Study On Optimal Design Of Thrust Ripple Of Permanent Magnetic Synchronous Linear Motors

Permanent magnet linear synchronous motors (PMLSMs) have many applications in high performance linear direct drive systems due to certain outstanding features such as the high force density, rapid dynamic response, simple structure and free-maintenance. However, there is a significant drawback of large thrust ripple, which deteriorates control performance of motors, limits the improvement of positioning accuracy and causes vibration and noise of motor apparatus. End-effect force, including end force and partially EM ripple, is a unique and main disadvantage of PMLSMs in comparison with rotary motors; accordingly, it is the reason that thrust ripple in PMLSM has larger influence than in a rotary counterpart. An intensive investigation both by analytical and numerical methods on minimization of thrust ripple is comducted in this thesis.It is difficult to obtain an analytical solution of magnetic field by conventional analytical methods due to longitudinal opening of the primary iron-core. This paper proposes a new equivalent method of permanent magnets. The magnet array is treated as independent field source at the analytical region boundary, and then, the Poisson equation of describing magnetic field is simplified into second order homogeneous partial differential equation and can be easily analytical. Analytical of air-gap magnetic field is conducted and the analytical solution is conveniently obtained within a compact mathematical expreesion. The proposed method could be further applied to explore analytical of magnetic fields for different regions of permanent magnet linear synchronous motors.Analytical of end force is carried out based on the analytical of air-gap magnetic field and Maxwell Stress Tensor Method. Analytical of end force is obtained and correctly reveals the basic characterisitics of end force. The optimal iron-core length for minimization of end force is further analysized using the analytical results of end force and therotical predicting formula of the optimal iron-core length is provided. Finite-element method (FEM) is further employed to verify the correctness of analytical conclusions. The analytical of cogging force of motors is derived in the expansion form of Fourier series based on superposition principal. The analytical result, for the first time, reveals harmonic content and periodic characteristic of cogging force of motors and the conclusion is obtained that an optimal combination is that when the slot number and the pole number are prime numbers based on the analytical result and characteristic analysis of cogging force. A method of quantitative prediction of cogging force for assessment of various combinations of slot and pole is developed, avoiding repetitive modelling and computations by finite-element method on different whole physical models for various candidate combination schemes.A novel approach of combinational iron-cored primary for a thrust ripple-free PMLSM is proposed and investigated in details. By the approach, multiple end sides of primary iron-cores are constructed, and armature windings can be arranged to form a circulating sequence at side edges. Therefore, low order harmonics of thrust ripple will be directly eliminated from a PMLSM. For further eliminating remained thrust ripple, optimal design of structure parameters is conducted. A design strategy of assimilation between the end force and slot force is proposed and remainder of thrust ripple is successfully removed. The evaluation of design result gives that the thrust ripple is under 1%of the mean value of thrust.Based on theories and methods of miniming thrust ripple developed by the thesis, various structure types of PMLSMs are presented to satisfy different demands of applications. A novel constructive three-phase six-slot PMLSM by utilizing the approach of combinational iron-cored primary is proposed for developing slotted PMLSM with a small scale, large thrust and high thrust performance. The novel constriction could extend slotted PMLSM to be applied into application field of small-scale automatic systems to replace slot-less PMLSM.Prototype PMLSM with combinational three iron-cores primary is constructed, and its static thrust ripple characteristics are tested by a testing apparatus of linear motors. Testing results verifies correctness of theorotic analysis mehhods of minimizing thrust ripple and demonstrates effctivness of the approach of combination iron-cores primary. In conclusion, research achievement of this thesis provides a systematic and effective design theory and method for minimizing thrust ripple of PMLSMs. The approach of combinational iron-cored primary could provide a thrust-free PMLSM, and a linear thrust model can be applied to implement a reliable, high efficiency and high performance control of PMLSM.