This paper proposes a new type of dual-armature alternating pole bearingless magnetic flux reverse permanent magnet machine, which combines the advantages of the alternating pole structure and the dual-armature structure. This machine shows the advantages of high torque density and good fault tolerance, which can not only reduce the number of permanent magnets but also further improve the machine’s torque. It is suitable for use in wind power, aerospace, and other applications. The dual-armature alternating pole bearingless magnetic flux reverse permanent magnet machine is used to improve the torque performance and suspension force. Based on the dual-armature alternating pole magnetic flux reverse machine, by adding an additional set of stator teeth to suspended windings, a new type of dual-armature alternating pole bearingless magnetic flux reverse permanent magnet machine is obtained. The number of permanent magnets is halved, and each permanent magnet has the same polarity. The ferromagnetic pole piece next to the permanent magnet automatically acts as the other pole. Based on the introduction of its related structure, the stator flux, rotor flux, stator back electromotive force, cogging torque, electromagnetic torque, etc., are analyzed. This new machine can reduce the number of permanent magnets and has a higher torque conferring advantages of output capacity and low torque ripple.
I. INTRODUCTION
In recent years, stator permanent magnet brushless machines have received extensive attention from scholars due to their high torque density, wide speed adjustment capability, and high operational reliability.1,2 Among them, the dual-armature flux reversal permanent magnet (DAFRPM) machine is a typical topology of the stator permanent magnet brushless machine. The pivot windings are placed in the stator and rotor slots, which provides a good heat dissipation performance.3,4 A DAFRPM machine has three magnetic fields, namely, a permanent magnet magnetic field, stator armature magnetic field, and rotor armature magnetic field. Its operating principle is different from that of traditional magnetic flux reversing machines, and its magnetic field is more complicated. Therefore, the working principle and harmonics of the DAFRPM machine analysis are important. The permanent magnet structure of the DAFRPM machine is similar to that of a surface-mounted permanent magnet machine. It is mounted on the surface of the stator teeth, and the rotor is only composed of silicon steel sheets. The structure is simple and suitable for electric vehicles and hybrid electric vehicle drive systems.5 Bearingless machines have the characteristics of a high critical speed, low mechanical friction, no need for lubrication and maintenance, etc. Since their proposal, many related studies have been conducted.6,7 In the context of dual-armature alternating poles without bearing magnetic flux, the dual-armature consequent-pole bearingless flux reversal permanent magnet (DACPBFR) machine came into existence. The machine can be used in flywheel energy storage, ultra-clean, aerospace, and other fields. Herein, we design and analyze a DACPBFR machine, which has the advantages of high torque density and good fault tolerance.
II. BASIC MECHANISM AND MAGNETIC FIELD ANALYSIS
A. Basic structure
Based on the traditional alternating pole flux reversal machine with 12 stator slots and 14 rotor slots, an additional set of armature windings is added to the rotor teeth to obtain a new type of DAFRPM machine. A set of suspension force windings is added to the stator windings to obtain a DACPBFR machine based on the bearingless concept (Fig. 1). Both the stator and rotor slots of the DACPBFR machine are equipped with armature windings, and the stator slots contain suspension windings at the same time.
B. Magnetic field analysis
The torque of the DACPBFR machine is mainly composed of two parts, namely, the torque component produced by the permanent magnet magnetic field and the stator armature magnetic field, and the torque component produced by the permanent magnet magnetic field and the rotor armature magnetic field. The principle of the former to generate torque is similar to the traditional magnetic flux reversal machine based on magnetic field modulation. As shown in Fig. 2, the DACPBFR machine has different stator magnetic field distributions at different rotor positions. Taking the stator winding flux linkage identified in the figure as an example, as shown in Figs. 2(a) and 2(c), when the rotor tooth or rotor slot is facing the stator tooth, the stator winding flux linkage is 0, and the permanent flux linkage generated by the magnet is basically a leakage flux; as shown in Figs. 2(b) and 2(d), the stator winding flux linkage reaches the maximum value in the forward and reverse directions. In a stator electrical cycle, the stator flux linkage of a double-armature magnetic flux reverse machine is shown in Fig. 3. Since the rotor has 14 poles, the mechanical angle corresponding to a stator electrical cycle is 25.7°.
Stator magnetic field distribution at different rotor positions. (a) Rotor position: 0°mechanical angle. (b) Rotor position: 6.4°mechanical angle. (c) Rotor position: 12.9° mechanical angle. (d) Rotor position: 19.3° mechanical angle.
Stator magnetic field distribution at different rotor positions. (a) Rotor position: 0°mechanical angle. (b) Rotor position: 6.4°mechanical angle. (c) Rotor position: 12.9° mechanical angle. (d) Rotor position: 19.3° mechanical angle.
The principle of the torque generated by the permanent magnetic field and the rotor armature magnetic field is similar to that of an external rotor surface-mounted permanent magnet machine. The permanent magnets on the stator can be regarded as six pairs of poles. The electrical periods of the rotor and the stator are not equal, so four special cases are selected. Angle analysis of the magnetic field of the rotor winding is performed. The rotor magnetic field distribution of the DACPBFR machine at different rotor positions is shown in Fig. 4. Taking the magnetic linkage of the rotor winding identified in the figure as an example, when the rotor position is as shown in Figs. 4(a) and 4(c), that is, when the rotor teeth are facing the stator teeth, the rotor winding flux is 0; when the rotor position is as shown in Figs. 4(b) and 4(d), that is, when the rotor teeth are facing the stator slots, the flux linkage of the rotor winding reaches the maximum value in the forward and reverse directions. In a rotor electrical cycle, the rotor flux linkage of the DACPBFR machine is illustrated in Fig. 5, showing periodic changes, so brushless control can be realized.
DACPBFR machine rotor magnetic field distribution at different rotor positions. (a) Rotor position: 0° mechanical angle. (b) Rotor position: 7.5° mechanical angle. (c) Rotor position: 15° mechanical angle. (d) Rotor position: 22.5° mechanical angle.
DACPBFR machine rotor magnetic field distribution at different rotor positions. (a) Rotor position: 0° mechanical angle. (b) Rotor position: 7.5° mechanical angle. (c) Rotor position: 15° mechanical angle. (d) Rotor position: 22.5° mechanical angle.
The no-load magnetic field lines of the DACPBFR machine are illustrated in Fig. 6—the DACPBFR machine has small magnetic flux leakage between poles due to its alternating pole structure. In addition, the DACPBFR machine with two sets of windings has wider stator teeth and thinner rotor yokes. Owing to the additional rotor armature windings, the magnetic flux that passes through the stator teeth is increased, and the smaller rotor yoke facilitates the placement of the rotor armature winding.
In the stator permanent magnet machine, the cogging torque exerts a significant influence on the control performance of the machine. The disadvantage of cogging torque is that it will cause torque fluctuations in the machine, which hinder the smooth running of the machine, and will affect its performance. Figure 7 shows the stator opposite potential waveform of the DACPBFR machine and its Fourier decomposition. Due to the coordination of the axial modules, the air-gap flux density modulation harmonics that generate the flux harmonics cancel each other out, which improves the sinusoidal characteristics of the flux linkage. The stator back-EMF of the magnetic flux reversal machine with alternating pole structure demonstrates better sinusoidal properties and can achieve better brushless control. Figure 8 illustrates the cogging torque waveform of the DACPBFR machine—the DACPBFR machine has a smaller cogging torque, which reduces the fluctuations in the machine’s output torque.
Stator back EMF of the DACPBFR machine. (a) Waveform of stator back EMF. (b) FFT decomposition of stator back EMF.
Stator back EMF of the DACPBFR machine. (a) Waveform of stator back EMF. (b) FFT decomposition of stator back EMF.
Like the traditional magnetic flux reversal permanent magnet machine, the permanent magnetic field interacts with the corresponding harmonics in the armature magnetic field to produce electromagnetic torque. The torque waveform of the DACPBFR machine is shown in Fig. 9. The average torque of the DACPBFR machine presents a state that changes with the current cycle, and the machine’s torque reaches 6 Nm. The DACPBFR machine has a better overload capacity.
III. CONCLUSION
A new type of DACPBFR machine based on the topology of the magnetic flux reverse machine that integrates the toroidal winding structure and the axial modular design is proposed. Based on the theory of air-gap magnetic field modulation, the working principle of the machine is studied, the principles of magnetic link harmonic cancellation and cogging torque suppression mechanism are clarified, and the electromagnetic characteristics of the machine are simulated and elucidated. The study shows that the DACPBFR machine has the following characteristics: (1) DACPBFR machine adopts toroidal winding, which improves the winding factor of the magnetic flux reverse machine and has a higher torque output capacity. (2) The permanent magnetic flux linkage mainly arises from the modulation harmonics in the air-gap flux density. (3) The cogging torque is mainly generated by the modulation harmonics in the air-gap flux density, which exerts a suppressive effect on the cogging torque in the DACPBFR machine.
AUTHOR DECLARATIONS
Conflict of Interest
The authors have no conflicts to disclose.
DATA AVAILABILITY
The data that support the findings of this study are available from the corresponding author upon reasonable request.