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-In-wheel type permanent magnet synchronous motors for electric commuter cars are required to be compact and light to use limited wheel space effectively and reduce unspringing weight. Therefore, our previous papers presented a low-cost in-wheel axial-gap motor that uses ferrite permanent magnets. This motor adopted an open slot structure to reduce production costs. However, to achieve further size and weight reduction, we examine here in detail the adoption of a semi-closed slot structure instead of an open slot structure. Experimental results show that the semi-closed slot structure is effective in realizing size and weight reduction.

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In this study, we examine how to increase the operating speed of a consequent pole axial gap motor to achieve higher output density. Our research group has been developing a consequent pole axial gap motor with field windings as the traction motor for electric vehicles. A smaller and lighter traction motor is required for better fuel economy and layout of such vehicles, and it is profitable to manufacture a smaller motor with an increased operating speed. To achieve high-speed operation, it is necessary to suppress the line-to-line voltage, and to that end, we examine the slot/pole combination. Moreover, because the rotor outer diameter of an axial gap motor is larger than that of a radial gap motor, the rotor strength should also be considered. We study the use of non-magnetic high-tensile-strength steel for the component that supports the rotor. We present the motor design and the results of spin burst tests, and we confirm that the motor can operate at the target maximum speed and output power density.

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