Rotor

Since the first bounding constraint is already accepted as the maximum diameter of the working space for the developed motor, the second restriction represents materials that can be applied for the parts of the rotor before final design and implementation could be started. Further rotor calculation depends directly on these two boundary conditions. As an additional restriction, a minimum distance between the rotor outer housing and the outer diameter of the considered working space of 3 mm is assumed.

4.1.1.1. Materials and Designs

For the development phase, firstly, standard materials are taken for all parts to define the main concept of the in-wheel motor and to have a reference for the next analysis.

Figure 4.1 demonstrates the biggest parts of the motor, which are the covers and the housing of the rotor. These parts are very suitable with more potential in the area of weight reduction. They present a thin-walled structure that ideally fits for an implementation of lightweight concepts like aluminum foams or carbon fiber reinforced plastics.

Figure 4.1 – Selected parts of the motor for implementation of lightweight concepts

48 In a first step, a new design of rotor housing and cover parts made of aluminum foams was developed according to the requirement profile. The concept presented in Figure 4.2, (a) includes two metal foam parts – right and left covers. For both covers SAS-structures were selected which consist of 3 layers: 0.5 mm thick facings in this case and 5 mm aluminum foam builds the core. The stability of the rotor housing made only from CFRP is not especially high and can cause vibrational loads. The use of the sandwich for the rotor housing consisting of aluminum foam core and CFRP facings increases the stability of the parts. Both designs also include parts for the bearing from one side and the threaded connection from another side (see Figure 4.2, (b)).

a b

Figure 4.2 – Concepts of rotor: a - Rotor housing of aluminum foam (sandwich), b - Development of a hybrid of aluminum foams and CFRP

Table 4.1 presents a comparison of the approximate weights of designs for the motor parts using lightweight technologies. Compared to standard materials, aluminum foams and CFPRs are lighter and have a potential for weight reduction, but technologically they are not characterized as the easiest way during the manufacturing process. This points out the fact that standard materials represent a more realistic approach for the manufacturing of the first prototypes of the developed in-wheel motor and the usage of the presented lightweight technologies, as it is explained in the following sections.

Component / Variant of version

Standard

materials CFPR Aluminum

foams

Combination, CFPR + Aluminum foam Weight, kg

Rotor assembly 10.3 8.84 8.07 8.45

Table 4.1 – Calculated weights of rotor assembly

Details regarding the considered material for rotor back iron is presented in the literature (see e.g. [16] [17]

[21]). The rotor back iron is made of a ferromagnetic material with nonlinear permeability, which provides a 1.97 T magnetic flux density and saturation at 50 kA/m. Variants of rotor back iron and its relevanceare more detailly described in the following section.

As it was presented in 3.2.1 and 3.4.2, permanent magnets have a direct influence on the motor development and future driving performance. The best magnetic material in terms of remanence flux density and coercivity is NdFeB [34]. That is why sintered NdFeB magnets currently appear to be the most promising solution. The main advantage of these magnets is the obtained value of the energy product (𝐵𝐻)_𝑚𝑎𝑥, which is the largest of all known materials with up to 5 T by ambient temperature and higher [28]. Secondly, NdFeB magnets are characterized by a high Curie temperature of about 160-170°C for types with an operating temperature of 80 ºC [43]. The remanence flux density depends on the magnet temperature and as the magnet heats up, it is reflected in an increase in current to produce an output torque [179]. For this reason, the main requirement for the operation of the motor is to create operating conditions under which the magnets never enter the irreversible demagnetization zone to avoid their demagnetization. However, NdFeB magnets are produced to cope with operating temperatures up to 200°C nowadays, which opens up a broad perspective for their application. One of the most important and essentially determining advantages of NdFeB magnets in terms of economics is their

49 relatively low price in comparison with other types of known magnetic materials. NdFeB also has better mechanical properties compared to e.g. SmCo-magnets, which is important for the production process [4]. As mentioned by Hackmann [61], other magnets like ferrites or AlNiCo-magnets provide a low energy product (𝐵𝐻)_𝑚𝑎𝑥 and these magnets would require too much material to achieve the desired energy density in the air gap. In order to make a choice with regards to the requirement on maximum temperature (see 3.4.3) for prototyping purposes, the generic material N45SH as a material for permanent magnets was used. NdFeB materials are sensitive to corrosion and react with oxygen in an untreated state resulting in very fast oxidation.

Therefore, NdFeB magnets need a corresponding surface protection. This is realized by using Ni-Cu-Ni coating. The parameter values of accepted material are presented in Appendix C.

4.1.1.2. Number of Magnets and Magnetization Type

The number of magnets 𝑛𝑀 has a significant influence on the motor performance. According to Equation (3.14), the pole pairs number or doubled number of magnets is proportional to the torque 𝑀_𝑒𝑙 and can be recognized as a pure geometrical coefficient. On the one hand, a maximization of the number of magnets leads to increasing motor torque, but on the other hand there is a complex of restrictions caused by the geometrical limitations of the inner diameter and the length of the rotor part and manufacturing limits of the magnet width 𝑤_𝑀. Considering the given rotor outer diameter in accordance with physically limited working space of the wheel, the first restriction on the rotor inner diameter 𝐷_𝑖𝑟 has its maximum at 350 mm. Therefore, the distance of 3 mm is provided as a gap between wheel rim and rotor housing.

In case of a high number of magnets (𝑛𝑀>100), the difference between the width of the magnet and the arc length required for its mounting consists of less than 0.001 mm. Assuming that the length of the arc is not relevant for the calculation of the number of magnets, this difference can therefore be neglected for further calculations.

The distance between the two magnets must consider the tolerances in magnet manufacturing. The tolerance range used for the manufacture of 10 mm wide magnets is ±0.1 mm. By reducing the tolerance range, the manufacturing of magnets becomes considerably more expensive [164]. According to the relevant tolerance range and to the dimension chain, the minimum distance 𝑑𝑀 between the magnets requires at least a value of 0.2 mm. Under these circumstances, this parameter may have a higher value and may be adapted for the required even number of magnets. The potential number of magnets can be calculated as:

𝑛_𝑀 = π ∙ 𝐷_𝑖𝑟 𝑑𝑀+ 𝑤𝑀

(4.1) In this work, two magnetization types – a conventional radial magnetization and a Halbach array magnetization – are presented with the target to minimize the weight of the rotor part of the developed motor, while the magnetic flux density in the air gap between stator and rotor is maximized and maintained.

Conventional radial magnetization requires a back iron of the rotor. Therefore, the magnets are mounted on the back iron with alternating polarity as shown in Figure 4.3, (a). The function of the back iron is to provide the containing and conducting of the magnetic fields. To describe the behavior of the magnetic flux density in the air gap by the variation of magnets, the parametric model was built upon a numerical finite elements’ method analysis (see [21] for a model description). According to this model, a height of 5 mm for the back iron allows the magnetic flux density to reach up to 1.10 T in the air gap.

As the next step of development, the inner diameter 𝐷𝑖𝑟 for the mounting of magnets has to be reduced by the height of the back iron and has a value of 340 mm. The rotor back iron as a fully cylindrical part is presented in Figure 4.4, (a). According to Equation (4.1), the developed motor by conventional radial magnetization can be implemented with the number of magnets 𝑛𝑀 = 104.

50

a b

Figure 4.3 – a - Magnetic circuit design for conventional radial magnetization [21], b - Shaping of the back iron of the rotor

A possible solution to minimize the weight of the back iron of the rotor is shaping, as it is showed in Figure 4.3, (b) and Figure 4.4, (b). From this point of view, it is possible to make radial grooves with 𝑅𝑟𝑎𝑑 of 4 mm into the rotor iron. The grooves must be positioned exactly radial to the magnet centers. A comparison of the weight calculation of the back iron in CAD shows that the mentioned solution allows to reduce a rotor back iron weight of about 2 kg, so the shaping can reduce the weight by 40%.

a b

Figure 4.4 – Variants of the rotor back iron: a - As fully cylindrical part, b - With radial shaping Halbach array magnetization, first presented by Halbach has an arrangement of permanent magnets resulting in a non-symmetric magnetic field [62]. Halbach magnetization consists of alternating axial and radial magnets as it is shown in Figure 4.5. The key advantages of the Halbach array are that the field on the one side is approximately doubled by the confined flux and the opposite side has a minimal stray field, which helps with confinement and consequently reduces the chance of a flux leakage. These advantages have a significant impact on the motor design, because the use of magnetic materials other than permanent magnets is not necessary.

Thus, there is no need to use back iron for retaining and the conducting of magnetic fields.

Further presented analyses are based on the assumption that for convenience of calculation and an aliquot number of magnets the inner diameter 𝐷𝑖𝑟 is 342.6 mm. Based on this limitation, the outer diameter of the rotor part as a support housing for magnets is considered to be 350.4 mm, which allows a 2.8 mm gap to the wheel rim and a thickness of the rotor housing of ℎ_𝐻 = 3.9 mm. As it can be derived from Figure 4.5, the Halbach array is represented by a double number of physical magnets. To receive the magnetic field of a magnet, two halves of axially magnetized magnets and a fully radially magnetized magnet are required. Thus, the magnets with axial and radial magnetization become the half of the width compared to conventional radial magnetization. According to this, also a smaller tolerance range of ±0.05 mm can be used. According to Equation (4.1), the number of magnets 𝑛_𝑀 for Halbach array can be calculated by 112 pieces. In terms of the tolerance range, the width of one magnet with radial or axial magnetization 𝑛_𝑀𝑝ℎ is 4.7±0.05 mm. It is important to highlight that the weight of the rotor housing for magnets is only 1.38 kg if it is made of conventional aluminum alloy.

51 Figure 4.5 – Halbach array magnetization

Under the considered conditions, remarkable differences by the magnetization require additional weight and volume by conventional radial magnetization. Research by Borchardt et al. [16] suggests thatthe comparison of the weight of magnetization arrangements by conventional radial magnetization and Halbach array provides an option of a weight reduction of about 51% of the rotor unit weight.

A simple comparison can be organized using Maxwell to observe the difference in magnetic fields by using dipole and Halbach array arrangement. The following simulation shows two identical configurations according to material and geometry, whereby the plate of the back iron in dipole variant is made of steel (Figure 4.6, (a)) and for Halbach array made of aluminum as depicted in Figure 4.6, (b).

a b

Figure 4.6 – FEM-simulation of magnetic flux distribution for: a - Standard dipole, b - Halbach array

a b

Figure 4.7 – Results of simulation of magnetic flux density for: a - Standard dipole, b - Halbach array The simulation results shown in Figure 4.7 provide that Halbach arrays have clear advantages over a standard dipole magnet configuration. The back iron of the rotor proves to be more effective in reducing leakage, although its influence on improving the magnetic field density in the air gap is not as significant. Based on the results of the simulation, a Halbach array is offered for further use in the developed motor, which can effectively reduce the weight.

52

4.1.1.3. Necessary Magnet Weight

In order to meet the required performance and lightweight requirements, great attention must be paid to the required volume and the respective weight of the magnets 𝑚_𝑀 during further development. This can be realized using the magnet’s geometric parameter – height ℎ𝑀 and the length 𝑙𝑀.

A geometric parameter related to the driving performance of the developed motor is the length of the permanent magnets. According to Equation (3.14), the length of the magnets has a proportional impact on the motor torque. Based on the requirements for the working space of the motor and taking into account the additional geometric spaces in the axial direction for the position of the windings, the length of the permanent magnets 𝑙_𝑀 has been assumed to be 100 mm. In case of insufficient torque of the motor, this parameter can be used for a detailed calculation of the motor parameters.

For the variant with a height value of the magnet ℎ_𝑀 of 5 mm, a translational model of the magnetic circuits for the Halbach array arrangement with similar geometry boundaries was presented by Borchardt in [21] in order to simulate the magnetic flux density in the air gap. The translational model in this case is preferable, while the differences to the rotational model are irrelevant. According to this approach, the mean magnetic flux density𝐵_𝐻𝑚 for Halbach configuration with 1.5 mm air gap ℎ𝑎𝑔 is to be expected at 1.1 T.

To analyze the influence of different heights of the magnets ℎ_𝑀 on the magnetic flux density, six different heights were studied over the range of 2.5 mm to 5.0 mm (2.5, 3.0, 3.5, 4.0, 4.5, 5.0). In order to limit the leakage flux and to minimize reluctance in the magnetic flux path, the value of the air gap was fixed at 1.5 mm.

Figure 4.8 shows the results of the FEM-simulation of the magnetic flux density for different heights of the magnets ℎ𝑀. The software program used to analyze the data was Ansys Maxwell.

ℎ_𝑀= 5,0 𝑚𝑚 ℎ_𝑀= 4,5 𝑚𝑚 ℎ_𝑀= 4,0 𝑚𝑚 ℎ_𝑀= 3,5 𝑚𝑚 ℎ𝑀= 3,0 𝑚𝑚 ℎ_𝑀= 2,5 𝑚𝑚

Figure 4.8 – Results of the simulation of the magnetic flux densities for different heights of magnets The analysis identifies a significant difference between approachable magnetic flux densities for different magnet heights. The higher the value of ℎ_𝑀, the higher the magnetic flux density in the air gap. For the development process, the height of the magnet ℎ_𝑀 of 3.5 mm was considered as the value that meets the production and machining limitations due to the high brittleness of thinner magnets and also due to the handling that causes swarf/sludge formation [129]. From this point of view, the total weight of the magnets 𝑚_𝑀 can be reduced by 31.5% to 1.35 kg in comparison to the height of 5 mm.

In addition, as shown in Figure 4.6, (a), no ferromagnetic impurities adhere to the outer surface of the rotor in the Halbach array, since the low magnetic flux is almost non-existent on the weak side - considering the thickness of the rotor housing.

All results were obtained without the winding in the air gap, because the windings in deenergized condition do not have a significant effect on the magnetic flux density [95]. Simulated results can then be used to determine the geometry of the windings in the air gap.