Wheel hub drive offers superior power density at low cost

September 07, 2015 // By Christoph Hammerschmidt
Wheel hub drives hold out the promise to reduce cost and energy consumption of vehicles because the enable designers to omit many components of the classical powertrain. However, the high torque requirements of these drives require costly raw materials and a complex design with water cooling which wipes out most of its benefits. Researchers from Fraunhofer IFAM now have introduced a wheel hub motor that combines high torque density with air cooling.

The LARA concept contains development, manufacturing and evaluation of a robust air-cooled wheel hub drive. The characteristics of the motor make it ideal for deployment in a lightweight city vehicle. The main challenge was to achieve the high torque necessary for vehicle usage in urban environments and to combine this high torque with an equally high energy efficiency, the institute said in a press release. An additional condition was to implement a simple air cooling mechanism that was effective enough to remove the heat caused by the thermal power dissipation.

Fraunhofer IFAM which focuses on applied material sciences and manufacturing technology, used cast coils which allow extraordinarily high slot fill factor. To minimise cost and weight, the coils have been implemented with aluminium instead of copper.

The cast coil technique has been developed also at Fraunhofer IFAM. It enables the conductor profile to exactly match the available space, leading to slot fill factors of 80% and more with optimum thermal conductivity between coil and cooling path. Despite a slightly higher specific resistance of aluminium compared to copper, this approach makes it possible to achieve power density with air cooling that is equally high as in conventional copper coils with water cooling. Overall, the technique enables reducing the weight and cost level while at the same time increasing torque and power. Combined with the specific insulation technology and a power converter in innovative topology, also developed by Fraunhofer IFAM, the impact of faults through component failures or short circuit can be minimised. This leads to high functional safety as required in automotive applications.

Besides lowering power consumption and cost, wheel hub drives offer a range of additional benefits. Since the conventional array of powertrain components can be omitted, the space inside the vehicle is increased. In addition, they enable individual torque control at each single wheel.

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