## 6 Conclusions  
This paper designs a novel efficiency optimization control named system efficiency improvement control strategy for the PMSM direct drive systems which aims to increase the system efficiency by optimizing the motor losses and inverter losses together. As the traditional linear loss model can only accurately determine the inverter loss at rated operation conditions, a nonlinear inverter loss model which applies polynomials to fit the nonlinear characteristics of the power device is built in this paper. The proposed nonlinear model can obtain a more accurate loss of power converter is obtained over the whole operation condition of PMSM drive system. As the PMSM is fed by the switching-mode inverter, the PWM output voltage of inverter will cause the harmonic current in the PMSM, which will lead harmonic motor loss and affect the system stability. To obtain a more accurate harmonic motor loss, this paper uses the double Fourier integral analysis to analytically calculate the fundamental component and harmonics of inverter output voltage, by which the fundamental motor losses and harmonic motor loss can be accurately analyzed. From the inverter loss model and motor loss model, this paper presents a whole-frequency-domain system loss model which consider both fundamental loss and harmonic loss. Based on the whole-frequency-domain system loss model, this paper applies the system efficiency improvement control strategy to optimize the motor current and PWM frequency together. The PMSM direct drive system can achieve higher efficiency without serious influence on the system stability under each operational condition of EVs. Compared with the  
Figure 15: System efficiency of the Si-IGBT-based PMSM drive system and SiC-MOSFET-based PMSM drive system by MTPA control and the proposed control. (**a**) System efficiency of the PMSM drive system by MTPA control; (**b**) System efficiency of the PMSM drive system by the proposed control; (**c**) System efficiency difference between the traditional MTPA control and the proposed control method in the Si-IGBT-based system and SiC-MOSFET-based system. traditional MTPA control strategy, the proposed control method can improve the system efficiency by 3.1% in the SiC-MOSFET-based PMSM direct drive system and by 8.38% in the Si-IGBT-based PMSM drive system. Both theoretical analysis and experimental results prove that, with the consideration of the coupling relationship between the power converter and PMSM, the proposed efficiency optimization control strategy can synthetically optimize the fundamental motor loss, harmonic motor loss and inverter loss. Without serious influence on the system stability, the system efficiency improvement control strategy can minimize the system losses. By decreasing the energy consumption of the PMSM system, the proposed optimization control method will significantly increase the endurance mileage in one charge and improve the performance of EVs. This paper provides a more accurate whole-frequency-domain system loss model, and as the control strategy is based on the system loss model, the optimized performance is limited by the motor parameters and inverter parameters. The experimental results show that although the SiC-MOSFET-based PMSM direct drive system has higher system efficiency than the Si-IGBT-based PMSM drive system, the Si-IGBT-based system has better optimized performance in the proposed optimization control method than the SiC-MOSFET-based system. Therefore, from point of view about reducing the energy consumption, the Si-IGBT-based system is more suitable for the urban complex traffic situation. Otherwise, the SiC-MOSFET-based system is more suitable for high speed road conditions and industrial application. Future research will attempt to find a establish a optimization design method which can optimize the motor parameters, inverter parameter and control method from a systems perspective. According to the operation condition characteristics, the system parameters and control method should be more specifically designed.