[1] Yancheng Inst Technol, Sch Elect Engn, Yancheng 224051, Peoples R China
[2] Huazhong Univ Sci & Technol, Sch Elect & Elect Engn, State Key Lab Adv Electromagnet Engn & Technol, Wuhan 430074, Peoples R China
[3] China Univ Geosci, Sch Automat, Wuhan 430074, Peoples R China
[4] Univ Liverpool, Dept Elect Engn & Elect, Liverpool L69 3GJ, Merseyside, England