting wires (LTS) and the high temperature superconducting wires (HTS). The low temperature hard superconductors, made of alloys of metals such as Nb and Ti, must be cooled down to temperatures below 10 degrees Kelvin for the material to be in the superconducting state. To ensure the stable operation of the LTS superconductor, the wire is made of many fine Nb-Ti strands, six microns in diameter, and embedded in a tube of copper. The other class of superconductors, made of alloys of earth metals (such as the so-called YBCO compounds) exhibit superconductivity at temperatures below 90 degrees Kelvin. The material, however, is very brittle. In order to shape it in the form of a long wire, it is encased in a silver tube and the whole matrix is extruded.
When an alternating current is caused to circulate in either class of wires (LTS or HTS) eddy currents are induced in the body of copper or silver. These secondary eddy currents lead to excessive Ohmic losses which will heat the superconductors and cause them quench (i.e. lose their superconducting property). This parasitic heating effect has discouraged the use of commercially available superconducting wires in motors excited with alternating current.
Because of recent advances in power electronics, alternating current drive systems have become a viable alternative to direct current motors for variable speed applications. Therefore there is a great deal of interest in searching for new methods of constructing superconducting alternating current motors that would incorporate two desirable features: the ability to exploit the attractive properties of available electronic power drives presently used with conventional alternating current motors and the use of HTS conductors. The HTS conductors require low cost liquid Nitrogen coolant and much simpler and more economical refrigeration systems that do LTS conductors.
The physics of the superconducting state are such that a superconducting ring behaves as a perfect diamagnetic body. Consequently, a current cannot be induced in a closed loop of superconducting material by using a moving magnetic field because the magnetic field cannot penetrate the superconducting loop. Conversely, if a magnetic flux had already penetrated a superconducting ring, the magnetic flux is frozen-in and cannot be destroyed.
The freezing of magnetic flux in a closed loop has been exploited by several inventors in different applications. Rabinowitz (U.S. Pat. No. 5,325,002) proposed to induce a current in a circuit of superconducting material that is above the critical temperature and therefore in the normal state. Once the current is induced, the circuit is then cooled to below its critical temperature at which time the material switches to the superconducting state and the circuit becomes a permanent electromagnet. A serious drawback of the Rabinowitz concept, if applied to the rotating field coils of a synchronous motor, is the significant amount of time it takes to cool the entire superconducting coil and its support. The long time constant renders the control of the motor completely impractical.
An extension of the Rabinowitz concept was applied by Leonard (1969), by Brechna and Kronig (1978), by Lipo (1987), by DeDonker and Novotny (1987) and by Tubbs (1990) to the construction of a synchronous/induction motor. This is a motor that is started as an induction motor and subsequently, when the rotor approaches the synchronous speed, the motor mode of operation is converted to that of a synchronous motor. In all of the above-mentioned motors, two sets of coils are installed on the rotor. The conductors of the first set are placed deep in the slots of the rotor laminations. The set of coils, made of superconducting wire, are inserted in the same slots and on top of the first set. When the motor is started all coils are at a temperature above critical. Because the Ohmic resistance of the superconducting coil at a temperature above critical is much higher than that of the non-superconducting first coil, upon starting the first coil carries most of the current and is responsible for the production of torque. Once the motor reaches the synchronous speed, the rotor is cooled to below critical temperature. As the second, superconducting coil is cooled, its resistance continually drops and a greater share of the current is transferred to it. Upon reaching superconductivity, the magnetic flux linking the coil becomes trapped in the coil and the motor behaves now as a synchronous motor.
All models of superconducting synchronous/induction motors constructed so far have displayed a disappointing performance because of two drawbacks. The first is due to the losses in the conductors. These consist of eddy current losses in the copper (or silver) matrix in the superconductor. The other drawback is the very long cooling time of the superconducting coil, which makes it impossible to alter the current in the coil while the motor is running in the synchronous mode.
It is possible, however, to induce a current in a circuit that is already below its critical temperature by creating a gap in the circuit to allow the magnetic field to penetrate the circuit. Once a current is produced in the circuit, the gap is closed and the current will continue to circulate indefinitely as long as the circuit remains superconducting. Furthermore, the Ohmic losses in the superconductor can be reduced or eliminated if the thermal stability of the conductors can be improved by proper shaping of the superconductors.
BRIEF SUMMARY OF THE INVENTION
The present invention solves the two main problems that arise from the use of superconductors in the rotor of a synchronous electric motor. First, rather than incurring the costs and complexity involved in forming HTS coils, this motor uses superconducting material deposited in a thin film on the outer surface of a rotor of insulating material. The use of superconductors in the form of thin films removes the need for sheaths of copper of silver to provide mechanical integrity and added heat capacity. With the absence of the sheath, eddy currents
Gerasimow Alexander M.
Scheuermann David W.
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