Contributions to optimisation of interior permanent magnet synchronous machine for electric vehicles

Decreasing fossil fuels, increasing cost of these fuels and environmental pollution, research into more efficient and more dynamic systems are effective in the development of hybrid vehicles and later in electrical vehicles. Electric drive systems develop parallel to the development of these vehicles. Therefore electric motors of these electric drive systems are being studied lots of the researchers. Different types of electric motors have been used in electric and hybrid vehicles. Direct-current motors, induction motors, switched reluctance motors, permanent magnet assisted synchronous motors, interior permanent magnet synchronous motors are the main ones. Direct current motors are no longer preffered by the electric vehicle manufacturers. Other type of machines are being used by different manufacturers in different rate. Internal permanent magnet synchronous motors are the most preffered ones on electric cars. This type of electric motors have high torque density, high efficiency, high power factor and good field weakening ability. In this study, the effect of the primary and secondary design variables on the operating performance of the interior permanent magnet synchronous machine was investigated. The main dimensions of electric motor and stator slots are defined as primary design variables. Primary desing variables are calculated with analytic equations. Internal permanent magnet rotor has a complex geometry so it is hard to design and optimize with analytic equations. These rotor dimensions are defined as secondary design variables. Dimensions of magnet holes and location of magnet are directly effect the electric motor performance. However, it is difficult to quantify the effect of these design parameters analytically. From here, it is aimed to analyze the effect of 7 different rotor parameters on internal permanent magnet rotor. Firstly, design spesifications and rotor structures of commercial electric car electric motors which are mostly interior permanent magnet synchronous motors are studied. Also the masses and the motor power ratings of commercial electric car models are studied. The power rating of EM and the V-shaped interior permanent magnet synchronous motor geometry is determined in design with this information. Permanent magnets are the most expensive component in an electric motor. Therefore commercial car manufacutrers are trying to reduce the amount of permanent magnet used in electric motors. Also their second purpose is to produce high torque density from their motors. These challenging goals are achieved by using V magnet shaped interiro permanent synchronous motors which are the most magnet efficient electric motors. Analytic equations are used for initial design of PMSM. Also some coefficients such as torque per volume and the split ratio are determined from the commercial car electric motors. Before the optimization of permanent magnet synchronous motor, some design cirteria must be defined. Firstly the effect of the magnet length on the moment component is very small. It is directly related to the demagnetization of permanent magnets. Secondly, magnet bridges are critical in terms of machine relaibility. Magnet bridges must be design as thick as possible considering mechanical loads on rotor due to the centrifugal forces however, must be as thin as possible considering electromagnetic performance. In order to minimize the leakage flux of the magnets which is one of the most effective factors on the performance of the machine, the smallest limit of the magnet bridge dimensions must be determined by static mechanical analysis with finite element method. The mechanical stress and deformantion of rotor due to centrifugal force were investigated. Magnet bridge thickness determined as small as possible considering mechanically safe rotor design. However, high mechanical stress occured at initial design due to the sharp cornered geometries. These high mechanical stresses at the inital design have been eliminated at the optimizated geoemtry with rounded corners. In this way, the preliminary design limits are kept constant in parametric optimization. The machine geometry parametrically created in order to design and analyze the machine with different power ratings and sizes very quickly and optimize the machine performance. Within the scope of this thesis, computer assisted optimization of the rotor was carried out by using some of these parameters with constant stator dimensions. Permanent magnet synchronous motor geometry is built in parametric using Ansys Electronic Desktop. In this study the rotor is optimized with 7 parametric variable. High torque density, high torque per magnet volume and low torque ripple are the main goals of optimizastion. 1000 different electric motor are analyzed with the help of Ansys Response Surface Optimizastion tool. Each parametric variable have different effect on optimization goals, so these sensivity analysis are given in this project. Results are showed that all aims could not be reached at the same time. For example most torque density design has very high torque ripple.That's why it is hard to determine the most suitable design. Ansys Response Surface Optimization suggests the most suitable 5 designs to make the selection easier. The optimized machine detailly analyzed using Ansys Electronics Desktop. This computer application solves non-linear electromagnetic problems with finite element analysis. 2D machine geometry is used in the analysis. Transient solution is applied and the EMs electromagnetic performance analysis. Flux lines, flux vectors, flux density of stator and rotor are given. Also air gap flux density along a pole is determined with its harmonics. It is important to obtain sinusoidal flux density with low harmonics in the air gap. But it is not possible to obtain pure sinusoidal wave shape on the air gap. Stator slot openings with high reluctance cause deformation on the air gap flux. The back electromotiveforce of the machine at maximum speed is determined. Its maximum dc value is greater than battery voltage. So the flux weakening must be apllied when using this electric motor at its full speed range. In addition to this, torque speed chactersitic with efficiency map and torque ripple map are determined. Maximum efficiency of the motor is 97,2 % and maximum torque ripple is lower than 25 %. Finally, the liquid cooling channels of the machine designed. Infolytica MotorSolve application is used in themal analysis. Motorsolve uses finite element method to solve the electromagnetic problems. After calculation of losses with electromagnetic analysis, the thermal behaviour of electric motor is calculated. The Motorsolve can perform succesive electromagnetic and thermal iterative solutions to obtain the most accurate result. The electric motor is analyzed at the speed of 4500 min-1 with 25%, 50%, 75%, 100% torque respectively. It is analyzed for 90 minutes in each load. The end-coil and magnet temperature at different power ratings were obtained by finite element method. Also the EM is analyzed at the speed of 7000 min-1 with 25%, 50%, 75% torque respectively. The end-coil and magnet temperature at different power ratings were obtained. In conlusion, designed and optimized EM has 125 kW power. The most efficient operation point is 97,2% and the maximum torque ripple is lower than 25%. As a result of the work, the result that contribute to the determination and calculation of rotor geometric design parameters which are effective on electric motor performance have been reached leading to a higher design performance.