Permanent Magnet Linear Machine Research Articles

A Novel Linear Permanent Magnet Vernier Machine With Consequent-Pole Permanent Magnets and Halbach Permanent Magnet Arrays

The linear permanent magnet (PM) vernier machine (LPMVM) can reach a higher thrust density at low speed compared to the regular linear PM machines due to the utilization of the magnetic gear effect, which makes it a good solution for low-speed direct-drive applications. In order to further improve the machine thrust density, a novel LPMVM with consequent-pole and Halbach PM array is proposed and optimized in this paper. Then, in order to show the superior thrust performance of the proposed LPMVM, it is compared to two conventional LPMVMs in terms of back electric motive force (back EMF), thrust force, and power factor. It is found that machine no-load back EMF can be improved by 72.5% and its thrust density is 46% higher than that of the conventional LPMVM.

Comparative Study of E-Core and C-Core Modular PM Linear Machines With Different Slot/Pole Combinations

Four modular permanent magnet linear machines (PMLMs) with E-core and C-core primary segments and 12-slot/13-pole (12s/13p) and 12-slot/14-pole (12s/14p) slot/pole combinations are compared in this paper. The influences of flux gap width on the electromagnetic performances and power losses have been investigated based on 2-D finite-element modeling. It has been found that, by choosing proper flux gap width, not only the average thrust force can be improved but also the force ripple and iron losses can be reduced for all the machines. Also, the E-core modular PMLM has higher thrust force and lower detent force compared with the C-core modular PMLM with respect to the same slot/pole combination. Moreover, the 12s/13p modular PMLMs can achieve better electromagnetic performances than the 12s/14p ones regardless of E-core or C-core primary structures. Finally, by shaping the end teeth, the detent force and force ripple of the four modular PMLMs are further mitigated. The 12s/13p E-core modular PMLM is proven to be the best type among these four modular machines.

Thrust Force Ripple Reduction of Two C-Core Linear Flux-Switching Permanent Magnet Machines of High Thrust Force Capability

Linear flux-switching permanent magnetic (LFSPM) machines are good choices for long stroke applications. These machines deliver high thrust force density in addition to the machine structure where permanent magnetics (PMs) and windings are all on the short mover. For LFSPM machines, their performance is always affected by big thrust force ripple. In this paper, for two C-core LFSPM machines of high thrust force capability, including a 6/13 C-core LFSPM (6/13LFSPM-C) machine and a sandwiched C-core LFSPM (SLFSPM-C) machine, and a thrust force ripple reduction method is proposed. The proposed method is developed by reducing the slot effect component of the cogging force based on staggered stator tooth, and suppressing the thrust force ripple caused by unbalanced three phase back-electromagnetic forces (EMFs) based on two end PMs. Based on finite element analysis (FEA) results, both C-core LFSPM machines can achieve small thrust force ripples as well as high sinusoidal back-EMFs, and at the same time, maintain high thrust force capability with the proposed method. It was also found that, the improved SLFSPM-C machine exhibited the same thrust force capability as the improved 6/13LFSPM-C machine, but with a much smaller thrust force ripple.

Optimization design of E-core hybrid- excitation linear switched-flux permanent magnet machine

Optimization design of E-core hybrid- excitation linear switched-flux permanent magnet machine

Fast Calculation of Detent Force in PM Linear Synchronous Machines With Considering Magnetic Saturation

The detent force of permanent magnet (PM) linear synchronous machines is the main source of thrust ripple. Its calculation becomes significant, especially under different load conditions. By using frozen permeability (FP) method, it can be obtained based on either the integral of Maxwell stress tensor or the virtual work principle. However, the calculated result from the former has nonzero average value, and the latter is complicated with general commercial finite element (FE) packages. For fast calculation and high precision, a hybrid FE-analytical method based on d-q axis flux linkage model is proposed, which only needs to calculate the thrust force and the flux linkage of two FE models. One is the nonlinear model with both PM and armature current excitations, and the other is the linear model with armature current only by using the FP method. The predicted result is compared with those of two existing approaches, which validates that the detent force can be calculated properly under different load conditions by the proposed method.

Force Ripple Minimization of a Linear Vernier Permanent Magnet Machine for Direct-Drive Servo Applications

Linear vernier permanent magnet machines (LVPMMs) for direct-drive servo applications are attracting more and more attentions due to its high force density, easy maintenance, rapid dynamic response, and so on. This paper proposes an LVPMM structure, which has a similar secondary as that of a regular linear PM machine, while its primary mover is composed of three modules, and the modules are separated by flux barriers. Since high-precision servo systems have rigorous demands on the force ripple of the drive motors, two methods are introduced to minimize the force ripple of the LVPMM. One is to choose an optimal length of flux barriers; the other is to change the three-phase winding connection. The effectiveness of both methods is verified by time-stepping finite element analysis. It is found that the force ripple can be reduced from 10.6% to 2.4% by the proposed two methods.

Analytical Calculation of the Magnetic Field Distribution in a Linear and Rotary Machine with an Orthogonally Arrayed Permanent Magnet

In this paper, an analytical model is proposed to analyze and predict the characteristics of a double stator linear and rotary permanent magnet machine (DSLRPMM). In order to simplify the magnetic field calculation, the DSLRPMM is cut along the axial direction (z direction) and transferred into a planar one. Hence, an analytical model of the machine considered the orthogonal effect (OE) is proposed based on the combined solution of Maxwell’s equation, conformal mapping, and equivalent magnetic circuit model (EMCM). The magnetic field distributions of the DSLRPMM are calculated with and without considering the OE, and some important electromagnetic parameters, including the back electromotive force (EMF), detent force, cogging torque, and output torque and thrust, are also predicted and compared to the 3D finite element analysis (FEA). The results show that the errors between the proposed analytical model and the 3D FEA results are less than 0.2% and even less than 0.1% for certain parameters, that is, the results obtained from the proposed analytical model agree well with that of the FEA. Moreover, the analyzed and predicted results are also verified by the experimental results on the prototype of the DSLRPMM.

Detent Force Reduction of a C-Core Linear Flux-Switching Permanent Magnet Machine with Multiple Additional Teeth

C-core linear flux-switching permanent magnet (PM) machines (LFSPMs) are attracting more and more attention due to their advantages of simplicity and robustness of the secondary side, high power density and high torque density, in which both PMs and armature windings are housed in the primary side. The primary salient tooth wound with a concentrated winding consists of C-shaped iron core segments between which PMs are sandwiched and the magnetization directions of these PMs are adjacent and alternant in the horizontal direction. On the other hand, the secondary side is composed of a simple iron core with salient teeth so that it is very suitable for long stroke applications. However, the detent force of the C-core LFSPM machine is relatively high and the magnetic circuit is unbalanced due to the end effect. Thus, a new multiple additional tooth which consists of an active and a traditional passive additional tooth, is employed at each end side of the primary in this paper, so that the asymmetry due to end effect can be depressed and the detent force can be reduced by adjusting the passive additional tooth position. By using the finite element method, the characteristics and performances of the proposed machine are analyzed and verified.

A single-phase axially-magnetized permanent-magnet oscillating machine for miniature aerospace power sources

A single-phase axially-magnetized permanent-magnet (PM) oscillating machine which can be integrated with a free-piston Stirling engine to generate electric power, is investigated for miniature aerospace power sources. Machine structure, operating principle and detent force characteristic are elaborately studied. With the sinusoidal speed characteristic of the mover considered, the proposed machine is designed by 2D finite-element analysis (FEA), and some main structural parameters such as air gap diameter, dimensions of PMs, pole pitches of both stator and mover, and the pole-pitch combinations, etc., are optimized to improve both the power density and force capability. Compared with the three-phase PM linear machines, the proposed single-phase machine features less PM use, simple control and low controller cost. The power density of the proposed machine is higher than that of the three-phase radially-magnetized PM linear machine, but lower than the three-phase axially-magnetized PM linear machine.

A tubular hybrid Halbach/axially-magnetized permanent-magnet linear machine

A single-phase tubular permanent-magnet linear machine (PMLM) with hybrid Halbach/axially-magnetized PM arrays is proposed for free-piston Stirling power generation system. Machine topology and operating principle are elaborately illustrated. With the sinusoidal speed characteristic of the free-piston Stirling engine considered, the proposed machine is designed and calculated by finite-element analysis (FEA). The main structural parameters, such as outer radius of the mover, radial length of both the axially-magnetized PMs and ferromagnetic poles, axial length of both the middle and end radially-magnetized PMs, etc., are optimized to improve both the force capability and power density. Compared with the conventional PMLMs, the proposed machine features high mass and volume power density, and has the advantages of simple control and low converter cost. The proposed machine topology is applicable to tubular PMLMs with any phases.

Design and Analysis of a Dual-Stator Spoke-Type Linear Vernier Machine for Wave Energy Extraction

In general, the conventional permanent magnet (PM) linear machine can reduce the energy conversion steps by eliminating the rotary to linear conversion mechanism which in turn reduces the maintenance cost in the wave energy extraction, while it suffers from bulky size due to a large number of poles for low speed operation. Therefore, the permanent magnet linear vernier machine has been regarded as a suitable alternative, which can produce high thrust force at low speed by using the “magnetic gearing effect”. In this paper, a dual stator spoke type linear vernier machine (DSSLVM) for the wave energy extraction application is proposed, which produces a competitively high thrust force and force density at low operation speed. To predict the performance of the proposed machine in perspective, the transient 2D finite element method (FEM) is utilized and main performance is compared with a presented linear primary permanent magnet vernier machine (LPPMVM) [1].

No-Load Analysis of PMLSM With Halbach Array and PM Overhang Based on Three-Dimensional Analytical Method

This paper presented a 3-D analytical method, based on transfer relations for permanent-magnet (PM) linear synchronous machine (PMLSM). The PMLSMs in this paper have a double-sided mover structure with Halbach array PMs and a slotless stator core. In addition, in order to complement the leakage and fringing flux density, we use a PM overhang in the mover structure. The analysis of this method's phenomenon is the proposed 3-D analytical method in this paper. Furthermore, we present experimental verification using a prototype model.

Study on a Novel Pseudo-Six-Phase Linear Flux-Switching Permanent-Magnet Machine for Direct Drive

A novel pseudo-six-phase flux-switching permanent-magnet linear machine (FSPMLM) for the direct-drive servo system is proposed in this paper. A six-phase structure is utilized to increase the fault tolerance capability and reduce the detent force. The proposed linear machine has the electromagnetic performance of a traditional six-phase linear machine, but its structure is as simple as that of a three-phase linear machine. First, the structure of the proposed linear machine is presented, and the operation principle is analyzed. Second, the finite-element method is implemented to investigate the thrust performance of the proposed machine. Finally, the results of the finite-element analysis are indirectly validated with an existing three-phase FSPMLM in a laboratory. Results indicate that this type of linear machine fits the direct-drive servo system very well.

A Linear Quadratic Regulator-Based Optimal Direct Thrust Force Control of Linear Permanent-Magnet Synchronous Motor

Linear permanent-magnet machines are often characterized by low inductance and short pole-pitch which leads to a small operational range of load angles. The resultant control performance using conventional direct thrust force control (DTFC) techniques is poor with high force ripple. This research improves this aspect of DTFC. A novel multiple-input multiple-output (MIMO) state-space model, independent of the mover’s speed, having stator flux and thrust force as states, is formulated for the linear permanent-magnet synchronous motor (PMSM). An optimal linear state feedback control scheme is then designed using the optimal linear quadratic regulator technique. Integral action is added to the designed control scheme by state augmentation to minimize the steady-state error and reduce the force ripple. Experimental results clearly prove that the proposed optimal control scheme results in a faster transient response of speed and force with improved steady-state regulation of force and flux when compared to the state of the art.

A Novel Partitioned Stator Flux Reversal Permanent Magnet Linear Machine

A new flux reversal permanent magnet (FRPM) linear machine known as the partitioned stator FRPM (PS-FRPM) linear machine is proposed. The structure and the operation principle of the machine are described, and the major design parameters are optimized for maximum thrust force. A periodic model is also used in the analysis in order to highlight the significant effect of the longitudinal end effect on the machine performance, particularly in terms of unbalanced magnetic circuit and cogging force. Furthermore, the proposed machine is compared with the conventional FRPM linear machine. It is concluded that higher thrust force, less cogging force, and consequently less thrust force ripple can be obtained by the proposed machine.

Research on a Tubular Yokeless Linear PM Machine

A tubular yokeless linear permanent-magnet (PM) machine is proposed for free-piston energy converter (FPEC), where high mass and volume power density are required. Topology, operating principle, design, and optimization of the tubular yokeless linear PM machine are studied. With the quasi-sinusoidal speed characteristic of the resonant piston considered, a single-phase yokeless linear PM machine is designed for FPEC by finite-element method. Some main design parameters, which include air-gap diameter, PM thickness, axial lengths of the radially magnetized magnets at center and two ends of the mover, pole pitch, and tooth pitch, are optimized to achieve higher power density. Compared with the conventional linear PM machines, the yokeless linear PM machine has the advantages of higher efficiency, higher mass, and volume power density. The utilization of space and machine material is also improved by adopting the yokeless machine topology.

Optimization and Analysis of a Yokeless Linear Flux-Switching Permanent Magnet Machine With High Thrust Density

This paper proposes a new yokeless linear flux-switching permanent magnet (LFSPM) machine topology with double magnets per mover module for maximizing thrust density. The optimization and analysis for the proposed yokeless LFSPM machine are carried out by the finite-element method. The comparison between the proposed yokeless LFSPM machine and the optimized conventional yokeless machine under the same constraints is conducted, showing that the proposed machine exhibits higher thrust density and low force ripple, while the product of power factor and efficiency of the new machine is marginally higher than that of the conventional yokeless machine.

Analysis of Transverse-Flux Linear Switched-Flux Permanent Magnet Machine

The transverse-flux linear switched-flux permanent magnet machine is investigated by 3-D finite-element analysis in this paper. The topology of the machine, as well as the machine operation principle, is introduced first. In order to improve the thrust force performance of the machine, two aspects are analyzed. One is the average thrust force, which is maximized by optimizing the mover stack length and the stator pole stack length. Not only does the average thrust force increase, but the force ripple slightly decreases as well. The other side is further lowering the force ripple. The segment group shift method is introduced and employed. After using this method, the force ripple has been reduced to an acceptable value, though the average thrust force will be slightly reduced. The analysis is the guidance for better understanding this novel machine.

Design of an Electric Machine Integrated with Trans-Rotary Magnetic Gear

Design aspects of the trans-rotary magnetic gear (TROMAG) integrated rotary machine are discussed in this paper, with particular focus on optimizing system cost and weight. Analytical models are used for design of the TROMAG. Optimal designs of the rotary machine are found by using a population-based genetic algorithm and two-dimensional finite-element analysis and thermal considerations. Weight, volume, and cost of the resultant system are then compared with the Pareto-optimal set of a permanent magnet linear tubular machine that is designed for the same force and speed specification. It is shown in this paper that, for high-force low-speed reciprocating motion applications, an electromechanical motion system consisting of a TROMAG and a rotary electric machine can far surpass a conventional direct drive linear machine in terms of weight, volume, and initial and operating cost.

Flux Field and Thrust Analysis of Permanent-Magnet Linear Machines With Isolated Movers

A novel permanent-magnet linear machine that consists of several isolated movers and one single stator is proposed in this paper. It is especially suitable for tasks requiring multiple drive machines. Each mover can produce independent thrust and flexible motions by interacting with the stator poles. Compared with conventional multimotor driven system, it helps to achieve more compact size, and thus improve the system working efficiency. The design concept is presented, and the magnetic field distribution in the machine is formulated analytically. The influence of magnetization patterns and back iron is included in the analysis. The research result indicates that the employment of same-magnetization pattern helps to improve the magnetic flux density and thus the thrust output of the system. A research prototype of the linear machine with two isolated movers is developed. Numerical computation and experiment measurement are carried out to verify the analytical models. Comparison shows that the analytical model fits with finite-element analysis and experimental results well, and the influence between the mover motions is negligible. The similar design concept of multimover and single-stator can be implemented into other rotary electromagnetic machines.