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A compact and high performance electric motor, called the 3D motor and designed to achieve output torque density of 100 Nm/L, was developed for use on electric vehicles and hybrid electric vehicles. The motor adopts an axial flux configuration, consisting of a disk-shaped stator sandwiched between two disk-shaped rotors with permanent magnets. It also adopts 9-phase current with a fractional slot combination, both of which increase the torque density. The rated torque output of this high power-density motor is achieved by applying a hybrid cooling system comprising a water jacket on the outer case of the stator and oil dispersion into the air gaps. The mechanical strength of the rotors against centrifugal force and that of the stator against torque exertion were confirmed in mechanical experiments. Several measures such as flux barriers, a chamfered rotor rim, parallel windings, and radially laminated cores were adopted to suppress losses.

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.