Permanent Magnet Linear Machine Research Articles

Thermal Analysis of a Novel Cylindrical Transverse-Flux Permanent-Magnet Linear Machine

This paper presents a novel staggered-teeth cylindrical transverse-flux permanent-magnet linear machine (TFPMLM), which aims to improve the power factor and force density. Due to the compact structure and high performance requirement, thermal problems should be seriously considered. The three-dimensional (3-D) temperature field model is established. The determination of convection heat transfer coefficients is discussed. Equivalent thermal conductivities of stator core and winding are given to simplify the analysis. With the thermal effect of the adhesive coatings among permanent magnets (PMs) and mover yoke taken into account, the temperature field distribution and variation rules of the TFPMLM are obtained using the finite volume method (FVM). The influences of slot filling factor and air flow velocity on the temperature field distribution are analyzed. It is found that the hottest spot of the TFPMLM appears in the middle of the end winding; and there is no risk of demagnetization for PMs.

Design Considerations of Tubular Transverse Flux Linear Machines for Electromagnetic Launch Applications

Due to the achievable high force densities, permanent magnet linear machines with transverse flux configurations are gaining increased interest in electromagnetic launch (EML) applications, in which the force density demand is crucial. This paper deals with the design of a tubular transverse flux linear machine (TFLM) targeting EML applications, and the issues pertinent to its topology selection, design optimization, and thermal modeling are discussed. By overall comparison of practical topologies and technologies of the TFLM, a tubular moving-magnet topology with separated flux paths is believed to be more appropriate. Design optimization is then performed to maximize the thrust force under specific volumetric constraints. It is shown that the thrust force is more sensitive to the stator segment length, instead of the magnet thickness. Regarding the overload requirement inherent in EML systems, special focus is given on the thermal characteristics of the TFLM at overload conditions, and it is proved that the machine shows appreciable overload capability. Measurements carried out on a prototype TFLM have validated the predictions.

Open Circuit Performance Analysis of a Permanent Magnet Linear Machine Using a New Hybrid Analytical Model

This paper presents the analysis of open circuit performance of a permanent magnet linear machine using a new hybrid analytical model (HAM). The goal is to show the capabilities offered by this new modeling approach. The new HAM is based on a strong coupling of magnetic equivalent circuits method and analytical models. The analytical model, based on the formal solution of Maxwell's equations, is established because of the separation of variables method. The open circuit performance are: 1) cogging force; 2) electromotive force; and 3) open circuit iron loss, obtained from the HAM are compared with corresponding results obtained from a finite-element analysis. A very good agreement has been obtained.

Comparison of Linear Primary Permanent Magnet Vernier Machine and Linear Vernier Hybrid Machine

This paper presents the comparison of linear primary permanent magnet vernier (LPPMV) machine and linear vernier hybrid (LVH) machine. The LPPMV machine and the LVH machine both operate based on the magnetic gear principle, hence possessing the advantages of low speed and high thrust force density. In addition, both machines employ similar configurations between which the key difference is that a one-piece primary iron core is employed in the LPPMV machine instead of the modular cores in the LVH machine. Using the finite element method (FEM), the characteristics and performances of two machines are analyzed and compared. The results show that the LPPMV machine exhibits higher thrust force and lower cogging force. Finally, the FEM results are validated by experiments based on a prototype of the LPPMV machine.

A Yokeless Linear Permanent Magnet Machine

This paper presents a novel yokeless linear permanent magnet (PM) machine topology. The armature windings employ fractional slot concentric configuration and can be prewound on separate teeth consecutively or alternatively with a high slot filling factor. A number of advantages are obtained from the proposed yokeless topology. First, the volume of the iron core is reduced and more space is available for the windings, which leads to higher force density, lower copper loss, and higher efficiency. The absence of the yoke iron also reduces the iron loss and like-hood of saturation, and hence improves efficiency and linearity of thrust force characteristic. Without the yoke, the slot leakage flux and the magnetomotive force drop in the iron core are also reduced and consequently the utilization of PMs and the machine force capability are improved. These benefits are demonstrated through a design case study.

Field structure for tubular linear permanent magnet machines

A novel structure for the field component of a class of linear tubular permanent magnet machines is presented. The new structure offers a reduction in the required steel for a given design, and offers the possibility to achieve a more efficient cooling of the field component, and therefore magnets. Finite element simulations of example designs are presented, which show that the form and magnitude of the flux linkage in the coils can be maintained with the reduced mass structure.

Study on the Characteristics of a Novel Six-Phase Fault-Torrent Linear Permanent Magnet Machine for Linear Oil Pumping

A six-phase fault-torrent primary permanent magnet linear machine is proposed for linear oil pumping. The proposed linear machine can save the permanent magnet with both the magnets and armature windings placed in the primary. Six-phase structure is used to increase the fault tolerance capability and reduce detent force. First, the structure of the proposed linear machine is given out, and the operation principle is analyzed as well as compared with an existing three-phase flux-switching permanent magnet linear machine. Second, the transient finite-element method is implemented to study on the thrust performance under normal and fault condition. In addition, based on the static characteristics, the motion law of landing top of linear pumping unit is analyzed, and the process is simulated by using the proposed linear machine. All the results indicate that the proposed machine has advantages of simple structure, minor thrust ripple, higher fault tolerance capability, and low cost. This kind of linear machine is well suitable for the application of linear oil pumping.

A novel single-phase flux-switching permanent magnet linear generator used for free-piston Stirling engine

This paper investigates a novel single-phase flux-switching permanent-magnet (PM) linear machine used for free-piston Stirling engines. The machine topology and operating principle are studied. A flux-switching PM linear machine is designed based on the quasi-sinusoidal speed characteristic of the resonant piston. Considering the performance of back electromotive force and thrust capability, some leading structural parameters, including the air gap length, the PM thickness, the ratio of the outer radius of mover to that of stator, the mover tooth width, the stator tooth width, etc., are optimized by finite element analysis. Compared with conventional three-phase moving-magnet linear machine, the proposed single-phase flux-switching topology shows advantages in less PM use, lighter mover, and higher volume power density.

Using Third Harmonic for Shape Optimization of Flux Density Distribution in Linear Permanent-Magnet Machine

This paper presents a sinusoidal permanent magnet (PM) shaping technique with third harmonic to improve the electromagnetic thrust force in linear slotless PM machines without sacrificing the thrust force ripple. Slotless PM linear machine possesses relatively low thrust force ripple due to the absence of cogging force, compared with slotted topology. However, thrust force ripple of the machine with rectangular PM shape still exists due to nonsinusoidal airgap flux density distribution produced by PMs. Sinusoidal shaping techniques can be used to reduce the thrust force ripple but the average thrust force is reduced as well. Therefore, a simple PM shaping technique with optimal 3rd harmonic is presented to improve the output thrust force but not to increase the thrust force ripple. The sinusoidal plus 3rd harmonic shaping technique is analytically demonstrated together conventional sinusoidal shaping method and verified with finite element method. The results show that the electromagnetic performance can be significantly improved.

Comparison of a High Thrust Force Density Segmental Stator Linear Switch Reluctance Machine for with Different Topologies

Linear switch reluctance machine (LSRM) has been an alternative to linear permanent magnet synchronous machine and linear induction machine. However, the nature of reluctance force production, together with the air gap which is always much larger than rotary SRM, result in low propulsion force density in LSRM. This is an inherent demerit of LSRM, limiting its applications where the machine size is strictly set. This paper presents a novel linear switch reluctance machine with segmental stator, which consists of series of ferrite-magnetic segments. The principle of the proposed LSRM featuring high force density has been introduced. A variety of topologies of the proposed machine have been presented including single stator, dual stator and non-stator yoke structure. This paper deals with the comparison of the proposed segmental stator linear switch reluctance machine and traditional teeth type linear reluctance machine in terms of the propulsion force, normal force and force ripple as well as the force density and active pay load ratio. Finite Element Method is implemented to obtain the correlated results. Some conclusions are drawn to verify the analysis above.

Starting Performance of Linear Flux-Switching Permanent Magnet Machine Applied in Marine Direct Drive for Electric Propulsion Applications

A double-sided linear flux-switching permanent magnet machine applied in marine rudder direct drive applications is presented. The starting performances of the machine are calculated by finite element method. The simulation results of the proposed DLFSPM are verified by the experiment. The results have verified that the machine is suitability marine rudder direct drive applications.

Research on Combinations of Primary and Secondary Pole Numbers for Linear Switched-Flux PM Machines with Odd Pole Number of Primary

Based on a simple analytical model, combinations of primary and secondary pole numbers for linear switched-flux permanent magnet (LSFPM) machines with odd pole number of primary are compared, with respect to back electromotive force (EMF) and electromagnetic thrust force. The winding configurations are also determined according to the coil-EMF vector diagram. Based on optimized LSFPM machines, some basic electromagnetic performances are compared in LSFPM machines with 9 primary poles. It is shown that the secondary pole number should be close to the primary pole number and the higher secondary poles can lead to a better performance.

General Analytical Method for Magnetic Field Analysis of Halbach Magnet Arrays Based on Magnetic Scalar Potential

This paper presents a general analytical method for predicting the magnetic fields of different Halbach magnet arrays with or without back iron mounted on slotless permanent magnet (PM) linear machines. By using Fourier decomposition, the magnetization components of four typical Halbach magnet arrays are determined. By applying special synthetic boundary conditions on the PM surfaces, the expressions of their magnetic field distributions are derived based on the magnetic scalar potential (MSP), which are simpler than those based on the magnetic vector potential (MVP). The correctness of the method is validated by finite element analysis. The harmonics of airgap flux density waveforms of these Halbach magnet arrays with or without back iron are also compared and optimized.

Cogging Force Reduction of Double-Sided Linear Flux-Switching Permanent Magnet Machine for Direct Drives

This study proposes a double-sided linear flux-switching permanent magnet machine for direct drives in precision servo applications. The method of asymmetry distribution of stator teeth (ADST) is introduced to reduce the cogging force. Moreover, cogging force, back-EMFs, and thrust force are calculated by using finite element analysis. In addition, the performance comparison is carried out to verify the accuracy and effectiveness of the method of cogging force reduction. Finally, the experiment confirms the reasonability and feasibility of the ADST method.

Enhancement of a Thrust Force of a Tubular Electromagnetic Launcher With Transverse Flux Configuration by Leakage Flux Suppression

This paper presents an approach for enhancing the thrust force of a tubular transverse flux permanent magnet linear machine (TFPMLM) that has been developed for electromagnetic applications. It is shown that, by introducing auxiliary cores to suppress the leakage flux, the thrust force of the tubular TFPMLM can be significantly enhanced, by ~ 130%, under the same volumetric constraints. The effectiveness of the proposed technique is validated by extensive three-dimensional finite element computations.

Thrust Optimization of a Flux-Switching Linear Synchronous Machine With Yokeless Translator

Linear permanent magnet machines have been increasingly used in several applications that require high reliability and good dynamic characteristics. In this paper, a novel double-sided design of a 3φ permanent magnet flux-switching linear synchronous machine (FSLSM) with a yokeless translator is proposed. It is found that the proposed design can achieve more than 50% higher thrust than a conventional FSLSM and excellent linearity even under high excitation currents. Optimization of the proposed machine is performed using individual parameter optimization and is compared with results from global optimization using genetic algorithm (GA). Force analysis of the proposed design is performed to establish the mechanical integrity of the design. The proposed FSLSM is most suitable for long-stroke applications and intermittent oscillatory type applications.

Flux Field Formulation and Back-Iron Analysis of Tubular Linear Machines

In electromagnetic machines, ferromagnetic material is one of the key components that affect the magnetic field and thus the output performance significantly. The objective of this paper is to study the influence of ferromagnetic material on magnetic field distribution of tubular electromagnetic linear machines. Generic magnetic field models are obtained for linear machines with various structure designs based on magnetic vector potential and Bessel functions. The magnetic field model is validated with numerical results from finite element calculations. The comparison between them shows that the flux model fits closely with the numerical computation. The obtained analytical model can be employed to study the flux density distribution of machine designs with single-sided, double-sided, and non back irons. It shows that the use of back iron, especially for the internal side, helps to greatly enhance the radial component of magnetic field. This component contributes to the axial force generation of tubular linear machines. The study could be useful for the design optimization of permanent-magnet linear machines.

A Method to Segregate Detent Force Components in Permanent-Magnet Flux-Switching Linear Machines

Detent force in permanent-magnet (PM) linear machines is caused by both end effect and slot effect. Segregation of the detent force into its end-effect and slot-effect components is helpful to better analysis and design. For conventional PM linear machines, the end-effect component of detent force can be easily calculated in a related slotless linear machine model, where the slot effect is eliminated. However, the slotless model is not applicable to permanent-magnet flux-switching (PMFS) linear machines, where the magnets and core teeth are set together on either a same stator or a same mover. This is because the PMFS machines cannot work at all without the teeth or slots. Therefore, a new method for segregating the detent force in the PMFS linear machines is proposed in this paper. Since a linear machine can be considered as a part of a particular rotary machine with large radius, and furthermore, cogging torque in the PM rotary machine is caused by the slot effect only, thereby the slot-effect component of the detent force in the PMFS linear machine can be calculated from the cogging torque in a related rotary machine, while the residual of the detent force is the end-effect component. Both finite element analysis (FEA) and experimental results show that the proposed method works well for the PMFS linear machines.

Cryogenic Characterization of Copper-Wound Linear Tubular Actuators

Electrical machine power density can be increased by liquid nitrogen (LN) cooling, since these machines operate at low resistivity and have a high heat transfer coefficient. This paper describes the experiments carried out on a short stroke tubular linear permanent magnet machine with copper windings and LN cooling. A cryogenic test rig for linear actuators was designed and built, and tests were carried out aimed at highlighting inherent current limiting capability, electric parameter variation in the fall from room to cryogenic temperatures, and actuator high thrust density. Shear stress reaches 17.8 kN/m2, and thrust-to-copper-loss ratio is 4 N W-1/2, which are significantly better compared to conventional actuators.

Nonlinear Equivalent Magnetic Circuit Analysis for Linear Flux-Switching Permanent Magnet Machines

A double slot-pitch and multitooth linear flux-switching permanent magnet (FSPM) machine, which is appropriate for high thrust force and low speed direct-drive applications, is proposed. Moreover, a nonlinear equivalent magnetic circuit (NEMC) model is developed for the analysis of the linear FSPM machine. The permeance of the repetition-unit (RU) in the NEMC model, of which the magnetic saturation as well as the field interaction is taken into account, is calculated by finite element (FE) analysis. Finally, the NEMC method is applied to the linear FSPM machine for calculations of the static characteristics, and the method is verified by FE analysis and experiment.