Junjie Jiang

Design and analysis of interior-magnet outer-rotor concentric magnetic gears

In this paper, a new topology of concentric magnetic gears is proposed and implemented. The key of the new topology is to bury permanent magnets (PMs) of the outer rotor into the iron core in a new way so that the mechanical integrity can be improved, and the PM material can be saved while the torque density is maintained. The proposed gear is designed with the speed reduction ratio of 7.33 and optimized by using the three-dimensional finite element method (3D-FEM). The key of the 3D-FEM is to employ scalar magnetic potential to reduce the required memory and time for data manipulation and computation. After prototyping, the measured maximum static torque well agrees with the calculated one, hence verifying the proposed design and analysis.

A flux-mnemonic permanent magnet brushless motor for electric vehicles

In this paper, a new permanent magnet (PM) brushless motor is proposed for electric vehicles. The key is to incorporate the concept of memory motors, namely, the online tunable flux-mnemonic PMs, into the stator doubly fed doubly salient PM motor, hence achieving effective air-gap flux control. By further employing the outer-rotor and double-layer-stator topology, the proposed motor takes the definite advantages of compact structure, low armature reaction, and direct-drive capability. Increasingly, this motor can offer the unique features of pole dropping and pole reversing. Finite element analysis and, hence, computer simulation are given to verify the validity of the proposed motor.

A flux-mnemonic permanent magnet brushless machine for wind power generation

In this paper, the concept of flux mnemonics is newly extended to the wind power generator. By incorporating a small magnetizing winding into an outer-rotor doubly salient AlNiCo permanent magnet (PM) machine, a new flux-mnemonic PM brushless wind power generator is proposed and implemented. This generator can offer effective and efficient air-gap flux control. First, the characteristics of the proposed generator are analyzed by using the finite element method. Second, the closed-loop flux control is devised to achieve a constant generated voltage under time-varying wind speeds. Finally, the experimental results are given to verify the validity of the proposed generator and control system.

Dynamical spin reorientation transition in NdFeO3 single crystal observed with polarized terahertz time domain spectroscopy

Temperature-induced spin reorientation transition in NdFeO3 single crystal is studied by terahertz time-domain spectroscopy in the temperature range from 50 K to 290 K. Following the resonant excitation of quasi-antiferromagnetic (AF) mode, the nature of temperature dependence of emission from AF-mode is investigated systematically in the spin reorientation temperature interval. The emission frequency is observed at 0.485 THz for both Γ4 and Γ24 phases, and it shifts abruptly to 0.456 THz (around 110 K) corresponding to Γ2 phase. The evolution of vector G is obtained from the temperature-dependent polarization changes of the AF-mode excitation. Our results demonstrate that the polarized terahertz time-domain spectroscopy is a sensitive tool to explore the dynamical spin reorientation transition in RFeO3 crystals, and the terahertz magnetic pulse shows potential application for non-thermally manipulating ultrafast spin reorientation.

Design of permanent magnet brushless motors with asymmetric air gap for electric vehicles

This paper proposes a cost-effective approach to design permanent magnet brushless dc motors for electric vehicles. The key is to shape the pole arc in such a way that the air gap length is at a maximum at the leading edge of each rotor pole arc and at a minimum at the trailing edge of the same pole arc, hence resulting in an asymmetric air gap. Thus, for a specified rotational direction, the distortion of air gap flux density and hence the torque ripple can be significantly suppressed. Also, with the use of advanced conduction angle control, the motor can achieve a wide speed range. The proposed motor drive is designed and implemented for a low-voltage battery-powered electric motorcycle.

Design and analysis of a transverse flux permanent-magnet machine using three-dimensional scalar magnetic potential finite element method

In this paper, a new transverse flux permanent-magnet machine is proposed and implemented. It features a unique configuration that it is composed of assembled stators and flux-concentrating rotor, hence offering low manufacturing cost while retaining high torque density and low cogging torque. Because of its unique configuration, the proposed machine is analyzed by a newly developed three-dimensional scalar magnetic potential finite element method. Both calculated and experimental results are given to support the validity of the proposed design and analysis.

Selective excitation of spin resonance in orthoferrite PrFeO3 with impulsive polarized terahertz radiation

Single cycle terahertz (THz) pulses were employed to excite coherent spin waves in (110)-oriented PrFeO3 single crystal. The free induction decay radiations at frequency of 0.34 THz (quasi-ferromagnetic mode, FM mode) and 0.41 THz (quasi-antiferromagnetic mode, AFM mode) were observed arising from the coupling of magnetic moment with the impulsive magnetic field of polarized terahertz radiation. These two spin modes in PrFeO3 can be excited and modulated by the magnetic field of THz pulse with a specific polarization with respect to the crystal axis. The extracted complex dielectric permittivity and magnetic permeability dispersion in THz range suggest a higher efficiency of energy transfer from the impulsive THz pulse into the AFM than the FM spin system.

Terahertz probes of magnetic field induced spin reorientation in YFeO3 single crystal

Using the terahertz time-domain spectroscopy, we demonstrate the spin reorientation of a canted antiferromagnetic YFeO3 single crystal, by evaluating the temperature and magnetic field dependence of resonant frequency and amplitude for the quasi-ferromagnetic (FM) and quasi-antiferromagnetic modes (AFM), a deeper insight into the dynamics of spin reorientation in rare-earth orthoferrites is established. Due to the absence of 4f-electrons in Y ion, the spin reorientation of Fe sublattices can only be induced by the applied magnetic field, rather than temperature. In agreement with the theoretical predication, the frequency of FM mode decreases with magnetic field. In addition, an obvious step of spin reorientation phase transition occurs with a relatively large applied magnetic field of 4 T. By comparison with the family members of RFeO3 (R = Y3+ or rare-earth ions), our results suggest that the chosen of R would tailor the dynamical rotation properties of Fe ions, leading to the designable spin switching ...

Design of a novel phase-decoupling permanent magnet brushless ac motor

This paper presents a phase-decoupling permanent magnet brushless ac motor which can offer better controllability, faster response, and smoother torque than its counterparts. The key is due to its different motor configuration and simple scalar control. The motor configuration is so unique that it inherently offers the features of phase decoupling, flux focusing, and flux shaping, hence achieving independent phase control, fast response, and smooth torque. The scalar control is fundamentally different from the complicated vector control. It can achieve direct torque control through independent control of the phase currents. The proposed motor is prototyped and experimentally verified.

Magnetic-field dependence of strongly anisotropic spin reorientation transition in NdFeO3: a terahertz study

One of the biggest challenges in spintronics is finding how to switch the magnetization of a material. One way of the spin switching is the spin reorientation transition (SRT), a switching of macroscopic magnetization rotated by 90°. The macroscopic magnetization in a NdFeO3 single crystal rotates from Γ4 to Γ2 via Γ24 as the temperature is decreased from 170 to 100 K, while it can be switched back to Γ4 again by increasing the temperature. However, the precise roles of the magnetic-field induced SRT are still unclear. By using terahertz time-domain spectroscopy (THz-TDS), here, we show that the magnetic-field induced SRT between Γ4 and Γ2 is strongly anisotropic, depending on the direction of the applied magnetic field. Our experimental results are well interpreted by the anisotropy of rare-earth Nd(3+) ion. Furthermore, we find that the critical magnetic-field required for SRT can be modified by changing the temperature. Our study suggests that the anisotropic SRT in NdFeO3 single crystal provides a platform to facilitate the potential applications in robust spin memory devices.