Design Of Permanent Magnet Machine For Electric Vehicle Based On Litz Wire

The transportation industry occupies a large proportion of the energy consumption of the entire society.With the increasingly serious energy crisis and the country’s emphasis on energy conservation and emission reduction,pure electric vehicles with zero-emission and highefficiency have ushered in a development boom,which has drawn the attentions of many countries.With the strong support from the governments,pure electric vehicles gradually replaced fuel vehicles as the mainstream of the automotive industry.Permanent magnet synchronous motors have higher efficiency because they have no excitation loss,and have become the optimal solution for drive motors of electric vehicles.After years of development,the current permanent magnet drive motors for electric vehicles have also shown the trends of high speed,high power density and high efficiency.In order to achieve higher power density,major automobile manufacturers have carried out research on hair-pin windings to increase the slot fill,thereby increasing the power density of the drive machine.However,the skin effect of the hair-pin winding is much stronger.The higher the frequency,the greater the AC copper loss.Under the trend of increasingly higher frequency,this contradicts the goal of high efficiency and restricts the application of the hair-pin winding.The Litz wire with insulated strands does the best in suppressing the AC copper loss,which is in line with the trend of higher-frequency of drive motors for electric vehicles,so it has more value to research and more engineering significance.There are very few researches about the application of Litz wire in electric vehicle motors.Hence,this paper studies the types of Litz wires and their impedance performance,and establishes a calculation model of the AC and DC copper losses of Litz wires to select the type of Litz wire windings.Then,based on the actual motors,this thesis analyzes and discusses the impact of the application of Litz wire on its operating performance.First,combined with the structure of the Litz wire,the factors affecting the copper loss of the litz wire are analyzed.Compared with the random wound windings used on traditional motors,Litz wires are very different in production technology,3D structure,electrical properties,and mechanical properties.The main counting factors of different types of Litz wires on AC copper loss are skin effect and proximity effect.This thesis establishes mathematical models of skin effect and proximity effect,then analyzed its countermeasures and suppression effects of Litz wire,and analyzed the DC copper loss of Litz wire using filling factor and twisting coefficient.These methods are used in analyzing Litz wire with various twisting methods.The performance of the filling factor and the twisting coefficient has established the copper loss analytical model of the Litz wire,which provides a reference to determine the strand diameter,twisting method and number of stranding steps of the Litz wire.Secondly,for the current wide variety of Litz wires,6 most representative Litz wires are selected to build a 3D model.Under the frequency of the electric vehicle drive motor,this thesis analyzes their current distribution characteristics and their losses,and calculates their fill factor to summarize each their advantages and disadvantages of each Litz wire,providing guidance for the application of the Litz wire in the drive motor for electric vehicles.Finally,based on a low-to-medium speed motor with a maximum speed of 3500r/min and a high-speed motor of 12000r/min for electric vehicles both of which have practical application background,the loss analysis of the Litz wire was carried out,and compared with the traditional random wound winding.It is discussed whether Litz wire could improve the efficiency of the motor.