Design, modelling, and fabrication of a ferrite magnet axial flux in-wheel motor

Abstract

The uncertainty in the availability and the price of rare-earth (RE) metals required to produce high-energy density magnets prompted the search for RE free high-efficiency machines. This work investigates the suitability of ferrite magnet machine as a substitute to RE magnet machine in an electric two-wheeler application. The two-wheeler selected for this work is an electric scooter used for the city commute, which belongs to a group of vehicles known as light electric vehicles (LEV). The objective of the work is to develop a ferrite magnet direct drive in-wheel motor for an electric two-wheeler and compare its performance with RE magnet in-wheel motors. Currently, an electric scooter costs twice the internal combustion engine (ICE) vehicle of similar class. The existing LEV mostly use a powertrain with a lithium-ion battery as the power source and a motor using RE magnets like NdFeB as the energy converter to meet the range and the performance of ICE counterparts, and these two parts accounts the major share of the cost of the vehicle. When comparing a ferrite magnet and an NdFeB magnet of same dimensions, the ferrite magnet produces around one-fourth of the flux, and it can be demagnetized by a much lower demagnetizing field. Motor designers workaround the inherent deficiencies of ferrite magnets by selecting a motor topology that allows placing more magnets and ensuring that the motor field poles are not exposed to an armature field of magnitude that can cause demagnetization. After reviewing many motor types and topologies, a permanent magnet brushless DC (PMBLDC) motor with a segmented armature torus (SAT) topology is selected for developing the ferrite magnet in-wheel motor. The SAT motor topology is a variation of axial flux motor and offers advantages such as high torque per unit mass, high-efficiency, and a dual rotor structure to accommodate more magnets. A finite element (FE) based design-synthesis program is developed and the program is used to generate designs of a ferrite magnet motor and a bonded RE magnet motor. A non-linear dynamic model of PMBLDC motor that includes the core loss calculation has been developed to simulate the system performance of the designs. The operating characteristics of ferrite magnets vary considerably with temperature, and therefore, the performance variation of the designed ferrite magnet motor with the operating temperature is studied using the dynamic model. From the results of dynamic analysis, it is concluded that the presented design will meet the tractive force requirement of the electric two-wheeler over the expected magnet operating temperature variation. The dual airgap assembly of SAT motor topology could present constructional difficulties, especially maintaining the proper airgap lengths. Two different assembly designs are considered for SAT motor prototypes. The first design is used in the construction of the bonded RE magnet motor, and the assembled motor has issues such as stator flexing and limited space for terminal connections. The second assembly of SAT motor addresses limitations of the first prototype, and it is used in the fabrication of the ferrite magnet motor. The operation of the first prototype is found to be restricted by its weak structure, and therefore, only the second prototype is installed in the electric two-wheeler. The ferrite magnet motor has been tested to measure back EMF, efficiency, and energy consumption when used as the powertrain of the prototype vehicle. The ferrite magnet SAT PMBLDC motor delivered a peak efficiency of 96.6 % when tested using constant torque loads in a test bench. Further, the energy consumption of the ferrite magnet motor as in-wheel powertrain of the scooter is evaluated using the basic urban driving cycle, and the result is compared to the results of two market sourced vehicles. It is found that the ferrite magnet powertrain consumes less energy to complete the test sequence, however, has 70 % more acceleration time. The structural optimisation of the ferrite magnet motor prototype could bring down the mass of motor and reduce the gap in acceleration time between the prototype vehicle and the market sourced vehicles. The development of a high-efficiency ferrite magnet motor for traction application and its positive test results provide a platform to expand the use of ferrite magnets in higher rated applications than the one covered in this work.