Magnetic-field-modulated Brushless Double-rotor Machine Research Articles

Investigation of the Power Factor of Magnetic-Field Modulated Brushless Double-Rotor Machine

The magnetic-field modulated brushless double-rotor machine (MFM-BDRM), getting rid of brush and slip ring, is a preferable selection for the power splitting device in hybrid electric vehicles. In practical application, however, the MFM-BDRM is facing the challenge of low power factor owing to its special operating principle—the magnetic field modulation principle. Although some researchers dedicate themselves to improving the power factor of MFM-BDRM, the cause of low power factor in the MFM-BDRM is still not clear. This article will investigate the power factor problem of MFM-BDRM, systematically. First, the inductance feature of MFM-BDRM based on the magnetic field modulation principle is investigated. Second, the fundamental cause of the low power factor of MFM-BDRM is analyzed. Furthermore, three feasible attempts are made and investigated to improve the power factor of MFM-BDRM. Finally, a prototype of MFM-BDRM is manufactured, and parts of the analysis are verified by experiments.

Design and Analysis of a Magnetic-Field Modulated Brushless Double-Rotor Machine—Part I: Pole Pair Combination of Stator, PM Rotor and Magnetic Blocks

The magnetic-field modulated brushless double-rotor machine (MFM-BDRM) is a promising candidate for the power splitting device in hybrid electric vehicles. Due to its special operating principle, the magnetic field modulation principle, various harmonic magnetic fields are produced in the MFM-BDRM. Therefore, it is crucial for the design of MFM-BDRM to reduce the influence of nonworking harmonic magnetic fields on the electromagnetic performance of MFM-BDRM, like back electromotive force (EMF) and torque ripple. In this paper, first, the distribution law of permanent-magnet (PM) and armature modulated magnetic fields is analyzed. Second, since the analysis and calculation method of harmonic back EMF in a traditional PM machine is not suitable for the MFM-BDRM, the formation mechanism of harmonic back EMF in the MFM-BDRM is further investigated. Third, combined with the characteristic of modulated magnetic field and formed harmonic back EMF in the MFM-BDRM, the optimal pole pair combination of stator, PM rotor, and magnetic blocks is investigated. Finally, analysis and calculation of a specific MFM-BDRM by analytical and finite- element method shows the MFM-BDRM with the optimal combination can obtain very sinusoidal no-load and load back EMF and quite low torque ripple.

Design and Analysis of a Magnetic-Field Modulated Brushless Double-Rotor Machine—Part II: Winding Configuration

The analysis of Part I shows that the integer-slot winding of the magnetic-field modulated brushless double-rotor machine (MFM-BDRM) can obtain very sinusoidal no-load and load back electromotive force (EMF) and quite small torque ripple under an optimal pole-pair combination of stator, permanent magnet (PM) rotor, and magnetic blocks. In practical applications, however, the integer-slot winding is usually replaced by the fractional-slot concentrated winding (FSCW), which can reduce end windings and easily realize automatic winding. This paper focuses on the influence of the FSCW on the electromagnetic performance of the MFM-BDRM and the application feasibility of the FSCW in the MFM-BDRM. First, the formation mechanism of the harmonic back EMF of FSCW is investigated. Then the pole-pair combination law of stator, PM rotor, and magnetic block for the FSCW of the MFM-BDRM is further investigated. Second, the specific FSCW of the MFM-BDRM with $Q = 2{p_s} \pm 1$ and $Q = 2{p_s} \pm 2$ and the integer-slot MFM-BDRM are comparatively investigated. It shows that the integer-slot winding of the MFM-BDRM is superior to the FSCW of the MFM-BDRM in no-load and load back EMF, torque ripple, maximum torque outputting capability, and power factor. Finally, the optimal winding configuration of the MFM-BDRM is determined. A prototype of the MFM-BDRM with the optimal winding configuration and pole-pair combination is designed and manufactured. The theoretical and finite-element analysis is verified by experiments.

Low-Loss Design and Analysis of Magnetic-Field Modulated Brushless Double-Rotor Machine

Due to the existence of large permanent magnet (PM) loss caused by its magnetic-field modulated (MFM) rotor in a traditional MFM brushless double-rotor machine (MFM-BDRM), a low-loss MFM-BDRM with the sinusoidal permeance characteristic of an MFM rotor is proposed and investigated in this paper. Since fan-shaped magnetic and nonmagnetic blocks in traditional MFM rotor cause many harmonic permeances and further produce large harmonic loss in the PMs, the ideal sinusoidal permeance model of an MFM rotor is first established and investigated by the analytical method. Considering that tangential leakage flux among magnetic blocks in the analytical method is difficult to calculate accurately, the influence of key parameters of the proposed MFM rotor, like the span ratio of magnetic block, minimum length of inner and outer air gap, on the electromagnetic performance of MFM-BDRM is further investigated and optimized by the finite-element method. Finally, to completely evaluate the performance of the proposed MFM-BDRM, a traditional MFM-BDRM with the same size is designed to compare performances under different operating conditions.

Research on a novel axial-flux magnetic-field-modulated brushless double-rotor machine with low axial force and high efficiency

The axial-flux magnetic-field-modulated brushless double-rotor machine (MFM-BDRM) is a possible alternative as a power-split device for hybrid electric vehicles (HEVs). However, the existence of large axial force may lead to assembly problems and rich inner air-gap harmonics could result in high PM loss and low efficiency. This paper proposes a novel axial-flux MFM-BDRM with improved PM rotor structure. 2-D analytical method to predict the magnetic-field distribution of the proposed MFM-BDRM is developed and the design procedure of the proposed machine is illustrated. The impact of key geometrical parameters on axial force and torque is investigated. To evaluate the advantage of the proposed machine, a comparison is made with a conventional one with respect to electromagnetic performances. Results show that the proposed machine is effective in reducing PM eddy loss and axial force by 60% and 35%, respectively.

Research on a new magnetic-field-modulated brushless double-rotor machine with sinusoidal-permeance modulating ring

The magnetic-field-modulated brushless double-rotor machine (MFM-BDRM), composed of a stator, a modulating ring rotor, and a PM rotor, is a kind of power-split device for hybrid electric vehicles (HEVs). In this paper, a new MFM-BDRM with sinusoidal-permeance modulating ring named Sinusoidal-Permeance-Modulating-Ring Brushless Double-Rotor Machine (SPMR-BDRM) is proposed to solve the problem of poor mechanical strength and large iron loss. The structure and the operating principle of the MFM-BDRM are introduced. The design principle of the sinusoidal-permeance modulating ring is analyzed and derived. The main idea of that is to minimize the harmonic permeance of air gap, thereby the harmonic magnetic fields can be restrained. There are comparisons between a MFM-BDRM with sinusoidal-permeance modulating ring and a same size MFM-BDRM with traditional modulating ring, including magnetic field distributions and electromagnetic performances. Most importantly, the iron losses are compared under six different conditions. The result indicates that the harmonic magnetic fields in the air gap are restrained; the electromagnetic torque and power factor are almost the same with same armature current; the torque ripples of the modulating ring rotor and the PM rotor are reduced; the stator loss is reduced by 13% at least and the PM loss is reduced by 20% at least compared with the same size traditional MFM-BDRM under the same operating conditions.

Analytical Investigation of the Magnetic-Field Distribution in an Axial Magnetic-Field-Modulated Brushless Double-Rotor Machine

The axial magnetic-field-modulated brushless double-rotor machine (MFM-BDRM) is a novel possible alternative power-split device for hybrid electric vehicles (HEVs). This paper proposes a two-dimensional (2-D) analytical method to predict the performance of the axial MFM-BDRM to reduce computing time. The computation is based on the solution of Laplace’s or Poisson’s equation with boundary conditions for each elementary rectangular region. By taking account of the existence of modulating ring and the stator slotting effect, the proposed model is able to calculate magnetic-field distribution with high accuracy. In order to assess the proposed method, the 2-D analytical and three-dimensional (3-D) finite element analysis (FEA) results have been compared, and good agreements have been achieved. As the analytical computation is much faster and more flexible, the proposed method can be used in the preliminary design process of the axial MFM-BDRM.

Investigation Into a Magnetic-Field-Modulated Brushless Double-Rotor Machine With the High-Strength and Low-Loss Modulating Ring Rotor

Aiming at the contradictory problem of low loss and high mechanical strength for the modulating ring (MR) rotor in traditional magnetic-field-modulated brushless double-rotor machine (MFM-BDRM), an MFM-BDRM with the high-strength and low-loss MR rotor is proposed. The key of the proposed MR rotor is to insert the nonmagnetic metal bar into magnetic block made by the silicon sheet laminations. First, to evaluate the influence of the proposed MR rotor on the performance of MFM-BDRM, a traditional MFM-BDRM with the same size is designed to compare their performances, like back electromotive force, torque, and torque ripple. Second, since the permeability of silicon sheet is not infinite, there are still some flux leakages passing through the nonmagnetic metal bar. The loss distribution law of the nonmagnetic metal bar under different permeabilities is investigated by the finite-element method. Besides, to ensure the safety of the proposed MFM-BDRM, the stress distribution law of the MR rotor is investigated under conditions of maximum speed, torque, and radial electromagnetic force.

Investigation of a Magnetic-Field Modulated Brushless Double-Rotor Machine With the Same Polarity of PM Rotor

Owing to getting rid of brushes and slip rings, the magnetic-field modulated brushless double-rotor machine (MFM-BDRM) can be a promising candidate for the electrical-continuously variable transmission (e-CVT) in hybrid electric vehicles (HEVs). However, since there are many magnetic-field harmonics with the high rotating speed in the air gap, the PM loss is a big problem in traditional MFM-BDRMs [1]. Therefore, a new MFM-BDRM with the same polarity of PM rotor is proposed, which has advantages of reducing PM loss, saving PM material and keeping good electromagnetic performance. The magnetic field distribution in the air gap, the back electromagnetic force (EMF) and torque performance of the proposed MFM-BDRM are investigated. To obtain the optimum design scheme, the influence of PM pole-arc coefficient on the torque performance is investigated. Then the torque performance is further optimized by the span ratio of magnetic blocks. To evaluate the overall performance of this new MFM-BDRM, a traditional MFM-BDRM with the same size is designed to compare their performances.

Characteristic Analysis and Verification of the Magnetic-Field-Modulated Brushless Double-Rotor Machine

The magnetic-field-modulated brushless double-rotor machine (MFM-BDRM), composed of the stator, the modulating ring rotor, and the permanent-magnet (PM) rotor, is a new power-split device for hybrid electric vehicles (HEVs). Compared with traditional double-rotor machines (DRMs), the MFM-BDRM shows more complicated electromechanical energy conversion relations, due to its special operating principle—the magnetic field modulation principle. To analyze the speed relation in the MFM-BDRM, a diagrammatized method is proposed. It shows that the speeds of stator magnetic field, modulating ring rotor, and PM rotor present a collinear speed characteristic. On this basis, the torque relations of stator, modulating ring rotor, and PM rotor are investigated from the view of a conservative lossless system. Then, a lever-balanced torque map is proposed to analyze their torque characteristic. It shows that the torques of stator, modulating ring rotor, and PM rotor can be calculated by the lever balance principle. The power flow map is further proposed to analyze the power flow characteristic among three ports. In addition, comparison of the MFM-BDRM and the planetary gear shows that the MFM-BDRM can be totally equivalent to an electrical machine and a planetary gear, making it gain a great advantage particularly when the MFM-BDRM is used in HEVs. The electromagnetic performance of MFM-BDRM is investigated by a finite-element method, which shows that the MFM-BDRM has advantages of fine sinusoidal back electromotive force and low torque fluctuation. Finally, the speed and torque analysis and FE results are verified by experiment.

Research on an Axial Magnetic-Field-Modulated Brushless Double Rotor Machine

Double rotor machine, an electronic continuously variable transmission, has great potential in application of hybrid electric vehicles (HEVs), wind power and marine propulsion. In this paper, an axial magnetic-field-modulated brushless double rotor machine (MFM-BDRM), which can realize the speed decoupling between the shaft of the modulating ring rotor and that of the permanent magnet rotor is proposed. Without brushes and slip rings, the axial MFM-BDRM offers significant advantages such as excellent reliability and high efficiency. Since the number of pole pairs of the stator is not equal to that of the permanent magnet rotor, which differs from the traditional permanent magnet synchronous machine, the operating principle of the MFM-BDRM is deduced. The relations of corresponding speed and toque transmission are analytically discussed. The cogging toque characteristics, especially the order of the cogging torque are mathematically formulated. Matching principle of the number of pole pairs of the stator, that of the permanent magnet rotor and the number of ferromagnetic pole pieces is inferred since it affects MFM-BDRM’s performance greatly, especially in the respect of the cogging torque and electromagnetic torque ripple. The above analyses are assessed with the three-dimensional (3D) finite-element method (FEM).

Investigation of a Novel Radial Magnetic-Field-Modulated Brushless Double-Rotor Machine Used for HEVs

A novel brushless compound-structure permanent-magnet synchronous machine (CS-PMSM) with six different topologies is proposed based on the magnetic field modulation principle. As the key part of the brushless CS-PMSM, the radial magnetic-field-modulated brushless double-rotor machine (MFM-BDRM) is investigated on the speed and torque relations of the first-stator magnetic field, the first-PM rotor and the modulating ring rotor by analytical method. Besides, on the basis of analyses of magnetic field distribution in the inner and outer air gap, the back electromagnetic force and torque performance of the radial MFM-BDRM are further studied by finite-element method (FEM). The results indicate that the low power factor is a major problem of the radial MFM-BDRM. Therefore, the influence of parameters, such as combinations of magnetic block number and PM pole pair number, the span ratio and radial thickness of magnetic blocks, and the length of air gap, on the power factor is analyzed. Additionally, to investigate the distribution law of core loss in the radial MFM-BDRM, amplitudes and frequencies of the magnetic fields in each part of the machine are analyzed. Furthermore, considering the weak mechanical strength of modulating ring rotor employing DW310 laminations, an attempt to use iron instead of DW310 in modulating ring rotor is investigated.