Superconducting Induction Motor

By now, the study of superconducting machine is mainly concentrated on the following aspects: superconducting homopolar machine, superconducting synchronous machine and superconducting special motor using bulk superconductor. Superconducting synchronous machine technology is the main trend of the study of superconducting machine today. Superconductor can substitute normal conductor which is usually used as field winding in synchronous machine. That can reduce the volume and the weight of the machine substantially, because of the large magnetic field excited by the superconductor without steel cores. Because the magnetic field is excited by the armature winding in induction machine, the steel cores should still be reserved when superconductors are used in rotor. But superconducting induction machine bears its unique advantages, such as not requiring auxiliary winding, simple structure, high reliability and fast starting. It also has high efficiency because it can attain synchronous speed. The volume of machine can be further reduced and the power density could be enhanced, once scientists and technicians solve the problem of ac loss in conductor. The study of superconducting induction machine is still inadequate. It's expected that the paper can serve as reference for further development in this field.This paper transforms the JS2-400M2-4 conventional induction machine which is already commercialized into superconducting induction machine by substituting the copper (Aluminum) rotor windings with Bi-2223/Ag superconducting material. The superconducting induction machine is analyzed through three methods and compared with conventional induction machine. Finite element method (FEM) is the first method. Firstly the non-load starting process and steady on-load status is simulated, then both the electromagnetic parameters and performance parameters of superconducting induction machine and conventional machine in rating state are computed. The result shows that the power density of superconducting induction machine is 10 percents higher than conventional machine. Equivalent circuit model is the second method. The equivalent circuit model of conventional induction machine can be used to analyze superconducting induction machine due to the similarity in their configurations and principle. However, rotor resistance varies with different operation speed. Therefore, Weibull function describing superconductor characteristics and equivalent circuit model should be associated together to analyze the superconducting induction machine. In other words, the superconducting induction machine can be analyzed through nonlinear equivalent circuit model. The steady state characteristic is presented through this method, and then the minimum starting voltage and the maximum synchronous torque are computed. As it shows, superconducting induction machine bears larger starting torque, maximum torque and higher efficiency than conventional machine. Establishing mathematical model which can simulate the transient process is the third method.This paper presents some improving methods for superconducting induction machine. If the inner diameter of stator is reduced, the area of stator slot and the conductor number per slot is increased, the power density can be higher, for example, the power density can be enhanced by 15 percents by changing the conductor number per slot from twelve to fourteen. Then the alternating component in rotor bars is analyzed. The alternating component can be eliminated via skewed slot method. At last, the qualities of the YBCO superconducting induction machine and BSCCO superconducting induction machine are compared. As it shows, the YBCO superconducting induction machine has better performance.Some analysis processes in this paper cannot design and analyze superconducting induction machine precisely like in conventional machine, because they are rough in some aspects, such as some assumption in nonlinear equivalent circuit model, etc. There are many reasons including the difficulty to describe characteristic of superconductor, defective of current simulation software in considering characteristic of superconductor, etc. But the design and analysis methods proposed in this paper could still generally realize and reflect the performance of superconducting induction machine.