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Wireless Power Transfer (WPT) promises automated and highly efficient charging of electric and plug-in-hybrid vehicles. As commercial development proceeds forward, the technical challenges of efficiency, interoperability, interference and safety are a primary focus for this industry. The SAE Vehicle Wireless Power and Alignment Taskforce published the Recommended Practice J2954 to help harmonize the first phase of high-power WPT technology development. SAE J2954 uses a performance-based approach to standardizing WPT by specifying ground and vehicle assembly coils to be used in a test stand (per Z-class) to validate performance, interoperability and safety. The main goal of this SAE J2954 bench testing campaign was to prove interoperability between WPT systems utilizing different coil magnetic topologies. This type of testing had not been done before on such a scale with real automaker and supplier systems.

This paper describes the magnetic circuit design of a coaxial AC motor system, comprising one stator and two rotors, and the test results obtained for a prototype motor. The rotors of the motor share the same stator core and coils, and each rotor uses its magnetic part as a yoke. Magnetic flux linkage of each rotor was determined in consideration of the maximum torque/power conditions and maximum motor speed. Finite Element Method were utilized to design a magnetic circuit for achieving the magnetic flux linkage specification. Tests conducted with a prototype motor showed that the torque characteristics can be divided into magnetic torque and reluctance torque, just like an ordinary IPM motor. Each torque level was improved through field-weakening control. The combined torque obtained when the two rotors were driven simultaneously approximately equaled the sum of the individual torques when the rotors were driven independently.

This paper describes two methods of driving a multi-motor system by one inverter set, which is intended for application to hybrid vehicles for improving fuel economy. With one method, a dual-rotor motor shares a single stator excited by one inverter, but the speed of the two rotors can be controlled independently. With the other method, two motors are connected in parallel by means of a special coil wiring technique that allows them to be controlled independently by one inverter. Simulations of the dual-rotor motor were performed with an equivalent electric circuit model to calculate the electric current, voltage, torque and speed. It was found that the magnetic reluctance balance among the stator teeth is an important variable for the high performance of this dual-rotor motor. The ability to drive two motors independently was then verified and demonstrated experimentally. An experiment was also conducted to verify the effect of harmonic electric current on reducing inverter loss.