Hitachi shrinks in-wheel drive for compact EVs

October 05, 2021 // By Christoph Hammerschmidt
Hitachi shrinks in-wheel drive for compact EVs
Wheel hub motors are considered the Egg of Columbus in electromobility: they allow vehicle architects to completely redesign the spatial layout of cars because the space for a motor on the front or rear axle is eliminated. However, they are considered problematic because they are difficult to cool and because they increase the unsprung masses of the wheels, which has a negative impact on road holding. Hitachi has now developed a particularly small, highly integrated drive system for installation directly in the wheel.

Together with its subsidiary Hitachi Astemo, the Japanese electrical company Hitachi has introduced what the company calls a Direct Drive System for electric vehicles. The system combines motor, inverter and disc brake in one unit. This enables the installation of the entire system into the wheel. Hitachi regards this a significant step towards mass production of such cars

The motor is a lightweight design with a remarkable high power density of 2.5 kW/kg which limits the weight increase traditionally associated with this type of drives. Hitachi even claims that adopting the in-wheel unit does not require a substantial change to an existing configuration of the suspension and related components. It eliminates driveshafts and other mechanisms related to the powertrain. According to Hitachi, the use of its new Direct-drive system helps car designers to reduce weight and the energy required for propulsion by 30% – resulting in an accordingly higher range on a single charge.

The unit is still a prototype, and Hitachi intends to further the development of this system. According to Hitachi, the maximum power is currently 60 kW per motor which translates into a system power of 120 kW for a two-motor system or 240 kW for a four-motor system. The Direct drive works with a voltage of 420 V and absorbs currents up to 280 A. The goal of Hitachi is to integrate the system into a standard 19” vehicle wheel. In order to achieve the high power density, Hitachi’s engineers hate placed the magnets of the motor in a so-called Halbach Array which improves the drive force by increasing the effective magnetic flux of each magnetic pole. In addition, they used beam welding to create a high-density array of flat coils, resulting in less weight without sacrificing performance.

The system uses direct-cooling technology in which high insulating cooling oil directly cools the power semiconductors and us cycled to the motor to also directly cool the magnetic

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