The Investigation Of Three-phase Permanent Magnet Linear Motors For Stirling Machines

Linear motor direct driving/energy conversion technology is one of the key technologies for the free piston Stirling machines. However, the major types of permanent magnet linear motor used in the free piston Stirling machines nowadays are the miniature single phase AC electric machines. These types of linear motors do not scale well above fractional horsepower sizes thus have limitation in output power and control performance. While using three-phase permanent magnet linear motor instead of the single phase types will promote the development of high power free piston Stirling machines as well as promote the application of the new control technology to improve the entire system efficiency. Currently, the research on the design technique and actuation technology of the free piston Stirling machines using three phase linear motors is still insufficient. In this dissertation several key problems and solutions about the using of three phase linear motors in the free piston Stirling machines will be pointed out and studied further. The main content and meaning are as following:(1) Based on equivalent magnetic circuit models, the expression formulas of magnetic fields and forces for five different motor topologies are derived. The mathematical relationships established between the design dimensions and magnetic fields as well as the detent forces contribute to intensively understanding the electromagnet characteristic of the three phase linear motors and provide a theoretical basis to propose methods in performance improving and design optimization.(2) Taking these effects as inhomogeneous distribution of magnetic fields, partial circuit saturation, edge effect, relative motion between stator and translator, and nonlinear characteristics of material, into considerations, the finite element models for five different motor topologies are established in order to calculate quantitatively the magnetic fields and the electromagnetic force. The distribution of magnetic field, the magnetic field intensity of air gap, and the saturation of iron core are compared and analyzed in order to investigate the influences of the different magnetic circuit structures on the motor magnetic load. At the same time, further calculation of the air gap flux density along the given path is also carried out to tell the influence of the amplitude of each harmonic under different magnetic circuit structures. The amplitudes of the electromagnetic force and detent forces are calculated and relevant fluctuation characteristics are compared and analyzed. The research results would provide the analytical theory for the design and the topology selection of electrical motors in various application needs.3) Aiming at the feature of multi-variables and multi-objective, an high efficient design optimization method is proposed for a permanent magnet linear motor combining the response surface technology with Design of Experiments (DOE). The method proposed has taken full advantages of the convenience and effectiveness of the magnetic circuit method in normalization of design parameters and the establishment of robust parametric machine models, and the high efficiency and high accuracy of the finite-element analysis method in the computations of the electromagnetic quantities. Moreover, this hybrid calculation method has good versatility and calculation accuracy, especially suitable to the analysis of a linear motor of free piston Stirling with quite different wave forms of electric current and electromagnet thrust from those of general servo system. This optimization algorithm is established based on the combining of response surface technology with Design of Experiments. The optimal results are selected and determined by the Design of Experiments approach through statistic analysis. Thus, this optimal method can meet the comprehensive demands better.(4) The rectangular wave voltage control strategy is used to establish the linear motor drive system for a Stirling cryo-cooler. The operating characteristic of this system with unloaded and loaded oscillating gas force is investigated, and also studied are the characteristic of the closed loop control speed with gas force load and the regenerative brake of the system. The simulation results demonstrate that the rectangular wave voltage control strategy for a three phase linear motor, generally used in the drive of a brushless rotary motor, can also achieve same functionality to drive a reciprocating piston of free piston Stirling machine with some necessary modification. Thus, with the help of the position sensors, the flexibly changing range for operating frequency and the movement of the motor translator can be achieved by the voltage control under the preset operating interval of electromagnetic force. The comparison of the experimental results with the simulation results has been made and demonstrates a reasonable agreement between both.(5) The direct torque control strategy is also used to establish the three phase linear motor drive system for a Stirling cryo-cooler. The implementing approach of the direct torque control in the reciprocating three phase linear motor drive system is studied, and the response characteristics are analyzed, respectively, for the unloaded, loaded and control speed operation mode of system. In the simulation the electromagnetic force is modulated to become a sinusoidal waveform with the phase 90°leads to the translator displacement by mechanical resonance principle. Then the speed control is implemented through such a way that the sinusoidal electromagnetic force is added to the electromagnetic driving force inversely proportional to the translator displacement. The simulation results demonstrate that the translator can rapidly reach a sustainable and stable reciprocating motion nearly at a resonance state spontaneously. Overall, the system performance and the efficiency of energy utilization for the direct torque control strategy is better than that for the rectangular wave voltage control strategy.