Design and Control of Novel Axial Flux Electric Motors for Electric Propulsion Systems

Abstract

Axial flux permanent magnet (AFPM) machines present a viable alternative to radial-flux machines for hybrid traction applications due to the adjustable air gap, higher torque density, modular design. Moreover, AFPM motors are ideally suited for spaces that require a short axial length and a large diameter. They perfectly match the design specifications for integrated motor-generators, fitting precisely into the allocated spaces. Therefore, multiple topologies have been developed such as single stator single rotor, single stator double rotor, double stator single rotor and multiple stator and rotor. These are widely applied to the transportation and aerospace fields.

Concentrated “tooth” windings are often used in axial machines since their radially short end windings are of benefit at the inner region. However, their use raises harmonic issues which reduces the quality of AFPM machines. Noise and vibration, high core loss and PM losses are the challenges. This, therefore, becomes the motivation of this research which is to find a method to mitigate the unnecessary harmonics and hence the losses.

An analytical approach has been used to examine the winding factors of concentrated windings. Through this analysis, a novel method has been developed to address harmonic issues in AFPM machines with concentrated windings. Specifically, for a 9-slot axial flux internal rotor (AFIR) motor, this method involves offsetting one of the stators by π radians and reversing the direction of the current. This adjustment effectively cancels the dominant 8-pole field generated by the armature winding, while maintaining the 10-pole fundamental harmonic.

To validate the analytical results, the dimensions of the AFPM and offset AFPM motors were determined, allowing for the construction and use of 3D FEA simulation models. The simulation results closely align with the theoretical predictions concerning airgap flux density, back-emf, and torque, thereby confirming the accuracy of the analytical derivations and the fidelity of the simulation models. Additionally, the simulation demonstrates the successful harmonic cancellation of the 8-pole field in the offset AFIR motor, validating the effectiveness of the offsetting technique for harmonic reduction.

Both the AFIR and offset AFIR prototype motors have been manufactured, and experimental tests conducted to further validate the analytical and simulation results. The experimental results closely align with the simulation results, particularly in terms of back-emf, PM flux linkage, dq-axis inductance, and torque, thus confirming the effectiveness of the simulations. A comparative analysis of the magnetic performance between the AFIR and offset AFIR motors reveals that the offset design not only achieves the performance levels of the standard AFIR motor but with greater efficiency and lower armature voltage. This enhancement reduces the load on the converter, cooling system, and battery. This thesis successfully demonstrates the capability of the proposed method to eliminate the redundant fundamental harmonic in concentrated windings of axial flux motors.

The main contributions of this PhD program are as follows: First, it introduces a novel offset method to mitigate harmonic issues for axial flux motors. Second, it quantifies the effectiveness of applying offset to one of the stators through analytical winding factor analysis. Third, it encompasses comprehensive work on axial flux motor design, modelling, manufacturing, control algorithm development, and experimental testing. Finally, it concludes that the prototype with the proposed offset method demonstrates lower armature voltage, higher power factor, reduced losses, and thus, greater efficiency compared to the original prototype without offset.

Date of Award	11 Sept 2024
Original language	English
Awarding Institution	University of Bath
Supervisor	Vincent Zeng (Supervisor), Xiaoze Pei (Supervisor) & Peter Wilson (Supervisor)