Research On Cogging Torque Characteristics Of Surface Mounted Permanent Magnet Synchronous Machines

With the popularization of vehicles,more attention has been paid to driving experience,so vehicles with good steering performance are preferred.Compared with traditional Hydraulic Power Steering System and Electric Hydraulic Power Steering System,Electric Power Steering System(EPS)has advantages such as more energy-saving,smaller in size,stronger resistance to low temperature,moreover,it's convenient to adjust the performance of Electric Power Steering System according to motorcycle type.The performance of Electric Power Steering System is mainly determined by EPS motor.Permanent magnet synchronous machines have smaller size,higher reliability,thus,they occupy the most part of the market of EPS motors.However,cogging torque of permanent magnet machines will affect driving experience.Thus,it is of great academic value and engineering significance to study cogging torque of permanent magnet synchronous machines applied to EPS.First,basing on the principle of virtual work,the analytical expression of cogging torque is deduced.Basing on the expression,the theory of common methods to suppress cogging torque is studied,such as choosing proper slot-pole combination,optimizing polearc coefficient,optimizing the width of slot opening,designing permanent magnet of uneven thickness,skewing,introducing dummy slots in stator teeth.Then a surface mounted permanent magnet synchronous machine which conforms to most design requeirements is designed,electromagnetic analysis,mechanical analysis and thermal analysis are performed.In order to suppress cogging torque,dimensions of the motor is optimized with above methods by finite element analysis.After optimization,the motor completely conforms to design requirements.Then a prototype motor is manufactured and tested,there are unexpected low-order cogging torque harmonics caused by manufacturing imperfections.Then how various manufacturing imperfections influence cogging torque harmonic component is studied respectively,including variations in stator inner radius,variations in stator slot opening width,uneven magnet remanence,errors in the circumferential position of magnets,errors in skewing angle.Then,the probability density function of cogging torque considering manufacturing imperfections is obtained through Monte Carlo simulation.Through measuring the actual dimensions of prototype machines,probability density function of each kind of imperfection is obtained,according to which a large set of random designs with manufacturing imperfections is generated.And cogging torque of each design is calculated by finite element analysis.Thus,the probability density function of cogging torque is obtained.Through this statistical method,the influence of each kind of imperfection on cogging torque is compared.And the probability density function of cogging torque considering all kinds of manufacturing imperfections is obtained,which allows a more realistic estimate of the cogging torque of mass-produced machines.Finally,two measures are proposed to suppress cogging torque harmonic caused by manufacturing imperfections.One is circularly rotating laminations when manufacturing stator,through which cogging torque harmonic caused by stator's imperfections is suppressed.The other is setting skewing angle specific to low-order harmonic caused by rotor's imperfections,with this method cogging torque is largely reduced at the sacrifice of rated load.