Review of Axial Flux Induction Motor 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 magnet 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 rotor. 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 electric motor 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 electric machine 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 978-1-5090-5853-2/17/$31.00 ©2017 IEEE 146 much larger than the shaft diameter. The possible use of high stator 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 Michael Faraday. 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 magnets 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.