| The permanent magnet machines has many advantages，such as high efficiency,small bulk, light quality, fine characteristic etc. With the development ofhigh-performance permanent magnet materials and the improvement of manufacturingtechnology, permanent magnet machines have been widely used in variousapplications， such as electrical products， traffic and transportation, industrialequipment， national defence, e.g.. Although Permanent magnet motor has manyadvantages, there are still some problems as an emerging technology. One of which iscogging torque. The interaction between the rotor magnets and the stator teeth of themachines will produce cogging torque inevitably. It exists even though there is noarmature current. These parasitic ripples can lead to mechanical vibration, acousticnoise. It always produces a pulsating torque ripple. In addition, it has a certaininfluence on the accuracy and performance of the speed control system and a positioncontrol system. Therefore, the research on the method of reducing cogging torque isof great significance to improve the performance of permanent magnet machines.Theobjective of this paper is to study simple and effective methods in the course ofmachines design， which can weaken the cogging torque and optimize machineperformance. The major works implemented in this paper are presentd as follows:The meaning and background of cogging torque is described in this paper, then areview on the development and the main types of permanent magnet machines arepresented. The various configuration and its traits of permanent magnet synchronousmachines, the design process and the analysis methods of the magnetic field arepresentd. Meanwhile, other performances of the machines, such as torque ripple andback-electro motive force (EMF) waveforms have been studied when applied thesemethods. The mechanism of the generation of the cogging torque is analysed,and thecogging torque calculated method：Flux-MMF graph method, energy method andmaxwell tensor method are discussed.In this paper, the analytical method for the cogging torque based on the energymethod and Fourier transform is used to give the expression of the cogging torque.The influence of changing certain design parameters on cogging torque, such as thenumber of slots per pole, pole-arc optimization, slot opening width optimization wasinvestigated. The calculated methods of these design parameters have been studied. Several methods of permanent magnet optimization have been studied to reduce thecogging torque. The methods were achieved developed by surface-mounted permanentmagnet machine by the results of finite element simulation by the Maxwell2D.The expression of cogging torque based on the Fourier series analysis is analyzed.Based on the analytical expression of cogging torque, the relation of cogging torqueand the permanent magnets/slot opening are studied. Through the theoreticalderivation and finite element study，the concept of repeat unit is proposed. It is foundthat the machine can be divided into a number of repeating units. The waveform ofcogging torque produced by each repeating unit is the same and in phase. While thewaveform produces by each magnet/stator slot in a repeating unit is the same, buthas a phase shift. The cogging torque of machine is multiple of the cogging torquegenerated of a repeating unit.Based on the repeat unit, an effective cogging torque reduction method based onshifting permanent magnets is presented. A certain harmonic of cogging torque couldbe eliminated by relative shifting of permanent magnet groups with proper angles.The method to obtain the angles is achieved，and the method is simple. With thismethod the cogging torque can be greatly reduced. The back-EMF and torque rippleof the machine with shifted magnets are smoother than the normal magnets machine.The slot-opening shift method, which is another effective method for coggingtorque reduction is presented. Stator teeth are divided into groups based on the repeatunit, and proper slot-opening shift is applied for each group. The cogging torque canthen be greatly reduced by relative shifting of slot-opening groups with propershifting angles while the back-EMF waveforms remain symmetrical. No additionalback-EMF harmonic components will be introduced and torque ripple will besmoother by this method. Several lower-order harmonics of cogging torque could beeliminated by the method. The method to obtain the slot-opening shift angles is given.Several methods analysised studied in this paper are used in a permanent magnetsynchronous machine. The computed cogging torque values and machine performancewere compared with the experiment values of the machine. The results show that thecogging torque decrease more using several methods in a machine and it is helpful toobtain the better performance. The cogging torque reduction method based on repeatunit is verified by the research results of computed values and experiment values. |
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