Stator PM Research Articles

Investigation of stator slot/rotor pole combination of flux reversal permanent magnet machine with consequent‐pole PM structure

Here, the influence of stator slot and rotor pole combination on performance of flux reversal permanent magnet (FRPM) machine with consequent-pole permanent magnet (CPM) structure is analysed. First, for 12-slot FRPM machines, the torque capabilities of CPM and conventional surface-mounted PM (SPM) structure having different rotor poles are compared. It shows that the torques of CPM machines are largely affected by the rotor pole number while the torques of SPM machines are similar. This is further explained by the equivalent pole-pair number and corresponding saturation condition. The influence of stator slot number and PM width ratio on torque performance of the CPM-FRPM machine is then investigated. Finally, detailed electromagnetic performance of the 12/14 SPM- and CPMFRPM machine is analysed and compared.

Magnetic Gearing as a Functional Principle in Electric Drives

The partitioned stator flux reversal machine (PS-FRPM) is a novel stator PM machine topology, which exhibits a higher torque capability than its single stator counterpart and the conventional permanent magnet synchronous machine (PMSM). The PS-FRPM consists of two stators, one which carries the armature winding, and one which is equipped with surface mounted permanent magnets. The rotor is sandwiched between the two stators. The separation of the stator allows a better utilization of the machine volume which results in a higher torque density. Furthermore, because the magnets are placed on a stationary component, they can be cooled effectively. Consequently, critical rare-earth materials can be saved.The structure of the PS-FRPM is very similar to that of magnetic gears. In this paper the torque production of a PS-FRPM is described by means of the magnetic gearing effect. First the magnetic gearing effect is introduced and then the corresponding analytics is transferred to the PS-FRPM. Based on Maxwell's stress tensor, the torque contributions of the individual space harmonics are determined. In contrast to conventional machines, the torque in PS-FRPM is produced by several space harmonics in both air gaps.

Design Optimization and Comparative Study of Novel Dual-PM Excited Machines

This paper systematically studies a new kind of PM machines with both stator and rotor PM excitations, namely, dual-PM excited machines. The key is to rely on the PM-iron structure in the machine to provide both PM excitation and flux modulation. Besides the fundamental field component in the air-gap, some other predominant harmonics introduced by the flux modulating effect can also contribute to the electromagnetic torque production. Therefore, this kind of machines can be designed with high torque density. Four dual-PM excited machine concepts with the same rotor configuration but different stator structures are comparatively studied, which include double-stator PM machine, stator multitooth-PM machine, stator slot-PM machine, and stator tooth-PM machine (STPM). Based on the flux modulating effect, the general design principle of the dual-PM machines is proposed in this paper. Through analytically investigating the air-gap field harmonics, the physical insight of the dual-PM machines is brought forward. All the four machines are optimized using an improved Tabu search coupled with finite element method, and their electromagnetic performances are comprehensively studied and compared. A prototype of STPM is manufactured. Experimental tests are conducted and the results well verify the electromagnetic design.

High‐torque‐density pseudo‐direct‐drive permanent‐magnet machine with less magnet

Magnetic gears, an alternative to conventional mechanical gearboxes, have been extensively studied in the last decade. By combining a magnetic gear and a conventional permanent-magnet machine into one frame, different magnetic-gear-integrated permanent-magnet machines with a high-torque density are obtained, among which the pseudo-direct-drive permanent-magnet machine features both a high-torque capability and a good power factor with an acceptable structural complexity. However, the stator PM array in the conventional pseudo-direct-drive permanent-magnet machine obstructs the magnetic path and reduces the excitation torque due to the coils. Another problem is the large permanent-magnet amount which increases the overall cost. The authors propose a novel topology which has an improved torque capability under the same excitation current with a reduced permanent-magnet amount. The working principles of the proposed machine are analysed theoretically using the linear-material assumption to decouple the permanent-magnet-machine part and the magnetic-gear part. Its performance is then validated by the finite-element method. A comparison study with the conventional topology is conducted to show its merits.

Comparative Analysis of Partitioned Stator Flux Reversal PM Machine and Magnetically Geared Machine Operating in Stator-PM and Rotor-PM Modes

In this paper, the partitioned stator flux reversal permanent magnet (PM) (PS-FRPM) machine and the conventional magnetically geared (MG) machine operating in both stator-PM (SPM) and rotor-PM (RPM) modes are comparatively analyzed in terms of electromagnetic performance to provide design guides for an MG machine regarding an SPM- or RPM-type machine and a higher or lower gear ratio machine. It is found that an SPM-type machine is recommended since both PS-FRPM and MG machines operating in SPM modes have a higher phase back-EMF and hence torque than their RPM counterparts, respectively, as a result of a similar phase flux linkage but a higher electric frequency since the iron piece number is larger than the PM pole-pair number. Moreover, a smaller gear ratio machine is preferred from the perspective of a higher power factor and hence a lower inverter power rating, as the conventional MG machines with higher gear ratios suffer from larger flux-leakage, higher synchronous reactance and hence lower power factors, as well as higher iron losses, than the PS-FRPM machines. However, higher gear ratio machines feature lower cogging torques and torque ripples due to the smaller difference between the PM pole-pair number and iron piece number. Both prototypes of PS-FRPM machine operating in SPM mode and MG machine operating in RPM mode are built and tested to verify the finite element predicted results.

Design procedure of coreless stator PM axial field synchronous machine for small-scale wind applications

Axial field machines are available in a wide range of configurations. It has flexible design restrictions. Several research efforts were performed to discover machine performance and design horizons. However, the common design process of axial field machines depends on numerical field analysis, due to the lack of trusted and generalized analytical and/or empirical formulas especially for field distribution. This paper aims to present the essential analytic formulation for dimension selection. In addition, it provides the normalized relations between dimensions and airgap flux density using 3D-FEM as an alternative for empirical formulas. The paper describes a design procedure for coreless stator axial flux permanent magnet synchronous machine. Definite equations are derived to help the designer select the proper number of PM poles and concentrated coils to get balanced multi-phase voltage out of the generator. Moreover, the paper defines the necessary limitations of the span of the concentrated coil for a certain pole pitch to reduce the harmonic content of the output voltage waveform. The paper proposes a new approach for selecting the dimensions of the magnetic circuit using proposed data curves instead of struggling with the 3D-FEM along with its high time consumption and the need for high computational power. The loss calculations and the armature reaction are also investigated as a performance analysis criteria for the designed generator. The paper investigates the design for small-scale wind applications. The design procedure is validated using both 3D-FEM simulations and experimental results. The results show the consistency of the proposed procedure. The procedure is described and applied for a certain configuration and materials. However, it is solely extendable to several other configurations.

A novel stator and rotor dual PM flux modulated machine

A novel structure of Veriner permanent magnet (VPM) machine is presented as an effective option for high torque density direct-driven applications. The key is to locate PMs on both sides of rotor and stator, and design the stator PMs as a cylinder shape, the rotation of which can be controlled by an external driven system. With this design, magnetic field in the airgap can be directly regulated by the rotation of stator PMs. The structure and operation principle are introduced and the performance is analyzed with time-stepping finite element method (FEM). Analysis results show that the novel structure has really high torque density and good field weakening performance.

A novel axial flux stator and rotor dual permanent magnet machine

A novel structure of axial flux stator and rotor dual PMs Vernier machine is presented as an effective option for high torque-density direct-driven applications. The key is to locate PMs on both sides of rotor and stator. With dual PMs, this proposed machine with a novel structure generates an improved torque per armature current at low speed. It is very suitable for direct drive applications with limited space such as the in-wheel electric vehicle (EV) motors. The theory of the structure with PMs on both sides is analyzed theoretically. The structure and operation principle of machine are introduced and the steady and steady performance of machine is analyzed and verified with time-stepping finite element method (TS-FEM).

Cogging Torque Reduction of Axial Field Flux-Switching Permanent Magnet Machine by Adding Magnetic Bridge in Stator Tooth

Axial field flux-switching permanent magnet machine (AFFSPMM) is a novel stator PM machine with high torque density and efficiency. The cogging torque in AFFSPMM is larger than that in the traditional PM machine because of the double salient structure. In order to decrease the cogging torque, the analytical expression of the cogging torque is deduced and a cogging torque reduction method by adding magnetic bridge in adjacent stator tooth is proposed. Based on the 3-D finite-element (FE) method, the influence of the magnetic bridge thickness on the cogging torque and the output torque is analyzed and compared with the rotor skewing and notching methods. The influence of the stator tooth and rotor pole width on the output torque is studied. The 3-D FE analysis is compared with the experimental results. The research results show that the proposed method is effective in the reduction of cogging torque for all kinds of stator structures. Moreover, the cogging torque can decrease greatly and the average output torque decreases slightly when combined with the optimization of the stator tooth width and rotor pole width.

Analysis of Dual Stator PM Brushless DC Motor

In this paper the analysis of dual stator permanent magnet BLDC (PMBLDC) motor has been reported. The proposed dual stator PMBLDC motor consists of two separate concentric stators having different number of slots with a common rotor on the same shaft. Two stator stampings of different radii are used for design of stators. The rotor is a hollow cylindrical rotor in the space between two stators. The analysis of the designed dual stator PMBLDC motor has been done. In dual stator machine, the total output torque corresponds to the algebraic sum of two independent torques.

Dynamic Permanent Magnet Flux Estimation of Permanent Magnet Synchronous Machines

This paper deals with rotor flux estimation in permanent magnet synchronous motor(PMSM). In most application, the rotor flux linkage is treated as a known constant. However a high operation temperature or a stator winding current impulse may cause magnet flux ripple or demagnetization which has direct impact on machines performance. To estimate the magnitude of rotor flux in this paper Kalman filter is used which chooses stator currents and PM flux as state variables. The estimation method can track the distribution of PM flux correctly and is robust to motor parameters uncertainty. The flux linkage harmonics can be derived from the back EMFs by means of FFT analysis. This method can be used to analyse the magnet flux ripple and demagnetization in PMSM. The simulation and experimental results show the effectiveness of the proposed rotor flux estimation method.

Reduction of Torque Ripple Due to Demagnetization in PMSM Using Current Compensation

Torque pulsations exist in permanent magnet synchronous motors (PMSMs) due to the nonsinusoidal flux density distribution around the air-gap. The rotor permanent magnet field is variable under the condition of demagnetization, which induces nonsinusoidal distribution of flux density. The torque pulsations vary periodically with rotor position and are reflected by speed ripple, which influence the PMSM drive performance. To estimate flux magnitude, an extended Kalman filter is built by using the state variables, such as stator current and PM flux. With this extended Kalman filter method we can accurately track flux linkage. A current compensation method is proposed to reduce the negative influence caused by demagnetization. The compensated currents are derived from the estimated flux linkage. Experiments on a DSP (Digital Signal Processor) controlled platform are developed for the evaluation of the proposed methods. In the experimental results smoother torque or reduced torque ripple is seen by using this current compensation and extended Kalman filter method.

Sensorless V/f control of high-speed surface permanent magnet synchronous motor drives with two novel stabilising loops for fast dynamics and robustness

This study introduces a novel fast dynamic sensorless control for high-speed surface permanent magnet synchronous motor (SPMSM) drives based on a V/f control with two stabilising loops to correct the stator voltage amplitude and phase by a closed loop for zero interior reactive power (implicitly pure i q control). This solution avoids Park transformations, speed and current closed-loop control - typical for vector control, and also rotor position and speed observers. It is robust to stator resistance and PM flux variations. Comprehensive experimental test results are given with a 50 Vdc MOSFET inverter and a SPMSM with very low electrical time constant of 0.5 ms and low inertia. Fast speed reversal of +10 000 rpm within 160 ms with good load rejection, starting from any initial rotor position and running to 20 000 rpm constitute the paper core. The speed (torque) dynamics are similar to those of vector control method with encoder, but online computation effort is notably lower, with superior control robustness. This solution should be particularly suitable for very high-speed drives with rated frequency of 0.5-10 kHz, where online computation time cycle is inherently small.