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The field modulation magnetic gear is a kind of transmission device with broad development prospects. It has the advantages of no friction， no pollution， less maintenance and easy installation. The magnetic gear models with different transmission ratio are established. The input and output torque waveforms of different models are compared. The influences of the number of pole pairs of inner rotor （P1） and the number of pole pairs of outer rotor （P2） on torque ripple are analyzed. According to the principle of magnetic field modulation and spatial magnetic field harmonic distribution， the torque ripple of magnetic gear is greatly affected by P1 and P2. The research results show that the torque ripple can be effectively reduced by selecting the magnetic gear with P1=4， P1:P2=1:n+0.25 or 1:n+0.75 （n is a natural number）.

In the transmission process of coaxial magnetic gear, aiming at the problems of large torque ripple and high magnetic density in air-gap, a magnetic gear with thin metal sheet is studied. The structure fixates a layer of conductive and non-magnetic metal material respectively on the internal and external permanent magnets of the rotor in the magnetic gear. A magnetic gear model with 4 opposite poles for inner rotor and 21 opposite poles for outer rotor is established via finite element method. The internal and external air-gap magnetic field and torque of the magnetic gear are calculated. Meanwhile, high magnetic density and torque ripple are analyzed. The results demonstrate that the magnetic gear with metal sheet can not only reduce the high-order harmonic components of magnetic density but also reduce the torque ripple. A reference for the improvement of the stability in the magnetic gear transmission is provided.

The magnetic gear has the advantages of no friction between the rotor,simple manufacturing and installation,low cost and so on,it is an excellent transmission mechanism. The development history of magnetic gears from time to time is briefly described,and the research status of magnetic gears at home and abroad is compared. The topological structure of several kinds of common magnetic gears are introduced,and the advantages and disadvantages of these kinds of magnetic gears are compared. Its future development direction and research questions are summarized.

Comparing with the mechanical gear contact transmission,magnetic gear is a new transmission device with a novel structure and a broad development prospect. The large torque transmission is achieved by using the magnetic field modulation principle. On the basis of the traditional magnetic gear structure,the inner and outer rotor permanent magnets are improved,which the inner permanent magnets are triangular-fan-shaped phase and the inner and outer permanent magnets are Halbach arrays. The two dimensional（ 2 D） finite element（ FE） method is used to simulate the modified magnetic gear model,and the electromagnetic field and electromagnetic torque of the magnetic gear are calculated. At the same time,the magnetic field and torque are compared with the conventional magnetic gear. The simulation results show that the output torque of the improved magnetic gear is greatly than the traditional magnetic gear,which can effectively enhance the air gap magnetic density and reduce the air gap harmonic content. It is a great significance to study and apply the torque density of concentric magnetic gear.

The concentric magnetic gear is a transmission gear with a novel structure and a promising prospect,by using the magnetic field modulation principle,the large torque transmission is realized. On the basis of concentric magnetic gear structure,the modulation magnet ring is improved,the silicon steel is added on the inside of the modulation magnet ring to constitute a mechanical bridge. The model of the improved magnetic gear is simulated by using two dimensional（2D） finite element（FE） method,and the magnetic field and electromagnetic torque are calculated. At the same time,the magnetic field and torque are compared with the common gear. The results show that improved magnetic gear magnetic field is higher than normal- type magnetic gear,lower harmonic content,and torque is also improved. It is significant for the research and application of the concentric magnetic gear.

An eccentric harmonic magnetic gear (EHMG) is better suited for situations requiring larger transmission ratios than magnetic-field-modulated magnetic gears. In the meantime, to increase the torque density even further, a new structure for EHMGs is presented in this paper. The stator's permanent magnets (PMs) are irregularly distributed, while the rotor's PMs are applied to a fan-shaped structure. Moreover, a Halbach array is adopted in both the rotor and the stator. A two-dimensional finite element (FE) model of the proposed EHMG is developed, and the flux density distribution and torque of the EHMG are calculated and verified via FE analysis. When compared to a conventional EHMG, the presented model's torque increases from 38.04 Nm to 50.41 Nm. In addition, for the sake of avoiding the oscillation and noise caused by resonance, a modal analysis of the proposed model is conducted and the consequences show that it has better antivibration properties. Finally, a prototype is made, a test bench is established, and the correctness and effectiveness of the proposed model are verified. [ABSTRACT FROM AUTHOR]