Theoretical Study And Electromagnetic Design For High-Speed And High-Thrust Linear Induction Motor

Linear motors (LMs) are the transmission devices that can convert electric power to mechanical work in the form of translational motion. Of the various types of LMs, the linear induction motors (LIMs) take the advantages of simple constructure and reliable operation, therefore, they are widely applied in servo drives, propulsion systems for ground transportation, industrial automation equipments and conveying and handling equipments, etc. To date, the extensive investigations and applications on the LIM have been mainly focused on the short primary LIM (SP-LIM) for its lower manufacturing and operating cost, as well as for the reduced energy consumption. However, although long primary LIM (LP-LIM) seems to be a costly choice as the primary windings need to be installed along the entire track, the weight of the conductive secondary can be significantly reduced, no sliding electric brushes or moving cables are required for power delivery and consequently, system reliability is greatly improved. These features make LP-LIM a prefer option in the applications with short-time or intermittent duty operation, such as numerical control machines, electromagnetic launching systems, impact extrusion test units, etc.The theoretical investigation and simulation analysis for the prediction of magnetic field distribution, performance calculation, electromagnetic design and control strategy of high-speed and high-thrust double-sided LP-LIM have been carried out by using the electromagnetic field technical theory. The research reported in this dissertation can be summarized as follows:The 1-d analytical governing field equations in the airgap region of LP-LIM, taking account of longitudinal dynamic end effect, are established and solved. Both the airgap magnetic flux density and the performances, such as thrust, active/reactive power, secondary eddy current, etc., under constant current excitation are formulated and validated against finite element calculations. The variations of motor design parameters on the end effect characteristic parameters as well as on the thrust and thrust ratio are analyzed. The influence of longitudinal dynamic end effect on the thrust-vs.-speed characteristics, with particular reference to the measures to improve the performances, based on the phasor diagram, is also investigated. The magnetic field distribution and thrust are also calculated in terms of quasi-2-d models, in which the short secondary is extended to infinity in the longitudinal direction.Since the complex power derived based on 1-d electromagnetic field is identical to that from the circuit, the influence of both longitudinal and transverse end effect can be explicitly taken into account in the equivalent circuit of LP-LIM as being the modified coefficients of the magnetizing reactance and the secondary resistance. In addition, the primary winding and iron core uncovered by the secondary are also represented in the equivalent circuit.The electromagnetic design procedure for LP-LIM is described. It involves selection of main dimensions, electromagnetic load, secondary specification and the airgap length, determination the primary windings and slots, arid calculation of magnetic circuit, equivalent circuit parameters, copper losses as well as associated performances. According to the flow chart, a design procedure, based on the MATLAB platform, is programmed to predict the overall characteristics of LP-LIM under either constant voltage excitation or constant current excitation. A 4kN and a 1.345MN prototype were designed specifically for electromagnetic launching applications, respectively.The dynamic model of LP-LIM, accounting for the end effect, is developed in arbitrary q-d-0 reference frame and validated by line starting simulation. To evaluate the dynamic response of variable-speed LP-LIM drive systems, the dynamic simulation with several types of control strategies, such as open-and close-loop constant volts/Hz control, secondary flux-oriented vector control, on the basis of the requirements to the fast tracking to the commanded acceleration, velocity and displacement, are implemented on the MATLAB/Simulink environment.The thrust-vs.-speed characteristics of LP-LIM show that the end effect will lead to wider statically stable speed range of operation and slightly lower breakdown thrust, the rated operating speed could be far away from the synchronous speed, thereby resulting in the higher secondary copper losses and hence lower efficiency compared to LIM with negligible end effect. In general, however, the influence of end effect on LP-LIM is much less than that on SP-LIM, therefore, to alleviate the end effect, it is preferable to select the proper motor design parameters rather than equip with additional compensation units.