A compact, lightweight direct-drive system designed by Hitachi Ltd. and Astemo Ltd. for electric vehicles (EVs) combines the motor, inverter and brake into a single unit. This enables the installation of the entire system into the wheel.

In conventional EVs, the drive system is placed on the chassis, which limits interior and battery space. Fitting the motor inside a wheel has been identified as a solution, but this increases the weight in the wheel and requires sweeping changes to the existing brake and suspension components. The new lightweight direct-drive system overcomes these challenges by combining motor, inverter and brake into a single in-Source: Hitachi Ltd.Source: Hitachi Ltd.wheel unit. Driveshafts and other indirect mechanisms have been eliminated, allowing motor power to be applied directly to EV operation. This reduces energy loss by 30% and increases the range on a single charge compared to existing EVs.

Increasing the number of magnetic poles is effective in improving a motor's drive force, but reduces the proportion of magnetic flux that can be effectively used and necessitates more coil weld points and welding space. By placing the magnets in a Halbach array, designers have improved drive force by increasing the effective magnetic flux of each magnetic pole, and made the motor lighter by using beam welding to create a high-density array of flat coils, thus achieving world-class 2.5 kW/kg power density. This limits the in-wheel weight increase of the EV, and avoids the increased energy consumption characteristic of conventional, heavier in-wheel systems.

The new system uses direct-cooling technology, in which high-insulating cooling oil directly cools the power semiconductors and is cycled to the motor to directly cool the coils. This combined with the single-unit drive system integrating a motor, brake and inverter significantly reduces the space taken up by cooling pipes, and enables in-wheel installation without having to significantly alter the existing configuration of suspension and other components.

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