Split Ratio Investigation of Double Stator Permanent Magnet Motor Considering Multimode Operation

Abstract

I. IntroductionWith the rapid development of electric vehicles and hybrid electric vehicles, they put forward more critical and complicated requirements for driving motor drive systems. Double stator permanent magnet (DSPM) motors have become promising candidates for driving motors, due to its advantages of high torque/power density and relatively compact structure. DSPM motor can be considered as the integration of inner and outer motors. By the flexible control of inner and outer motors of double stator motors, different operation modes of motors can be realized. In [1], a double stator permanent magnet motor is proposed, and corresponding multi-mode characteristics are investigated. It is noted that current research on double stator motors mainly biased on proposing novel motor topologies, realizing different operation modes, and verifying motor electromagnetic performances. However, the reasonable power split and allocation of inner and outer motors are seldom investigated, especially for the multi-mode operation. Hence, it is very necessary and essential to purposefully determine power ratio of inner and outer motors, considering design requirements of multiple operation modes.Split ratio, which termed as the ratio of inner and outer diameter of stator, often determines motor magnetic and electrical loading. It possesses significant influences on motor performances, including torque output capability, loss characteristics, and efficiency. So, motor split ratio is considered as one of most crucial motor design parameters and often investigated during the motor initial design stage. In [2], the split ratio of surface-mounted permanent magnet motor is analyzed and the optimal motor split ratio is decided by analytical method. While for double stator motors, split ratios of inner and outer motors determine not only motor magnetic and electrical loading, and but also the power split characteristics of the whole motor. However, it is noted current publications of motor split ratio mainly focus on the split ratio of single-stator motors which mainly work on rated condition. Since double stator motor are required to work in different operation modes, traditional design methods of motor split ratio cannot be applied for double stator motors directly. Consequently, it has been a hot but challenging issue on how to determine split ratio of double stator permanent magnet motors during the initial design stage, considering the design requirements of multiple operation modes. In this paper, a double stator permanent magnet (DSPM) motor is investigated, and the relationship between motor split ratio and power split characteristics of different motor operation mode is analyzed in detail. Furthermore, by the purposeful design and optimization of motor split ratio, design requirements of multiple operation modes can be satisfied comprehensively.II. Motor topology and split ratio investigationFig. 1 presents the configuration of the investigated DSPM motor. As can be seen, inner stator and middle rotor form inner motor, while outer motor consists of outer stator and middle rotor. In order to realize multimode operation effectively, the torque-speed curves of inner and outer motor are designed purposefully, as shown in Fig. 2. The inner motor is designed to have a relatively flat torque curve over a wide speed range, which is utilized to augment the output torque of outer motor, especially in the range of torque of outer motor is dropping off. In addition, driving requirements of whole DSPM motor are arranged purposefully as optimization objectives for the inner and outer motors of DSPM motor. So, by the purposeful and reasonable design and optimization of split ratios of inner and outer motors, design requirements of multi-mode operation can be satisfied.III. Optimization results and performances validationIn order to satisfy design requirements of multiple operation modes, response surface method and multi-objective genetic algorithm are utilized for DSPM motor, the optimal motor split ratio and pole teeth ratio can be determined. Distribution of optimization objectives and design parameters are presented in Fig. 3 and Fig. 4 respectively. And, based on finite element analysis, electromagnetic performances of DSPM motor can be obtained. Fig. 5(a) depicts the flux lines and flux densities distributions of DSPM motor after optimization. Table I presents the multi-mode performance comparison of initial and optimal DSPM motor. And, torque performances of overall DSPM motor before and after optimization are shown in Fig. 5(b). As can be seen, after optimization, the average output torque increases from 71.83Nm to 82.81Nm, while the torque ripple reduces from 10.93% to 7.78%. Electromagnetic performances verify that, by the reasonable design and cooperation of inner and outer motors of DSPM motor, design requirements of different operation mode can be realized more effectively, which provides a research path for the optimization design of multi-port PM motors. More detailed theoretical analysis, optimization design results, and experimental validations will be presented in the full paper. **