Flux-switching permanent magnet machine typically suffers from relatively high cogging torque due to its special doubly salient structure and high air-gap flux density, causing undesired torque ripples, as well as acoustic noise and vibration, especially at low speeds. In this paper, an analytical expression of cogging torque is derived based on a magnetomotive force-permeance model, through which the optimal design parameters and dimensions, such as combinations of stator slots and rotor poles, skewing angle, stator tooth width, rotor tooth width, and magnet thickness, can be determined. Based on the derived analytical model, two novel approaches for cogging torque reduction are proposed, i.e., asymmetric magnetomotive force and asymmetric permeance, respectively, and consequently, the optimal designs are also acquired analytically. However, it is also found that the optimal choice of the proposed techniques depends on the combinations of stator slots and rotor poles of the machines. Finally, the effectiveness of the proposed approaches is verified by both finite element analytical predictions and experimental results.

This paper proposes a topology of special rotor-type permanent-magnet (PM) machine named rotor-PM flux-switching (RPM-FS) machine, which is evolved from stator-PM flux-switching (SPM-FS) topology and exhibits high power density, wide-range speed regulation, and good overload capability. The operation principle of RPM-FS machines is investigated based on the field modulation theory. It can be found that the torque is mainly contributed by the dominant harmonics in rotor-PM field and armature reaction field with the same order and rotating speed, i.e., 10, 14, and 34 pole pairs. In addition, a comprehensive comparison of modulation principles between the RPM-FS and SPM-FS machines is conducted from three perspectives, namely PM field, armature reaction field, and torque production mechanism. It is found that the modulation principle not only reveals the similarities of operation principle between two PM-FS machines, but also provides guidance for the combination of stator slots and rotor poles. Further, the electromagnetic torque produced by the dominant harmonics in two machines is verified by finite-element analysis and experiments.

This paper proposes a novel modular spoke-type permanent-magnet machine for in-wheel traction applications. First, the topology and the operating principle are briefly introduced. Then, an analytical model of the proposed machine is deduced based on conformal mapping. A three-phase 48-slot 52-pole modular spoke-type permanent-magnet (PM) machine is designed based on the deduced mathematical model where the dimensions and power source are constrained the same as those of a commercial three-phase surface PM machine in electric motorcycles. Electromagnetic performance comparisons among the proposed machine, the commercial machine, and a conventional spoke-type PM machine are conducted based on finite-element analysis (FEA) with respect to magnetic field, back electromotive force (EMF), flux-weakening capability, torque capability, machine efficiency, etc. The results indicate that the proposed machine has an improved torque, a higher efficiency, and an extremely better flux-weakening capability than the conventional in-wheel machine. Finally, a prototype machine is manufactured and tested to verify both the analytically and FEA predicted results.

There is increasing popularity of using permanent magnet (PM) machines in vehicle propulsion systems. Due to the pre-excitation of PM, the machines could offer higher efficiency and torque density. However, their inherent torque ripple could be problematic for electric vehicles (EVs) due to low damping of torsional vibration which may affect passenger comfort. This can prohibit the use of PM machines to improve vehicle energy efficiency. This paper presents the application of resonant control (RC) to suppress the impact of the PM torque ripple, which aims to reduce the vibration of a vehicle. A prototype PM machine and driveline have been fitted to a light-duty off-road vehicle. Firstly, the drivetrain frequency response was analyzed. The main source of the vibration is identified as the 24th harmonic torque ripple of the 12-slot/8-pole PM machine, which originates from the cogging torque and air-gap flux harmonics. The vibration becomes more severe when the torque ripple frequency is close to the natural frequency of the drivetrain. Then, the RC in conjunction with PI controller (PIR) was introduced to be used at the outer speed loop, so that an accurate current reference signal can be generated for the inverter to mitigate the speed ripple. When compared to the conventional Proportional-Integral (PI) controller, PIR controller has demonstrated over 80% reduction in speed ripple, even when the torque ripple frequency is close to the natural frequency of the vehicle.

Axial flux permanent magnet (AFPM) machines are promising for hybrid electric vehicles (HEVs) due to the compactness, high torque density and high efficiency. However, poor thermal characterization leads to an over-sizing of these machines which ultimately compromises overall system efficiency. In this paper, the transient thermal behavior of all the components in the single sided AFPM machine are characterized in an accurate but computationally efficient lumped parameter thermal model (LPTM). For the first time, contact measurements on the rotor have been used in AFPM machines to demonstrate the ability of the model to predict all component temperatures to within 4 °C for steady state. The mean temperature error over a load step transient was less than 5°C with a maximum error less than 13.5 °C which was for the winding. The model has a running time of approximately 1000 times faster than real time on a desktop machine and is suitable for integration into system simulation tools and predictive control strategies to avoid over-sizing of the motor and improve the usage of the electric machine in dynamic duty cycles.

This paper compares two flux-switching machines, namely, one stator permanent magnet flux-switching machine and one rotor permanent magnet flux-switching (RPM-FS) machine, with the same overall dimensions, main material properties, and current density. The comparison of characteristics is conducted from two perspectives, i.e., the electromagnetic torque production mechanism and torque (power)-sizing equation. The contribution of harmonics to average electromagnetic torque is analyzed based on the modulation principle and gearing effect, which reflects the similarities and differences between two FS machines in torque production mechanism. Moreover, torque performances are investigated from the viewpoints of magnetic parameters and electrical parameters. Then, electromagnetic performances including overload capability, flux-weakening capacity, and efficiency are analyzed and compared further. The predicted results indicate the RPM-FS machine exhibits a larger torque capability, a lower torque ripple, and improved flux-weakening capacity. The finite-element analysis predicted results are validated by experiments on two prototype machines.