Radial Flux Permanent Magnet Machine Research Articles

A Review of Axial Flux Permanent Magnet Machine Technology

Axial flux permanent magnet machines (AFPM) are popular for applications that benefit from high torque density and an axially compact form factor, such as in-wheel traction drives. Although the radial flux permanent magnet machine (RFPM) and the AFPM work based on the same underlying principle, the differences in their geometry introduce complexities in analysis of the AFPM. In this paper, the different AFPM design variants, their sizing approaches, computationally efficient design optimization techniques, and manufacturing techniques reported in literature are reviewed. In addition to classical AFPM machines, emerging variants and research opportunities with potential to push the boundaries of electric machine technology are reviewed. These include bearingless AFPMs, magnetically geared AFPMs, and combined radial-axial flux machines.

Equivalent Transformation Between Axial- and Radial-Flux Permanent-Magnet Machines

In this paper, an equivalent transformation approach for permanent-magnet (PM) electrical machines using coordinate transformation theory is presented. With the proposed method, an axial-flux PM (AFPM) machine can be transformed to an equivalent radial-flux PM (RFPM) machine or vice versa. The main purpose of proposing such a transformation method is to simplify the design of AFPM for specific applications. Besides, according to the working space requirement, the shape of PM machines can be changed while maintaining magnetic performances. The proposed transformation theory is based on the basic magnetic theory and the theoretical derivation is presented in detail. The resulted transformation equations for RFPM and AFPM machines with surface-mounted PMs are friendly to engineers. Moreover, an AFPM machine is used to validate the proposed theory, both FEM and experimental results have good agreement with the results of the proposed method. With the equivalent transformation theory, engineers could design, simulate and evaluate the AFPM machines with an exist well-developed machines.

Comparison of Thermal Characteristics in Various Aspect Ratios for Radial-Flux and Axial-Flux Permanent Magnet Machines

This paper proposes an axial-flux permanent magnet machine (AFPM) that uses a neodymium-bonded permanent magnet (Nd-bonded PM) and a coreless rotor structure and assesses how it performs in industrial applications requiring high efficiency and low cost. The AFPM is able to achieve high efficiency in high-speed regions because its Nd-bonded PM can restrain eddy current loss. Additionally, the AFPM can continuously function at the rated operating point without a cooling system, so the expenses for one can be eliminated. Hence, the AFPM can simultaneously achieve high efficiency and low-cost mass production. In this paper, a comprehensive comparison including the thermal characteristics of the proposed AFPM and a conventional radial-flux permanent magnet machine (RFPM) is shown. Furthermore, this paper also focuses on how the aspect ratio influences the thermal characteristics of both machines. Finally, the effectiveness of the proposed AFPM in industrial applications is assessed using 3D-FEA and experimental results.

Analysis of a Combined-Type Axial-Flux Permanent Magnet Machine

A combined-type axial-flux permanent magnet machine is proposed in this paper. The structure of the proposed machine is introduced. The torque density characteristics of the proposed combined-type axial-flux permanent magnet machine are studied, and the torque density advantages of this kind of machine are analyzed by comparing with radial-flux permanent magnet machines. The magnetic field analysis method of the machine is studied. In the process of magnetic field analysis, an air gap permeance element is proposed to improve the traditional magnetic equivalent circuit method, which can facilitate the establishment of magnetic equivalent circuit model at different times. The calculation results of the proposed method are verified by finite element method. The cogging torque optimization of this machine is investigated, which includes magnet skewing with different reference modes. The optimal magnet skewing angle is calculated and verified, and the characteristics of different cogging torque optimization methods are summarized.

Modeling, Investigation, and Minimization of AC Winding Loss in Slotless PM Machines

The AC winding loss can be the primary source of losses in slotless permanent magnet (PM) machines. This paper presents the simplified model, investigation, and reduction method of the AC winding loss in slotless radial flux PM machines. Preferentially, the open-circuit AC winding loss is calculated in the rotor reference frame based on the resistance limited. Subsequently, a fast optimization model, including the AC winding loss, is developed by solving the field equations only once for a specific geometry. As a result, the influence of the design parameters on the AC winding loss can be investigated by taking advantage of the fast computational model. It shows that the concentrated winding with multiples of 2-pole/3-coil is the best selection to achieve the low AC winding loss due to the possibility of using multiple parallel paths. Finally, the finite element (FE) and experimental results demonstrate the validity of the proposed analysis.

General Analytical Modeling for Magnet Demagnetization in Surface Mounted Permanent Magnet Machines

This paper proposes an analytical model for the prediction of airgap magnetic field distribution for axial flux permanent magnet (AFPM) machine with partial magnet demagnetization. The AFPM machine geometry is first converted to a polar representation. Consequently, the subdomain model based on current sheet technique is developed. Then current sheet representation of PM is derived to consider the partial demagnetization using superposition principle. The back electromagnetic forces and cogging torque are obtained accordingly based on Maxwell's equations. The results show that the results of proposed approach agrees with that of finite-element method. The model is further validated by experiments under both healthy and demagnetized conditions, which can validate the proposed approach. Main contribution of the work is to consider the partial irreversible demagnetization. Moreover, the proposed method is applicable for both AFPM and radial flux permanent magnet machine.

Application of Soft Magnetic Composite Material in the Field of Electrical Machines

The property of the soft magnetic composite (SMC) material is introduced. Take the traditional material used for electric machine, i.e. the silicon steel lamination, as the reference, this paper summarizes the application advantages of SMC in the typical electric machine, such as tubular permanent-magnet liner machine, high-speed permanent-magnet machine, claw-pole machine, axial flux permanent-magnet machine and radial flux permanent-magnet machine. SMC material has much applied potential in the field of electric machine.

Dual-Rotor Multiphase Permanent Magnet Machine With Harmonic Injection to Enhance Torque Density

Torque density of a novel dual-rotor, radial-flux multiphase permanent magnet (PM) machine is enhanced by injecting stator current harmonics without increasing stator yoke thickness. In order to enhance a regular PM machine torque capability by injecting harmonic currents, the no-load air-gap flux density should be designed to be trapezoidal, which increases the flux per pole, and results in a thicker stator yoke. However, this drawback can be avoided in a dual-rotor, radial-flux PM machine. In such a dual-rotor PM machine, the main flux can be designed to go directly from the inner rotor poles, to stator inner and outer teeth, then to the outer rotor poles, or reverse, without through the stator yoke. Therefore, the stator yoke thickness is not affected by air-gap flux at all and can actually be reduced to minimum for mechanical reasons. This paper applies stator current harmonic injection technique to a dual-rotor, radial-flux PM machine. Sizing equations of the multiphase PM machines are deduced and used for initial design and theoretical comparison. The torque performance of the proposed dual-rotor, radial-flux multiphase PM machine with stator harmonic current injection is evaluated using finite element simulation, and compared to a traditional three-phase dual-rotor, radial-flux PM machine. Simulation results validate the feasibility of torque density enhancement. The same technique is proven that it is also suitable in a dual-rotor axial-flux PM machine topology.

Minimization of Cogging Torque in Axial-Flux Permanent-Magnet Machines: Design Concepts

Various techniques exist for reducing the cogging torque in radial-flux permanent-magnet (RFPM) machines. However, although some of these can be applied to axial-flux PM (AFPM) machines, the additional manufacturing complexity and cost may be prohibitive. Therefore, alternative low-cost techniques are desirable for use in AFPM machines. In this paper, the utility of employing cogging torque minimization techniques that have been developed principally for use in RFPM machines is examined by 3-D finite-element analysis, and several alternative cogging torque minimization techniques for AFPM machines are proposed.