1. Institute of Rail Transit, Tongji University, Shanghai 201804, China
2. Maglev Transportation Engineering R&D Center, Tongji University, Shanghai 201804, China
3. College of Electrical Engineering, Sichuan University, Chengdu 610065, China
4. School of Engineering, Sichuan Normal University, Chengdu 610101, China
5. Clare Hall, University of Cambridge, Cambridge CB3 9AL, UK