Review of Axial Flux for Automotive Applications F. C. Mushid and D. G. Dorrell

Abstract -- Hybrid and electric vehicles have been the focus axial-flux machine. of many academic and industrial studies to reduce transport However, the working principal of both axial and radial flux pollution; they are now established products. In hybrid and machines is obviously the same. They may be characterized electric vehicles, the drive motor should have high torque by their conductor geometry and field orientation as shown in density, high power density, high efficiency, strong physical structure and variable speed range. An axial flux induction Fig. 1: the radial-field machine, is where the airgap flux is motor is an interesting solution, where the motor is a double radial and the conductors are axial; and axial-field machine, sided axial flux machine. This can significantly increase torque is where the airgap flux is axial and the conductors are radial. density. In this paper a review of the axial flux motor for Axial flux are more commonly brushless permanent automotive applications, and the different possible topologies for machines. [4] compares the advantages of the axial flux the axial field motor, are presented. permanent magnet and the induction motor for radial flux

machines. It was discussed that due to limited rare-earth Index Terms-- Axial flux induction motor, radial flux machine, hybrid and electric vehicles, single and double side magnet material resources, an axial flux induction machine . could be a better choice for automotive applications; the use of high-power variable-speed induction motor drives have I. INTRODUCTION gained interest, particularly, in high speed compressor UTOMOTIVE vehicles are obviously a common means systems, for energy conversion units, and in high pressure Aof transport. The pollution caused by combustion pumps. The use of network frequency could allow these engines reduced air quality, increase the contamination machines to reach high speed in their operation [5][6]. The carbon dioxide in the environment, especially in large cities work in [7][8] proposed light construction and excellent where the concentration of vehicles can be very important. mechanical and dynamical performance are properties make An alternative sustainable solution for transportation is the axial flux induction machine well adaptable to medium therefore needed to reduce emissions. Plug-in hybrid and speed operation (3000-15000 rpm). It is interesting to note electric vehicles have been the focus of many researchers and that the rotor is disc shaped and is made from solid steel with automotive companies in the world to solve this problem. an inserted cage made from cut plate of good conductor. These are now commercially available. It should be born in There have been patents filed to address various aspects of mind that the electric energy used in these vehicles should be the machine [9][10] though the general geometry appears to sourced from renewable energy sources for these vehicles to have been postulated prior to these patents [11][12] so the be classed as “green”. In the design of the electric vehicles, patented aspects of the arrangement seems clouded. The energy and power density of energy storage and conversion machine continues to be studied and researched [13][14]. The units are important, and indirectly related to the size and permanent magnet version of the axial flux machine has been weight of the vehicle. The used in an electric commercialized [15][16][17] and [12][18] are seeking to do vehicle can be DC or AC and the controller of the motor is the same with the axial flux induction motor for specific associated to the motor type. These need to be very torque applications. dense and operate over a wide speed range. [1] Presented a review of diverse types of used in HEVs and EVs. In [2], in 1988, author proposed a toroidally-wound, slotless, permanent-magnet, brushless DC motor. This approach highlighted the potential of the axial- flux permanent magnet machine and its high efficiency and high power density, and to generate high torque at low speed. In [3] Platt proposed an axial-flux induction motor, which directly drove the two wheels. In the following discussion, major types of axial field motors are described; based on the Fig. 1. Flux orientation flux direction in the air gap electromechanical energy conversion, machines are classified as either a radial and It has been shown again in [19] that the construction of an axial field machine rotor could be readily varied, an axial

flux induction motor could be designed to have a small or  F. C. Mushid and D. G. Dorrell are with The University of KwaZulu- large inertia. It is can be seen that better ventilation and Natal, Howard College Campus, Durban 4041, South Africa (e-mail: cooling can be achieved as the axial field holds a greater [email protected] and [email protected]). diameter-to-length ratio and also its inner diameter could be

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much larger than the shaft diameter. The possible use of high by air-gap. Induction motors (IMs) are widely used in specific electric loading and high specific magnetic loading electric propulsion systems. They have a simple construction will further reduce the size of the axial-field machine [19]. and low cost, are reliable and robust, can work in harsh The reason why the axial flux induction motor has an environments, and virtually no maintenance. The lack of slip- advantage in this application is the short axial length of the rings in the rotor windings allows the squirrel cage induction machine, which is a requirement [5-7]. Research on the motor to be used, and to work up to higher speeds. The direct-drive high-torque wheel-motor was presented for the conventional control of an induction motor in variable speed hybrid and electric vehicle in [20-22]. The numerous operation uses a variable voltage frequency inverter. The advantages highlighted above for the axial flux induction desired performance for the traction systems under high load is the main reason for the non-linearity of the induction motor motor will be the focus of this paper in the context of the model; it must be used under high saturation conditions to automotive drive application; and on the analysis of the obtain high torque. However, the desired performance for the effects of different parameters on the performance drive system can be obtained from several controls, for characteristics of axial flux induction motor will be the focus example, oriented to the field weakening control (FOC - of further work. Field Orientated Control). Nevertheless, the cost of this type of control system is higher than that used for DC motors II. HISTORY OF THE AXIAL FLUX MACHINE [25]-[27]. Contemporary electrical machines are found in The axial flux machine is not a new technology. The axial various physical topologies. There are several aspects to flux machine has been presented in numerous topologies. The consider for the axial flux induction motor structure and they radial flux induction motor is the most popular motor type may be characterized by their conductor geometry and field due to its high reliability and low cost manufacturing – it is orientation. They may characterised by: the workhorse of industry. However, the flux path in a radial • The configuration of the stator and rotor flux machine is relatively long compared to the axial flux • The structure of the stator winding equivalent, and a large fraction of the length of a radial flux • The structure of the rotor induction motor is the end turn region of the windings. • The magnetic core Hence, the axial flux induction motor topology has attracted • The source of the motor some interest as a double sided axial flux machine which can • The stator slot arrangement significantly increase torque density and where the length of • The materials which are used in the motor the machine is a limiting design parameter. The history of • The number of the phases electrical machines shows that the earliest machines were the • The airgap length axial flux machines dating back to 1831 and were built by All these aspects need to be considered for good performance . Some years Late in 1837, Davenport and reliability of the machine. patented the first radial flux machine. There are several reasons why the axial flux machine was not used for many IV. TOPOLOGY years; for instance, there is a large attractive force between Axial flux induction machines have been presented in the stator and the rotor, causing difficulties in manufacturing, various topologies which give the axial machine the also there are high costs in the laminated stator core advantages to fit in different applications. There are three manufacturing and difficulties in assembly to maintain main arrangements: single sided machines, double sided uniform air gap. Thus, radial flux machines have come to machines and multistage machines. However, in axial flux dominate the market. However, these issues can be overcome machines the stator has a ring structure and rotor is disc with improved manufacturing techniques. The excitation of shaped. The radial length from the stator inner radius to the electrical machines with permanent took a outer radius is the active part which produces the torque and significant step forward in 1983, with the development of the axial length is dependent on the proper yoke design of the Neodymium-Iron-Boron (NdFeB), which belongs to a family stator and the rotor; i.e., the flux density in the stator and the of rare earth permanent magnets; this revived the use of rotor yokes. Nevertheless, as the number of poles increases, motors with axial flux permanent magnets. During recent the active radial part of the machine remains unchanged, and decades, permanent magnets have undergone much the axial length depends on the flux density in the stator development. This has shown the need to consider the yokes [28]. thermal stability of the permanent magnet, as well as the The different arrangements for single sided and double sensitivity of the specific parameters of permanent magnets sided machines are shown in Fig. 2. The single sided machine with increasing temperature. Axial flux motors with is as indicated but the double-sided arrangement can have permanent magnets are increasingly being used, for example, several different arrangements. incorporated into the wheels of the electric propulsion systems, wind generators, industry, among others [23][24]. A. Single Side Rotor Axial Flux Induction Motor This configuration of axial flux machine consists of a III. AXIAL FLUX MOTOR SYSTEM CONSIDERATIONS single stator and a single rotor and the stator core can be Induction motor is the most common type of motor used in slotted or slotless [29]. In the single-sided the machines the many applications. Induction machines consisted of a stator flux has to flow in the circumferential direction so that the and a rotor mounted on bearings and separated from the flux enters and leaves the stator and rotor on the same side.

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This will necessitate the use of magnet yokes behind the is then is closed through the rotor yokes. The magnetic flux windings and cage. The is substantial axial force between the path through the stator yoke is quite short, so this part can be rotor and stator when excited. eliminated, leading to a lighter stator core consisting only of ferromagnetic teeth, or no teeth at all, with the windings being airgap windings. The NS topology is not possible with toroidal windings since there is no effective flux linkage between the stator windings and rotors [29]. For the axial flux machine, all the flux distributed tends to keep the motor torque balanced so that the motor can deliver equal torque to both driving wheels of the vehicle. One possibility when using two rotors is to mechanically decouple the rotors so that the it could be used for the differential between the two drive wheels. However, this is quite advanced and more torque needs to be transmitted to the faster rotor, and an induction motor will give more torque to the slower rotor, so at this point this seems impractical. Hence, it is assumed that one inverter will be used to produce the drive to the motor and the rotors fixed together. C. Comparison between Axial and Radial Flux Machine At this stage, it is worth considering the axial and radial flux machines together and noting any relevant differences. Fig. 2. Single and double sided axial-flux induction motors. Obviously, the main difference between axial flux and radial B. Double sided Axial Flux Induction Motor flux machines is the orientation of magnetic flux. In the axial This kind of axial flux motor consist of three important flux machine the flux path in the core is very short, enabling elements, represented in two arrangements: the minimization of core loss and hysteresis currents. In a • Double stator with a single rotor where by the center double sided machine this can lead to core elimination in the rotor is between two : one issue in this kind of yoke for a double-sided stator, or solid core in double sided configuration is that the core of the rotor can be removed if rotor. The axial flux topology gives high torque and power the machine is a permanent magnet type to obtain a coreless density values, and a compact light-weight machine is rotor structure; i.e., the magnets will be non-ferromagnetic obtained which is suitable for medium or high speed motor material. In an induction motor with a cage rotor, the rotor when compared to their radial flux motor equivalents. This could be made from a solid steel disc with radial slots to feature is important for use in electric propulsion systems, locate the rotor bars. Since the rotor is between the two since there is a need to incorporate the drive system on the stators there is an equal distance between the two stators, this wheels or in a compact space; space and weight are critical in will result in no axial stress on the mechanical part of the a vehicle. machine since the force will be in the equilibrium. However, There are disadvantages to an axial flux motor; the this is an unstable point. This arrangement is shown in Fig. 2. magnetic force through the air gap is along the same plane as When the flux allowed to travel through the rotor, the axial the motor shaft; i.e. along the length of the motor. A radial length is less since the rotor is thinner. In addition, with this flux motor has magnetic forces that are perpendicular to the arrangement, the flux on either side of the rotor will be length of the motor or shaft and there will be diametrically almost equal so the net axial forces on either side of the rotor cancelling forces in a balanced machine with a centered will be approximately equal. If the flux path reverses in the rotor. Only in a double-sided machine are the equivalent rotor (left hand version of the double-sided machine in Fig. cancelling axial forces; so single sided machines require high 2) then the flux paths on each side are independent and it is rated thrust bearings. The discussion so far is mostly aimed highly likely that the flux will increase is the airgap is less on the axial flux induction motor; however, axial flux permanent one side and the rotor will be pulled towards that stator. magnet machines are more common and these axial forces • Single stator with double rotor where by the interior are always present since the permanent magnets cannot be stator is between the two rotors: in this configuration base on switched off [26][30]. In [29] a new topology of axial and the direction of the flux in the stator core, two topologies can radial flux induction motor was proposed in which the rotor be derived from this configuration. The North-North (NN) has an axial and a radial-cage winding and the motor has a topology and the North-South (NS) topology. In the North- single stator with a double rotor topology. This topology will North (NN) topology, the direction of the magnetic flux is reduce the air-gap length to give high efficiency and low closed along the stator yoke and the stator core is needed power factor. The issue of airgap length in an induction (flux paths as shown on the left in Fig. 2). The stator has a motor. High torque is obtained with a large airgap length; tape wound iron core and windings wrapped around the core. however, this leads to a large magnetizing current which The winding can be wound in a toroidal fashion [2]. For the affects the efficiency and power factor, especially at low NS topology, the magnetic flux passes through the stator and power, and this is an important design criterion which needs critical investigation.

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D. Automotive Drive Motor Options low power devices. Super- and ultra-capacitors are now The discussion so far has mentioned the different options considered as short term energy storage in vehicles because for the axial flux induction motor as well as mentions of the they have high power capabilities, although their energy . At this point it is interesting to density is still low compared to the batteries (which are address the range of common machines that are available for usually lithium ion). use as a drive. Fig. 3 gives a flowchart for the drive motor options. Note that most of these can be available as axial flux machines although the topological realization can be difficult in some instances.

Electric Machines for HEVs and EVs

Asynchronous Synchronous (induction) (PM and wound)

Cage Variable Salient pole PM Induction reluctance wound rotor (In HEV/EV production) Traditional but not widely used Wound Switched Brushed DC Fig. 4. Torque/speed envelope for the Toyota Prius Generation II drive Reluctance Rotor (Traditional but not efficient – motor. Induction (About to go into HEV/EV used on early HEV/EVs) production) B. Voltage source Inverter (Traditional machine – not Interior PM Doubly-fed yet used in HEV/EV; used (Brushless - most popular The voltage source inverter (VSI) is extensively used for as series generator in) Reluctance production motor – electric vehicles applications and these are invariably 3-phase Doubly-fed (New machine with reasonable reluctance torque applications being available) types. The ratings must be high so that IGBT or similar type Induction explored) Inset PM devices must be used. The DC bus (probably 300 to 600 V) (New machine with Synchronous applications being Reluctance (Low reluctance torque is direct fed by the high voltage bank in EV through a explored) component – sometimes called (Now produced as drive, consequent pole when only synchronous VSI. The induction motor is supplied by the Unipolar may be used eventually in one magnet per pole-pair) HEV/EV) PWM –VSI and is the best solution when for speed and (Appears not developed for Hybrid and torque regulation. The machine is AC controlled so the HEVs/EVs– more suited to Surface PM bearingless machines) PM assisted (In production as motors currents are sinusoidal; induction motors have a higher (discussed here) and generators) requirement for sinusoidal control with low harmonic content Fig. 3. Different machines available for HEVs and EVs. compared to a brushless permanent magnet motor. Although at high speed, 3rd harmonic injection can be used to increase V. AXIAL FLUX MOTOR CONTROL the apparent voltage and extend the speed range. Constant V/f To use an axial flux machine in an automotive drive will often be used up to the base speed where the power and application the system must be considered in the design torque are maximum and then constant voltage during the process and what is possible. In addition, the torque/speed maximum power range. Therefore, the biggest demand exists envelope that is required from the machine is also needed. at the maximum power and torque points. Induction motors Fig. 4 shows the torque speed envelope for the 8-pole have a large magnetizing current requirement so that at low machine used in a Generation II Toyota Prius. The points to load it may be more efficient to reduce the voltage to balance note are the narrow maximum torque range and the wide the magnetizing and load current requirements and reduce maximum power range. Above about 150 Nm the machine is machine flux and hence iron losses. This is at variance to duty cycling and cannot deliver this torque for very long. brushless permanent magnet machine where the Other components in the system need to be considered. The magnetization is fixed so reducing the voltage at low load is machine is also expected to regenerate when braking because not possible. The currents during start and acceleration can this can give about 25 % energy saving. Generally, these be into hundreds of amps among the control systems machines are fluid cooled due to the high power and torque available, field oriented control is often used to allow the required and efficiencies well over 90 % are reported for the control of the torque in steady-state and in transient permanent magnet type of drive motor. conditions [32][33]. A. Batteries As already mentioned, the most common structure of the inverter is to use six controllable power electronic devices to There are several rules to be followed in placing the form a three-phase inverter. The controllable power batteries in electric vehicles, such as a balanced weight electronic devices that function as static switches can be distribution in the electric vehicle; output terminals as close turned on and off with the switching patterns provided by the as possible to the engine controller; good accessibility; modulation method (SPWM). The output voltage waveform protection stroke, temperature and vibration. These rules may can be calculated as follows: vary from vehicle to vehicle and will be discuss further in another Section [31]. The capacity of the battery can limit the ma 3 VVL L() rms dc (1) regenerative braking capability since they are high energy, 2 2

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where ma is the amplitude modulation. The frequency parameters on the performance characteristics of axial flux modulation is assumed high. Fig. 4 illustrate the torque induction motor for automotive applications. characteristic over the entire speed range of the Prius drive motor; this is a brushless permanent magnet motor though there have been studies that design an equivalent induction motor [1]. It can operate under the maximum torque region, which is below the normal speed (base frequency). If an induction motor operates in the constant power region, the speed of the induction is above the base speed. The supply frequency is increased in order to achieve high operating Fig. 5. One sided section of an axial flux machine. speed of the induction motor. One of the advantages of the In Fig. 6, the top of the machine is shown. The machine induction motor over the brushless permanent magnet motor design has 48 slots so that that a full slot pitch is 6 slots. It is is that machine operates in field weakening mode above the known that a 2/3rd pitching can be used in a 3-phase machine base speed so that the voltage demand on the inverter will be and this isullustrated here. The reason for this is to produce a less. In other words, the flux in the air gap will decrease since compact winding. Since it can be an issue fitting the end the supply voltage is not proportional to the supply winding into a confined space. Also of interest is the ratio of frequency. The wide maximum power range in an automotive inner to outer ratio of the stator core. As illustated, the slots drive motor means that this is highly advantageous. The have to have constant width so the teeth are thinner on the torque produced by the induction motor decreases as a inner sirface compared to the outer surface so that the consequence of the flux reduction and there is less demand machine has low flux linkage in the inner region. The rotor for current. cage can be fabricated from an aluminium or copper plate C. Modelling of induction Motor in d-q axis with section cut out so that it can fit into the core which is It is now standard practice to operate a variable speed made from a magnetic plate with slots milled for the cage. induction machine using d-q theory and some sort of flux or . Typical d-q representation and torque calculation is given in the appendix for completeness.

VI. EXAMPLE OF AXIAL FLUX MACHINE In this section an example of the finite element analysis of an axial flux machine is outlined. Fig. 5 shows one section of a simulation. The pole number in an induction motor is very important. While to give high torque the pole number should increase, the magnetizing reactance will decrease. This example is an 8 pole machine and Fig. 5 illustrates one pole of a double sided machine. If the top of the problem (the Fig. 6. Aspects of FEA model of axial flux induction machine. stator yoke) is set to a Dirichlet boundary (constant magnetic vector potential so no flux crossed it), and the bottom is set to VIII. APPENDIX D-Q REPRESENTATION a Neumann boundary (so flux crosses normally), then a Here a summary of the equations needed to control the mirror image appears below hence making it double sided machine are given. A four-coil model is required. The stator with two stators. Conversely, a yoke could be put under the and the rotor voltage equations in the d-q axes in terms of rotor and the bottom boundary set as a Dirichlet boundary flux linkages are derived from: and the top boundary as Neumann boundary; then the mirror Vqs R s I qs  s  ds  p  qs (2) image appears above the stator and this becomes a double- V R I    p  (3) sided machine with one stator and two rotors. Further work is ds s ds s qs ds to look at the merits/demerits of the two arrangements. Iqr I qs () s   r  dr  p  qr (4)

Vdr R r I ds () s   r  qr  p  dr (5) VII. CONCLUSIONS where p is the differential operator. The stator and the rotor In this paper a review of the axial-flux motors is presented flux linkage equations in d and q axes are given by within the context of an automotive drive. The axial flux qs  LS Iqs  LmIqr (6) induction motor has been chosen as the motor to be used in   L I  L I (7) this work since it has the potential to be a compact and cost ds s ds m dr effective machine. A brief comparison between the axial flux qr  Lr Iqr  LmIqs (8) motors and radial flux motors was presented. The paper also dr  Lr I dr  Lm I ds (9) presented equations describing the induction motor circuit for The electromagnetic torque of induction motor with pole power and torque characteristics. Further work will be on the number P is obtained from: simulation and analysis of the effects of the different

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3 P [19] W. S. Leung and J. C. C. Chan, “'A new design approach for axial- TILe  qs ds  ds qs  (10) field electrical machines”, IEEE Transactions on Power Apparatus 22 and Systems, Vol. PAS-99, No. 4, pp. 1679-1685, July/Aug 1980. [20] P. J. Chrzan, J. Nieznañski and R. Szczęsny, “Induction motor control dm 2 TTJWWe L  and r  m (11) in hybrid vehicle”. Proc. of Control in Power Electronics and dt P Electrical Drives SENE’97 Łódź, November 1997 in polish And substituting (11) in (10) we get [21] Z. Szymański, “Adaptive control method of wheel electric vehicle”, Proc. of Acem’01, Kuyabashi, 2001. 2 d TTJ r (12) [22] Z. Szymański, “Application of electric and hybrid drive systems in eLP dt traction vehicles”. Proc. of SEMTRAK’2000, Zakopane, September, where P and J represents number of poles and moment of 2000, in polish. [23] J. F. Gieras, R. Wang and M. J. Kamper, Axial Flux Permanent inertia [34]. 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PAS-94, no. 5, 1975, pp 1500-1507. Franck Mushid obtained a BScEng in Electrical Engineering from The [13] Zahra Nasiri-Gheidari and Hamid Lesani, "A Survey on Axial Flux University of KwaZulu-Natal in 2016 and is currently studying from a Induction Motors," Przeglad Elektrotechniczny, (Electrical Review), Masters by research degree at the same University. vol. 88, no. 2, pp 300-305, 2012 [14] Zahra Nasiri-Gheidari and Hamid Lesani, “New design solution for David Dorrell (M 95, SM 08) is a native of St Helens, UK, and has a static eccentricity in single stator–single rotor axial flux induction PhD degree from The University of Cambridge in Engineering (1993). He is motors,” IET Electr. Power Appl., 2013, Vol. 7, Iss. 6, pp. 523–534. currently Professor of Electrical Machines with The University of KwaZulu- [15] YASA Motors Technol. http://www.yasamotors.com/technology/ Natal in Durban, South Africa, a post he took up in late 2015. He has held [16] Ashwoods Electric Motors, https://www.ashwoodselectricmotors. lecturing positions with The Robert Gordon University and The University com/axial-flux-motor of Reading. He was a Senior Lecturer with The University of Glasgow, UK, [17] Regal Beloit, Fasco motors, Axial Flux Brushless Pump Motor, for several years. In 2008 he took up a post as Senior Lecturer with The http://www.austfan.com.au/motors/fhp/imPower_AxialFlux.php University of Technology Sydney, Australia, and he was promoted to [18] N. Brown, “In-Wheel EV Motor From Evans Electric Unveiled In Associate Professor in 2009. His research interests cover the design and Australia”, Clean Technica, Aug 1st, 2013, https:// analysis of various electrical machines and renewable energy systems. He is cleantechnica.com/2013/08/01/in-wheel-ev-motor-from-evans- a Chartered Engineer in the UK and a Fellow of the Institution of electric-unveiled-in-australia/ Engineering and Technology.

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