This paper overviews the recent development and new topologies of flux-switching (FS) machines, with particularly emphasis on the permanent magnet (PM) type. Specific design issues, including winding configurations, combinations of stator and rotor pole numbers, rotor pole width, split ratio, etc., are investigated, while the torque capability of selected FSPM machines is also compared.

Advanced Flux-Switching Permanent Magnet Brushless Machines

Vernier permanent-magnet (VPM) machines are well known for high torque density but low power factor. This paper deals with the low power factor of VPM machines. The goal is not obtained by reducing the electrical loading or adjusting current advance angle but by proposing a novel vernier topology, i.e., a dual-stator spoke-array (DSSA) VPM topology. In this paper, the characteristics of the DSSA VPM topology, such as active part, auxiliary mechanical structure, and rotor anisotropy, are analyzed in detail. Performances are evaluated based on finite-element analysis, including power factor, torque density, and cogging torque. The results show that the DSSA VPM topology exhibits high power factor, viz., ~0.9, and significantly high torque capability. The verification of the mechanical structure scheme is also done in this paper. Finally, theoretical analyses are validated by the experimental results by a 44-rotor pole 24-slot DSSA VPM prototype.

High-Power-Factor Vernier Permanent-Magnet Machines

In this paper, switched flux permanent magnet (SFPM) machines are analyzed from the perspective of the air-gap field harmonics. It is found that the modulation of the salient rotor to PM and armature reaction fields in SFPM machines is similar to that of the iron pieces to those fields in the magnetic gear and magnetically geared machine. The magnetic gearing effect is analyzed in SFPM machines with different stator/rotor pole combinations, winding configurations, and stator lamination segment types by a simple magnetomotive force-permeance model, and validated by finite-element (FE) analysis. Different from fractional-slot surface-mounted PM machines in which the working air-gap field harmonic generates 95% of the average electromagnetic torque, 95% of the average electromagnetic torque in SFPM machines having ps stator pole pairs and n
 r
 rotor poles are contributed by several dominating field harmonics, i.e., rotating ones with |kn
 r
 ± (2i - 1)p
 s
| pole pair (k = 1, i = 1, 2, 3) and static ones with (2i - 1)ps pole pair (i = 1, 2, 3). The FE predicted average static torques in SFPM machines are validated by measurements on prototype machines.

Analysis of Air-Gap Field Modulation and Magnetic Gearing Effects in Switched Flux Permanent Magnet Machines

Vernier permanent-magnet (VPM) machines have been obtaining a lot of attention over the past few years due to several advantages, such as their high torque density and simple mechanical structures. Moreover, it is found that the torque ripple of VPM machines is ultralow, even without specific design measures such as a short pitch, skewing slots/poles, magnet shaping technology, etc. This paper presents theoretical analysis and comprehensive simulations on the torque ripple of VPM machines. First, a general instantaneous torque equation of VPM machines is proposed to analyze torque features and the effect of parameters on the torque performance of VPM machines. Subsequently, based on the general torque equation and a finite-element algorithm, it is verified that torque smoothness is the inherent characteristic of VPM machines, and the torque ripple of VPM machines can be below 0.2%. Furthermore, it is demonstrated that the torque density of a VPM machine is 40% larger than that of a regular permanent-magnet machine. All these advantages demonstrate that VPM machines can obtain much better steady and dynamic drive performance. Finally, all the theoretical analyses are verified by experiments on a VPM prototype.

Analysis of Torque Capability and Quality in Vernier Permanent-Magnet Machines

The paper deals with the possibility of replacing a mechanical planetary gear system by a magnetic planetary gearbox. The magnetic planetary gearbox has many advantages such as contact-free, no gear lubrication, high-speed-reduction ratio and high durability. Firstly, the principle of operation, design equations and Willis relation for the magnetic and the mechanical planetary gears are given. A comparative study between the two systems in terms of torque transmission capabilities is presented.

Comparative Study Between Mechanical and Magnetic Planetary Gears

In this paper, a novel direct-drive double rotor Vernier panent magnet (DR-VPM) machine is proposed and analyzed. The key of the design is to incorporate two concentric rotors and the Vernier structure within one permanent magnet (PM) machine, while keeping the machine volume and slot number unchanged. The main merits of this proposed machine are its compact structure, improved torque density, reduced stator end winding length, and reduced copper loss. The operating principle of the machine is discussed and its steady and transient performances are analyzed using circuit-field-motion coupled time-stepping finite element method (CFM-TS-FEM). A dual-excitation PM Vernier (DE-VPM) machine and a single outer rotor PM Vernier (SR-VPM) machine are designed and compared with this proposed Vernier PM machine using CFM-TS-FEM. Comparison results as reported are used to confirm and validate the advantageous performance of the proposed machine.

Quantitative Comparison of Novel Vernier Permanent Magnet Machines

High-torque and low-speed electrical drives are often employed for applications where mechanical gearing cannot be accommodated. On the other hand, permanent-magnet (PM) gear has drawn significant attention from both academies and industries due to the conspicuous merits, such as reduced acoustic noise, maintenance free, improved reliability, precise peak torque transmission capability, and inherent overload protection. In this paper, a magnetic-geared PM brushless motor is presented. It is a novel low-speed and high-torque motor which merges the advantages of conventional PM brushless motor and PM gear. Its topology and operation principle are introduced. Some techniques are employed to optimize and improve the motor performance, while the validity of the proposed techniques is verified with finite-element analysis. Moreover, an alternative operation condition, which can further reduce the motor speed and increase its output torque, is proposed and analyzed.

Development of a Magnetic-Geared Permanent-Magnet Brushless Motor

The permanent magnet (PM) flux-switching linear motor, which is developed from the PM flux-switching rotary machine, possesses similar advantages such as high power density and simple structure. Furthermore, the PM flux-switching linear motor has a potential for mass production at low cost, since the expensive coils and magnets are both set on the short mover. However, the detent force which is induced by both slot-effect and end-effect deteriorates the motor performance. Some methods to reduce detent force which are effective in PM linear synchronous motors do not work in PM flux-switching linear motors, due to their special structure. This paper describes a new method to reduce the detent force by fixing assistant teeth on the mover ends. Finite element analysis shows some interesting results which will be detailed in this paper.

A New Method for Reduction of Detent Force in Permanent Magnet Flux-Switching Linear Motors

The invention provides a birotor permanent magnet motor and a washing machine with the same. The birotor permanent magnet motor comprises an inner rotor, a stator and an outer rotator which are sequentially arranged from inside to outside, and is characterized in that both the inner rotor and the outer rotor are permanent magnet rotors, and the stator comprises a magnetic modulation ring and a stator winding arranged on the magnetic modulation ring in a wound mode. The birotor permanent magnet motor is based on the magnetic field modulation principle and has a birotor structure, the outer rotor and the inner rotor can rotate according to certain proportional relation, and high-performance output of the motor under operating conditions of a low speed and a high speed can be met at the same time. When the birotor permanent magnet motor drives the washing machine to operate, not only are design requirements for low-speed large torque output and high-speed small torque output of the motor under the optimal operating state met, but also the structures of a belt pulley, a reduction gearbox and the like in a traditional washing machine are eliminated, transmission efficiency of the motor is improved, and meanwhile the defects of high noise, machine body vibration and the like of the traditional washing machine are overcome.

Dual-rotor permanent magnet motor and washing machine

In this paper, a multi-power-terminals permanent magnet (PM) machine with magnetic field modulation is proposed. The advantage of this new integrated configuration includes not only the characters of permanent magnet synchronous motor (PMSM), but also the features of the no-contact transmission permanent magnet gear. Its topology and operation principle are introduced. Some techniques are employed to optimize and improve the motor performance, whilst validity of the proposed techniques is verified with finite element analysis (FEA).

Lili Wang

Papers

Quantitative Comparison of Novel Vernier Permanent Magnet Machines

Development of a Magnetic-Geared Permanent-Magnet Brushless Motor

A New Method for Reduction of Detent Force in Permanent Magnet Flux-Switching Linear Motors

Dual-rotor permanent magnet motor and washing machine

Design of a multi-power-terminals permanent magnet machine with magnetic field modulation