Research On Rotor Magnet Loss And Thermal Field In Permanent Magnet Synchronous Motor For Electric Vehicles

In recent years, with the rapid development of electric vehicles, electric drive systemshave been in a fast growing period. As Permanent Magnet Synchronous Motor (PMSM) isthe key component of electric driving system, numerous attentions and research have beenfulfilled on PMSM. Generally speaking, there are no loss and hard temperature rising onrotor of PMSM, because the permanent magnet on rotor is assumed to rotate with statorwinding magnetic field at the synchronous speed. However, with the study of high powerdensity and high torque density PMSM, it is demonstrated that considerable eddy currentlosses are induced in rotor permanent magnet. Consequently, the rotor will suffer severetemperature rising. Even worse, rotor permanent magnet will be demagnetized or be burneddown.There are a large amount of harmonics in magnet field of PMSM, which are arousedby permanent magnet, widing distribution and slot opening. The harmonics are rotatingwith the rotor in different speed. As a result, there will be considerable eddy current lossinduced in rotor permanent magnet. Meanwhile, when the motor is controlled using PulseWidth Modulation (PWM) control method, especially with large stator winding current andlarge amount of time harmonics, the armature reaction magnetic field will generate a lot ofharmonics, which will also induce considerable eddy current losses in rotor permanentmagnet. Unfortunately, the above design methods and high-current PWM control arewidely used in high power density PMSM. So studying the mechanism and affectingfactors of rotor permanent magnet eddy current loss is greatly significant.The rotor permanent magnet eddy current loss was studied based on fast-calculatedanalytical method and precise electromagnetic Finite Element Analysis (FEA) method. Thethermal field was investigated using thermal network method and thermal FEA method.Moreover, an inner rotor high power PMSM and an outer rotor in-wheel PM|SM wereanalyzed. At last test benches were set up to verify the analytical results and FEA resuts.Firstly, the principle and analytical method of rotor permanent magnet eddy currentloss were analyzed. An electromagnetic analytical method was proposed that it isconsidering the skin depth effect in rotor permanent magnet. The factors of rotor permanent magnet eddy current loss were analyzed considering permanent magnet, time harmonics inwinding, space harmonics in winding and slot opening.Secondly, based on electromagnetic FEA, a rotor permanent magnet eddy current lossmodel was proposed. The influence parameters on rotor permanent magnet eddy currentloss were analyzed, which contains permanent magnet, combination of slots and poles, timeharmonics of winding current, and slot opening. The waveforms of magnetic flux in air gapand magnets were investigated. Both the eddy current density and eddy current loss densityin magnets were achieved and graphed in detail. The relationship between rotor permanentmagnet eddy current loss and parameters was established.Thirdly, the temperature distribution in overall electric motor and rotor permanentmagnet were investigated in both inner rotor high power PMSM and outer rotor in-wheelPMSM. The lumped parameters thermal network method and thermal FEA method are used.In thermal network method, the convection, conduction and connecting heat transfers werestudied. The thermal network models were established in different cooling manners. Thetemperature distribution in overall electric motor and rotor permanent magnet wereinvestigated. In thermal FEA method, a thermal model based on FEA was established. Thetemperature distribution and thermal flux in two different motors were investigatedrespectively. Temperature distributions in different operating modes and cooling mannerswere revealed. Temperature distribution in magnets and airgap were also investigated.Fourthly, a measurement method for rotor permanent magnet eddy current loss wasproposed which is based on back electromagnetic force (EMF) descension. The rotorpermanent magnet eddy current loss was tested using decrement of back EMF andreversible demagnetizing factor. The temperature rising of the two motors was measured ontest bench. It is shown that there a good agreement in analytical method, numerical methodand measurement for rotor permanent magnet eddy current loss. Meanwhile, there is goodconsistency in thermal network, FEA thermal analysis and measurement.