Hitachi, Ltd. and Hitachi Astemo, Ltd. have developed a compact, lightweight direct-drive system for EVs, combining the motor, inverter, and brake into a single unit. This enables the installation of the entire system into the wheel.

The major features of the technology are:

1. Halbach array magnets and innovative flat coils produce world-class power density. Increasing the number of magnetic poles is effective in improving a motor’s drive force, but has shortcomings—namely, it 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, Hitachi and Hitachi Astemo have succeeded in improving 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 that’s characteristic of conventional, heavier in-wheel systems.

   (A Halbach array is a magnetic array in which the orientation of the N pole of each magnet is rotated 90 degrees to create high-density magnetic flux at each of the magnetic poles of a motor.)

2. Direct-drive system incorporating a single motor, inverter, and brake unit. Prior EV motors have low power density and monopolize space inside the wheel to provide sufficient drive force, which make it difficult to use existing brakes and suspension components. Space is also needed for a dedicated and electrically insulated coolant channel, which prevents electrical faults from occurring when power semiconductors in the inverter come in contact with coolant.

   The newly developed 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 significantly reduces the space taken up by cooling pipes, and enables in-wheel installation without having to alter the existing configuration of suspension and other components significantly.

The new motor transmits the drive force directly to the wheels, and its lightweight design and world-class 2.5 kW/kg power density significantly limit the weight increase traditionally associated with in-wheel units. Moreover, adopting an in-wheel unit does not require a substantial change to the existing configuration of the suspension and other components.

Driveshafts and other indirect mechanisms have been eliminated, allowing motor power to be applied directly to EV operation. This reduces energy loss by 30 percent and increases the range on a single charge compared to existing EVs.

Hitachi and Hitachi Astemo will continue research into practical implementation of the technology, which enables more expansive interior and battery installation spaces. In addition to the vehicle control technology it has been developing, Hitachi Astemo will apply the new direct-drive system to its global EV product lineup.

The technology will be exhibited in part at the 30th Aachen Colloquium, from 4-6 October in Aachen, Germany.