The electric vehicle PE (Power electric) system consisting of traction motor, inverter, battery, and reducer, which serves as a power source for internal combustion engine vehicles, is an important design factors of electric vehicle performance desired by customers: ① performance, ② AER : All Electric Range), ③ Cost has the greatest impact. Therefore, the competitiveness of the PE system is directly related to the marketability and profit creation of electric vehicle manufacturers. The common and traditional motor system topology of a typical electric vehicle consists of an inverter and a Y-connected motor. To overcome the electrical limitations of the traditional topology, the 2-stage topology operates by changing between CEW(Closed-end winding) mode, which is a high-efficiency mode, and OEW(Open-end Winding) mode, which is a high-power mode, according to the vehicle command by using two inverters and a mode-change switch. It is possible to improve the vehicle power and by increasing the motor inductance, it is possible to realize a high-efficiency motor system. In addition, it contributes to the reduction of inverter material cost by reducing power semiconductor switch(Silicon Carbide) by utilizing the reduced motor current and CEW mode features. The 2-Stage motor system can increase MI(Modulation Index) from 1.05 to 1.73. Using these advantages, Kia's CV GT EV, which was released in October 2022, 270kW rear wheel output power was realized through the application of a 2-Stage motor system compared to the 160kW rear wheel output power of the general model. Since the inductance of the motor was not increased compared to the basic model, it was possible to achieve high motor system efficiency compared to the increase in output power. Since the 2-stage motor system operates with CEW, which is highly efficient at the frequent normal driving point of the vehicle, it is a high-power system and can be operated with high efficiency. In the case of OEW mode using two inverters and an open end winding motor, there is limitation that do not need to be considered in the traditional topology. In the case of OEW, zero sequence current is formed which is a loss component that does not contribute to torque since each phase is independent. To solve this problem, a zero sequence suppression controller is required, and in some cases, the maximum voltage utilization(MI=1.73) rate may not be utilized to the maximum. In general, a method for designing a high-output motor is to increase the battery voltage, utilizing boost converter, adapting silicon carbide semiconductor with high cost or lower the inductance of the motor. Since all of these methods lead to an increase in cost and decreasing in efficiency, improvement through a new topological structure is required. The 2-Stage motor system topology presented in this paper is a groundbreaking structure that reflects all of these improvements. This paper proposes the world's first 2-stage motor system topology that has the greatest impact on the competitiveness of electric vehicles. It is possible to operate with high efficiency and high power at the vehicle driving point through two inverters and transfer switches. It is a competitive topology structure that enables high-efficiency operation and inverter power semiconductor reduction because the motor system output can be increased 1.73 times compared to the previous one through improved MI, and the inductance of the motor does not change or can be reduced.
Mr. KANGHO JEONG, Senior Research Engineer, Hyundai Motors Company