Existing in-wheel motors with air gap winding technology

As it was mentioned in 2.3 the main disadvantage of in-wheel motors is the additional weight which increases the amount of the unsprung weight in the vehicle. Therefore, by the development of the in-wheel motor a special attention on the lightweight design should be paid.

Since 2011, the Department of Mechatronics of Institute of Mobile Systems at the Otto-von-Gericke University Magdeburg developed, built and tested under the leadership of Prof. Dr.-Ing. Roland Kasper in-wheel motors with special winding – air gap winding (Figure 2.7). This special winding represents an ironless winding. An ironless winding is a winding that has no iron material between the conductors. The advantages of motors based on ironless winding technology are the following: lower losses compared to conventional motors, which enables to reach high efficiency in combination with a high power density. Relatively simple motor design is characterized by the fact that the motor requires only a low amount of conductor material and back iron, so by using of the innovative air gap winding in the permanent excited electric machine can be realized the material savings while keeping the equal magnetic flux density [20]. As a result, the motor with air gap winding has a compact lightweight design and therefore is especially attractive in the application area of electromobility. The first systematic study based on the air gap winding was performed by Borchardt in [17] and [19], where parametric model of a novel electrical machine that allows fast and precise magnetic circuit calculations was presented. And also, the ironless air gap winding technology was patented in 2013 under patent number WO2013/029579 A2 [92].

Figure 2.7 – Principle of the air gap winding [20]

The first prototype of an electric machine with the air gap winding, also known as Elisa I in-wheel motor, represents the lightweight construction concept for the application in automotive, shown in Figure 2.8, (a). The in-wheel motor Elisa I arose from the third-party funds research project sponsored by the state of Saxony-Anhalt in Germany in cooperation with the local state companies [54]. The Elisa I motor was planned as a pure motor for test bench measurements with the idea to use methods for the further serial production. Special feature of the motor is the near-series design, characterized by the integration of power electronics, universal connection to suspension system of the vehicle and the application of modern calculation methods for the simulation of mechanical, electrical and thermal properties of the motor. Therefore, it was particularly improved the magnetic components in terms of materials and geometry, as well as to reduce friction losses of the bearings by adding ball bearings in order to influence the efficiency and power density of the motor. The result of this project is the prototype of the in-wheel motor with such highlights as nominal torque of 300 Nm and nominal power of 40 kW provided by the casted stator with app. 300 mm outer diameter and the 100 mm magnet length [96]. The prototype with a weight of only 20 kg delivers a power-weight ratio of 2 kW/kg [97].

Nevertheless, project Elisa I met some problems in manufacturing method, whereupon was developed project Elisa II, see Figure 2.8, (b). In Elisa II design, the functional components were placed differently and calculated more specifically in order to meet the requirements for suspension space and weight. The in-wheel motor is calculated for the integration into a 15” rim. By using of a brake concept with internal brake caliper, the motor

14 can be combined with a wide range of common chassis systems. A further feature of the developed motor is the use of a lightweight sandwich structure consisting of balsa wood in the area of the side covers of the rotor to keep the unsprung weight of the motor as low as possible. Elisa II motor has more reliable construction and is made with higher accuracy. Power of the motor is declared by 40 kW by rated torque of 300 Nm. The motor is operated with maximal current of 100 A and voltage of 400 V. The motor weight is only 20 kg without breaks and 25 kg with breaks [188].

a b

Figure 2.8 – a - In-wheel motor Elisa I [188], b - In-wheel motor Elisa II [188]

As the next step of development based on the in-wheel hub motor Elisa II an in-wheel motor called Editha was developed, which can be mounted on the non-modified rigid axle of the Smart Fortwo. The service and parking brake system, the wheel carrier, the electrical connections and the cooling supply lines were designed to suit them in the given installation space of the vehicle. Another specific modification is running on the secondary bearing position that is designed as a thin-section four-point contact bearing with a flange ring to absorb tilting moments during braking [100].

In parallel with research activities aimed at improving the innovative winding technology, several projects have been initiated to implement this technology in other applications. The most technologically advanced follow-up project was 4.5 kW in-wheel motor for lightweight electric scooter with a speed range of follow-up to 45 km/h, as it shown in Figure 2.9, (a). Flexibility and scalability of the motor design and its highlight - air gap winding, made it possible to develop a perfect design by unique integration of the motor into the rim at the total weight of the motor by 2.7 kg. This allowed to keep the total weight of the scooter by 32 kg. The structural feature of this motor was the flat stator design, which allowed the motor to have air-cooled cooling system [96].

Another example of air gap winding application is the project dedicated to a generator integrated into trailer wheels. This generator is used to recuperate braking energy, which can be used for the operation of various additional units such as a refrigeration system. The designed generator can generate up to 30 kW of power at speeds of up to 350 rpm. This air-cooled generator, integrated in the trailer wheels, has a great advantage of operating as an additional equipment without significant changes in the design of the trailer axles [96].

The extremely low power consumption of flyboats was used in the next project to overcome the range limitations of battery-operated boats. Two slim cylindrical motors based on the air gap winding technology (see Figure 2.9, (b)) were integrated into the flyboat, providing 11 kW of power to drive the boat. Due to the optimization of the cooling system it allows the speed range of 600-2600 rpm for this power value. This significant advantage helps to use low energy in the flight mode for the maximum speed, what allows to save the energy and reduces the size of the batteries [96].

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a c

Figure 2.9 – a - In-wheel for electric scooter [163], b – Motor for flyboat [163]

Further examples of the air gap winding technology application in mobility are electric longboard and high torque motor for the trial motorcycle and in the area of the green energy are small wind and water mills generators.

As the next step in the development of winding technology, the new design concept combining air gap and slot winding with the same magnetic structure was presented by Kasper and Borchardt [91] and patented in WO2017125416A1. For this purpose, the current Elisa I motor with only the air gap winding was modified by the additional winding integrated in the stator back iron as a slot winding. The schematic representation of combined winding is presented in Figure 2.10. Besides the back iron of the stator, which was modified in the height, the active parts of the motor remained unchanged. Using this modification, the torque of the existing Elisa I motor was increased to 480 Nm and power to 62 kW. At the same time the weight of the new design of the motor has only 1 kg more compared to the original design.

Figure 2.10 – Scheme of the combined winding

In addition to the above-mentioned investigations directly into innovative winding technology, by the realization of projects and scientific activities, many issues related to different motor systems were identified and investigated, such as:

- Borchardt and co-workers presented in [17] and [19] design and model of the in-wheel motor with air gap winding, in [20] described the modelling and calculation of the motor with air gap winding, provided design optimization of electrical machines with air gap winding [16] and [21], introduced the winding machine for the automated production of the air gap winding,

- Kasper and Borchardt provided in [91] the study dedicated to the combination of slot and air gap winding types, Kasper et.al. presented FEM modelling and measuring process of the water generator in [95] and described a new mathematical approach for eddy current loss in the motor with air gap winding [94],

- Zörnig et al. [189] analyzed the different bearing systems of the different in-wheel motors, the frictional torques of the components and presented design aspects of the first generations of the in-wheel motors with air gap winding in [188],

- Golovakha and co-workers presented in [57] the control solution for the developed in-wheel motors,

16 - Vittayaphadung et al. [174] analyzed the deformation of the wheel hub bearing in the in-wheel motor by using of FEM simulations,

- Sauerhering and colleagues proposed in [145] an approach to investigate the influence of cooling channel geometry and thermal interface materials on the thermal load of an electric motor with air gap winding, - Hinzelmann et al. in [74] investigated and described the winding process application of the combined winding in the hydroelectric generator,

- Schmidt and co-workers in [150] defined and implemented measuring methods of an electric machine with air gap winding based on a mathematical model and in [149] provided a deep analysis of the losses in the motor with air gap winding,

- Stamann et al. investigated in [162] and [163] the joining technology for the air gap winding by usage of double-sided adhesive electrical insulation films with thermally conductive adhesive layers.