A wheel hub motor is a motor that is installed directly in a wheel of a vehicle and at the same time carries the wheel hub. Most designs are electric wheel hub motors, but there were also internal combustion engines. In the case of electric wheel hub motors, both internal and external rotors are conceivable. In principle, the hub dynamo is similar to the wheel hub motor, an exclusively regeneratively driven electric machine in external rotor design for drive applications with insufficient power. Wheel hub motors were already used in electric vehicles in the 19th century.
The main advantage of electric wheel hub motors in vehicles compared to drive concepts with a central motor is the elimination of the classic drive train with the necessary components (manual) transmission, cardan shaft, differential gear and drive shafts depending on the characteristics.
Since their transmission losses are also eliminated, there is potential for increasing the efficiency of the entire drive system. Braking energy recovery can also be achieved with an electric wheel hub motor. In principle, drive concepts with wheel hub motors are single-wheel drives in which the torque generated by the electric motor causes a tractive force at the wheel contact point of a wheel. Although it is also possible to drive on only one wheel in a conventional vehicle with usually four wheels, it makes sense to equip at least one axle with an independently adjustable wheel hub motor on the left or right wheel. With wheel hub motors, other vehicle concepts are possible that are practically impossible to implement with a conventional drive train due to lack of space. Since the drive is located directly in the wheel, vehicle concepts are conceivable, for example, in which a flat surface between the wheels can be used completely for the respective application. Furthermore, drive concepts with wheel hub motors offer significantly improved dynamics, which can be used, for example, for driving safety systems and vehicle dynamics control, among other things, due to reduced rotational inertia of the drive train and much faster control of the drive torque.
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The increase in unsprung masses – strictly speaking, these are masses suspended only by the tire – can have a disadvantageous effect, which tends to make the chassis less comfortable without changes to its design. Compared to body-resistant central motors, the wheel hub motors are much more exposed to environmental influences (e.g. splash or jet water, dust, salty media) as well as forces and accelerations acting in the wheel. Depending on the design, heat from the mechanical brake can also be introduced into the drive. Depending on the concept and depending on the respective driving situation, this can mean an additional temperature swing for temperature-sensitive components such as the frequently used permanent magnets or the wheel bearings, which must be taken into account when designing the drive. The principle-independent possibility of torque setting and control leads to increased demands on the functional safety of the drives and the associated control units.
