Research On Permanent Magnet Synchronous Motor For In-wheel Traction Application

In-wheel propulsion is a promising solution for fulture electrical drive system and its performance requirements are mainly focused on high torque density,wide speed range and intermittent peak power capacity.Combining equivalent magnentic circuit and finite element analysis,a 70kW prototype machine with low speed high torque density is designed according to machine specification for In-wheel propulsion.The main research contents are as follows:Comparing different motors used in In-wheel traction application and their pros and cons,it can be found that permanent magnet synchronous machine equipped with rare earth materials is perfered alternative as for meeting high torque density performance.Techinical feasibility study on target application is conducted on various motor structure.Althrough machines with axial flux or exterior rotor are of higher torque density theoretically,the inherent shortcomings limit their use in target application,the same is true for transverse flux machine.As for stator structure design,it is related to the slot pole combination.Overlapped distributed winding and fractional slot concentrated winding machine are common options for electrical propulsion system.As for target application,emphasis is put on torque density rather than power density,short axial dimension confine the use of overlapped distributed winding whose long end winding decrease the effective stack length and thus torque density.The slot pole combination are dertermined based on tradeoff between torque density,conductor current density and winding heat dissipation.After several rounds of calculation and comparision 32/36 is rational and an alternative with 30/36 is also invested.This two prototype machines have the same steel lamination and coil configuration and conversion can be made by changing coil connection sequence.Rotor structure is determined by permanent magnet position and can be devided into two main types:surface-mounted and interior permanent magnet.It is well acknowledged that surface-mounted permanent magnet machine has poor flux weanken ability and lower torque density than that of interior permanent magnet machine.This is true for traditional overlapped distributed winding but not for fractional slot concentrated winding machine.Fractional slot concentrated winding machines are permanent magnet dominant which mean that the reluctance torque contribute little to increasing torque density and can be neglected when considering more rotor flux leakage.Also,the harmonic and leakage inductance prominently increase the flux weaken ability and obtain a wide speed range of this kind motor.Electromagnetic design procedures are concluded as follows:performance is calculated and structure is optimized by equivalent magnetic circuit method;the results are then confirmed by finite element analysis.For thermal analysis,loss of high speed operating point is first calculated at supposed working temperature and then output to lumped parameter thermal network model to calculate motor temperature rise of different parts.The results are compared with the supposed values to set the new ones,this process continues iteratively until meeting the tolerance.As for transient thermal analysis,the S9 duty cycle has been investigated with simplified equivalent operation periods combination and further tests are needed for reliability demonstration.The prototype machine is manufactured and preliminary open circuit tests are finished,the tested back electromotive force shows good consistency with simulation results.This confirm the design method is applicable and subsequent onload test is under way.