Linear Electric Machine Research Articles

A Study on Linear Generator Characteristics in Free Piston Engine

The linear generator (LG) is integrated into a system that called the free piston linear generator (FPLG). In recent year, free piston engine that is studied by many researchers, has some advantages. Therefore, the application of the line generator in free piston for converting the chemical energy to electrical energy that can be used for plug-in hybrids to extend the range of operations as an alternative energy converter. Therefore, the optimizing of linear generator performance be able to significantly reduce the vehicle's fuel consumption. The parameters in permanent magnet linear electric machine (PMLEM) are designed by Maxwell software and the optimal parameters is carried out via the flux density. To achieve this goal, a linear generator in free piston engine with a flat structure and quasi-Halbach array flux structure was proposed. The simulation of linear generator is performed based on the speed ranges in the New European Driving Cycle (NEDC) for a mid-size car. In the study, a linear electrical machine (LEM) is designed by Maxwell software with translator that has various diameters. The results show that the suitable parameters for stator and rotor about 7.7 mm and 14 mm, respectively. Because the flux density for this design’s stator and rotor yoke avoids saturation state and generates the highest electrical output compared to the other states. Besides, the paper also examined the power output at different frequencies from 15 Hz to 50 Hz, corresponds to a velocity amplitude of 3.6 m/s to 12 m/s. The simulation result shows that the frequencies from 15Hz to 30 Hz with 4.8 m/s to 7.2 m/s (1200 rpm to 2100 rpm), respectively meets engine speed operation ranges in hybrid vehicle and these frequencies produce the power output that increases considerably about 7.5 kW at 30 Hz.

Design Principle for Linear Electrical Machines to Minimize Power Loss in Periodic Motions

This article describes a novel approach to the design of linear machines that are not run at a steady-state operating point. The focus lies on minimizing the power loss to achieve highest possible efficiency. First analytical considerations reveal the potential of designing electrical machines and spring mechanisms for periodic, accelerated motion. It shows the influence of the machine size on the power losses due to the dependence of machine force as well as moving mass on this parameter. Based on these findings, a simulation model was created that can be used with an optimization algorithm. The simulation consists of a stationary and a time-dependent finite-element analysis of the linear machine in conjunction with an operational model that includes a given movement cycle. It was used to design a flat-type linear machine for a free-piston linear generator. The simulation results validate the analytical calculations and illustrate the differences to conventional design approaches for linear machines.

A Linear Position Measurement Scheme for Long-Distance and High-Speed Applications

In the maglev train electric drive system, the linear electric machine is a welcome solution. However, for long railways, the mover position measurement is an issue, especially when the train speed is very high, e.g., 600 km/h. The position measurement method of the mover must be simple and precise at high speeds while maintaining a low cost for industrial applications. The principle of the stator permanent magnet (stator-PM) machine provides evidence that the magnets and windings can be mounted on the mover of the linear resolver, leaving a simple secondary-side structure on the railway. With this idea, this article presents a PM linear resolver that generates orthogonal back-EMFs as the sinusoidal and cosine (SIN/COS) signals. The train position can then be measured by decoding the PM back-EMF. On the other hand, the high-frequency injection is imposed to solve the mover position measurement at zero speed and low speed. This PM linear resolver is prototyped with printed circuit board windings instead of regular coil windings. The dedicated decoding firmware is developed to evaluate the measurement performance up to 60 km/h in the lab setup.

Longitudinal End Effects in a Linear Wave Power Generator

Even though the magnetic circuit of a linear electric machine is very similar to a rotating electric machine, they diverge in one fundamental property. The linear generator is open in both ends, i.e., the magnetic circuit is non-symmetric. This paper investigates and discusses the drawbacks of this non-symmetric design in a linear permanent magnet generator, installed in a wave energy conversion system. A two-dimensional geometry has been utilized for the numerical calculations in a finite element method simulation tool. The results present an increased cogging force and significant core losses in the translator as consequences of the longitudinal ends in the machine.

Study of the effectiveness of the electromechanical system of active vibration protection of a vehile with various regulators

The article considers a single-mass vibration protection system with an executive electromechanical device. The relevance of the research direction and the current state of its knowledge are presented. It is proved that in comparison with the controlled elements of viscous resistance, the controlled stiffeners provide the best quality of vibration protection. The prospects for the use of controlled electromechanical stiffeners are emphasized. The paper considers an actuator in the form of a linear DC motor. Examples of the use of active vibration protection systems with linear electric machines are given. In particular, an example of using as suspension of the vehicle is provided. The advantages and most significant drawbacks of such suspensions, which hinder their mass introduction, are indicated. The design scheme of a single-mass oscillatory system with parallel installation of a viscous friction element and a stiffener is presented. It is substantiated that, taking into account the accepted assumptions, the presented scheme is equivalent to vehicle suspensions. It is proposed to use a magnetorheological vibration damper as a controlled element of viscous resistance. The analysis of the state of the issue of active vibration protection systems with magnetorheological vibration dampers is given. The structure of closed by vibration acceleration of vibroprotection system protected object is developed. There are presented the mathematical models of its functionally necessary elements: vibration sensor, vibration protection system along the disturbance and control channel, linear DC motor. A technique for synthesizing a correction device that provides the desired resonant properties of a closed system is proposed. The results of simulation modeling of an active vibration protection system with a regulator synthesized according to the proposed technique and a regulator, the structure of which was obtained in the author’s previous works, are presented. Based on the obtained results, the conclusions on the presented materials are drawn and recommendations on the use of regulators are formulated.

Thermal model of a linear electric machine

A thermal model of a linear synchronous electric machine with permanent magnets has been developed. This model makes it possible to obtain the most accurate description of thermal processes, and to determine the maximum allowable output power, the parameters affecting heat transfer, and measures for thermal protection of a linear electric machine.

Enhanced Complex Space Vector Modeling and Control System Design of Multiphase Magnetically Levitated Rotary–Linear Machines

Rotary–linear electric machines can perform coupled rotary and linear motion. In addition, they can have magnetic bearings (MBs) integrated and magnetically coupled with the rotary, linear, or rotary–linear machine operation. Since rotary–linear machines with MBs have not been thoroughly analyzed in the literature, the models that provide understanding of their operation and give basis for the control system implementation are not entirely covered. Hence, in this article, an enhanced complex space vector-based model of the rotary–linear machine with MBs is derived and expressions for the torque, thrust force, and MB force are given. The rotary–linear machine complex space vector of the voltage, current, or flux linkage is defined using the proposed transformation with two complex frames: one related to the rotation and MBs and another to the linear motion. This results in complex space vectors with two complex units; however, the techniques used for a conventional complex space vector calculation can also be applied to the proposed complex space vector description. This is also experimentally validated on a hardware prototype of a magnetically levitated linear tubular actuator (MALTA), whose position control system is designed and implemented based on the enhanced space vector modeling approach, with the dynamic operation of the MALTA, including linear motor operation with an axial stroke of 10mm and a mechanical frequency of 17Hz.

A Cylindrical Linear Permanent Magnet Vernier Hybrid Machine for Wave Energy

In some wave energy converters, there is a need for linear electrical machines where all the active parts are mounted on the stator. For this reason the linear Vernier hybrid machine has been investigated. In other research, it is analyzed in 2D and assumed to have a flat cross section. In this paper a cylindrical version of the Vernier hybrid machine is presented and analyzed. A geometrical model is used to investigate potential mass savings of the cylindrical version, and a finite element analysis design study is used to demonstrate a potential improvement in performance. A proof of concept laboratory scale model has been built and tested to validate the predictions.

Mover guide in a linear electric generator with double‐sided stationary stators

This study addresses double-sided stationary stators in a linear electric generator with similar operation to a linear oscillatory motion electric machine. The plate mover in the linear electric generator's double-sided stationary stators is powered by the acoustic power output of the thermoacoustic engine's acoustic line. The main originality of this structure resides in the linear guidance used for the moving part. The mover is made up of a plate of sandwich magnets between two stators. This plate is in magnetic balance between the two stators through a controlled friction device.

Capability study of dry gravity energy storage

Abstract The increasing penetration of intermittent renewable energy sources has renewed interest in energy storage methods and technologies. This paper describes a gravitational potential energy storage method. A review of current storage methods that make use of the principle of gravitational potential energy is done, with a comparison given in terms of power, energy rating and round trip efficiency. One of these gravitational energy storage methods, involving moving a solid mass vertically up and down, is further analysed in terms of energy storage capacity, energy and power density and the levelised cost of storage. Two different hoisting methods are discussed, the first of which is the traditional drum winder hoist and the second is a proposed, multi-piston hoist based on the use of linear electric machines. The two hoist methods produce storage systems with distinctly different properties and storage applications.

Hybrid Analytical Modeling of Saturated Linear and Rotary Electrical Machines: Integration of Fourier Modeling and Magnetic Equivalent Circuits

This paper presents a 2-D hybrid analytical modeling method for the analysis of the magnetostatic field distribution with the capability of including nonlinear materials. The model combines Fourier modeling, which is accurate and fast, with meshed magnetic equivalent circuits that have unique permeability in mesh elements and, therefore, can model local saturation. To present the diverse applicability of the proposed method, it is applied to a linear machine with permanent magnet excitation and a rotary machine with current excitation. Magnetic field calculations are compared with finite-element analysis (FEA) with good agreement.

Design and Control for Linear Machines, Drives, and MAGLEVs—Part I

The 18 papers in this special section focus on the technology of linear electric (electromagnetic)machines (LEMs). LEMs were introduced at the turn of the last century. They have been conceived to convert electrical energy into mechanical energy (or vice versa) as a direct linear motion through electromagnetic forces. LEMs are reversible devices,meaning that they can work as bothmotors and generators. Although feasible, linear alternators are currently explored for limited applications (e.g., Stirling motor). On the contrary, linear motors or actuators are electromagnetic devices capable of producing direct (without any link or interposition) linear motions of the following types: in a unique direction, short-stroke motion, oscillatory motion, and step-by-step motion. Correspondingly, exactly as in the rotating counterpart, different types of linear motors have been devised: linear synchronous motors (LSMs) in its versions with excitation winding, with permanent magnets (PMs) LSMs or without both (linear reluctance motor = LRM), linear induction motors (LIMs), and linear stepper motors.

A Review of the Design and Control of Free-Piston Linear Generator

The Free-piston linear generator (FPLG) is a novel energy converter which can generate electrical energy and is regarded as a potential technology for solving the restriction of the short driving range of electric vehicles. Getting rid of the crank and flywheel mechanism, FPLG obtains some advantages of a variable compression ratio, compact size, and highly-efficient power generation. Linear electric machine (LEM) design and piston motion control are two key technologies of FPLG. However, they are currently the main obstacles to the favorable performance of FPLG. LEM being used to drive the piston motion or generate electric energy is an integrated design including a motor/generator. Various types of LEMs are investigated, and suitable application scenarios based on advantages and disadvantages are discussed. The FPLG’s controller is used to ensure stable operation and highly-efficient output. However, cycle-to-cycle variations of the combustion process and motor/generator switching make it difficult to improve the performance of the piston motion control. Comments on the advantages and disadvantages of different piston motion control methods are also given in this paper.

A Free-Piston Linear Generator Control Strategy for Improving Output Power

This paper presents a control strategy to improve the output power for a single-cylinder two-stroke free-piston linear generator (FPLG). The comprehensive simulation model of this FPLG is established and the operation principle is introduced. The factors that affect the output power are analyzed theoretically. The characteristics of the piston motion are studied. Considering the different features of the piston motion respectively in acceleration and deceleration phases, a ladder-like electromagnetic force control strategy is proposed. According to the status of the linear electric machine, the reference profile of the electromagnetic force is divided into four ladder-like stages during one motion cycle. The piston motions, especially the dead center errors, are controlled by regulating the profile of the electromagnetic force. The feasibility and advantage of the proposed control strategy are verified through comparison analyses with two conventional control strategies via MatLab/Simulink. The results state that the proposed control strategy can improve the output power by around 7–10% with the same fuel cycle mass.

Stirling Generators: Challenges and Opportunities

The current state of the market of Stirling generators shows that high-technology engines that have been tuned in the process of cooperation or competition between powerful corporations are winning. The main advantages of Stirling engines are the ability to use various heat sources and combustion chambers meeting environmental requirements, a low level of noise and vibration; favorable characteristics for both vehicles and stationary electric generators, and good consistency with a linear electric machine. High cost and weight—size parameters, as well as limited aggregate capacity and useful life, are constraints on the development of Stirling generators. It is a feature of a Stirling engine that the pressure-circuit, generator, and combustion-chamber parameters need to be regulated. The control system should be intelligent to provide starting conditions. Power control of a free-piston Stirling engine is usually accomplished by changing the piston stroke, which is realized a linear generator electronic control unit. At the moment, due to the development of alternative energy and distributed power supply, a potential niche for the use of Stirling generators is growing.

Performance Analysis of a Mutually Coupled Linear Switched Reluctance Machine for Direct-Drive Wave Energy Conversions

The requirements of high efficiency and environment-friendly behavior for wave energy conversions (WECs) have promoted improving investigations of linear electric machines. As an alternative to these existing linear machines, a mutually coupled linear switched reluctance machine (MCLSRM) for direct-drive WECs is proposed in this paper. An extensive comparative study is carried out between the proposed machine and other two comparative machines. By analyzing the operation principles of the three machines, the flux linkage and electromagnetic force formulas are theoretically derived based on equivalent magnetic circuit models, and discussed with the aid of finite-element analysis. Due to high utilization of the electrical and magnetic circuits, it is possible to find an MCLSRM design with comparable force capability to a traditional linear permanent-magnet (LPM) machine with the same electrical loading and the similar overall volume. Furthermore, two prototypes of different machines of the proposed machine and its LPM counterpart are manufactured for experimental validation. From the comparison results, the proposed MCLSRM has a good performance to extract more power from the prime mover above the rated velocity and is highly economic. The LPM counterpart has higher efficiency due to its lower fundamental flux density.

Sensorless control of a linear reciprocating switched-reluctance electric machine

It is proposed to use a simple switched-reluctance electric machine that does not have windings or permanent magnets on a movable part to develop a linear reciprocating electric machine intended for use as an electric generator together with a free-piston internal-combustion engine. It is noted that one of the most difficult problems in the development of such a linear electric machine is the development of a control system. A sensorless control algorithm is considered. Information on the stator-phase inductance of the electric machine and its variation is used to determine the position of the movable part. For its estimation, probing voltage pulses of known duration are applied to the phase. The amplitude of the current pulses caused by them will be proportional to the phase inductance. A mathematical model of electromagnetic processes is used to test the efficiency of the proposed control algorithm and determine appropriate control parameters. Since stator phases have separate magnetic circuits and are not magnetically connected, the electromagnetic processes in each phase are considered independently. Model parameters are determined experimentally using an experimental prototype of the considered electric machine, for which the dependences of the phase flux linkage and the generated force on the phase current for different positions of the movable part are obtained. The results of the investigation of processes in the considered electric machine at different frequencies of the movable part are given. It is found that, at frequencies of the movable part close to the nominal frequency, the phase should be connected to the power supply even before the movable part reaches the extreme position. Control using probing pulses applied to the operating phase is impossible, because the level of these pulses does not reach the maximum value until the phase is switched on. In this case, it is necessary to use the probing pulses of another, nonoperating phase to determine the position of the movable part. Such an algorithm makes it possible to control a linear reciprocating switched-reluctance electric machine at both low and high frequencies of the movable part. The obtained results confirmed the correctness of the adopted approaches to the development of a sensorless control algorithm.

Mathematical simulation and parameter determination of regulation of a linear electrical reciprocating machine

A linear switched-reluctance electrical machine (LEM) intended to be used as an electric-power generator together with a free-piston internal-combustion engine (ICE) has been analyzed. Taking into account the hard service conditions of an electrical machine (EM) in close proximity to a cylinder ICE, hottemperature zones and heavy mechanical loads of the EM of the vent-inductor type have been recommended to be used. Development of a control system for this type of machine is one of the most complicated problems when developing it. It has been suggested to determine the reasonable principles and regulating algorithms of the machine that is being considered, as well as its regulation system and energy-data evaluation, on the basis of a model of the electromagnetic process. Taking into account the characteristic properties of the linear switched-reluctance machine as a modeling object, a mathematical model has been developed to solve the problems mentioned above. As stator phases have separate coil-flux guides and are not connected magnetically, the electromagnetic processes in each phase are analyzed independently. For experimental corroboration of ideas for design calculation, an experimental model of the machine has been constructed. Parameters required for modeling the characteristics’ correspondences of flux linkage and phase current’s generating force at various positions of a moving element have been determined for this machine experimentally. Performance of the linear switched reluctance electrical reciprocating machine has been analyzed with the mathematical model with moving-element movement frequencies unfeasible for study on a proving stand. Reasonable parameters of the current pulses’ phases of the machine have been determined, and its energy data have been evaluated. The obtained results have confirmed the correctness of the design approaches, choice of regulation principles, and adjustment of a linear vent-inductor electrical machine of forward and backward action and the feasibility of using it as an electric-power generator together with a free-piston ICE.

Hybrid System Modeling and Full Cycle Operation Analysis of a Two-Stroke Free-Piston Linear Generator

Free-piston linear generators (FPLGs) have attractive application prospects for hybrid electric vehicles (HEVs) owing to their high-efficiency, low-emissions and multi-fuel flexibility. In order to achieve long-term stable operation, the hybrid system design and full-cycle operation strategy are essential factors that should be considered. A 25 kW FPLG consisting of an internal combustion engine (ICE), a linear electric machine (LEM) and a gas spring (GS) is designed. To improve the power density and generating efficiency, the LEM is assembled with two modular flat-type double-sided PM LEM units, which sandwich a common moving-magnet plate supported by a middle keel beam and bilateral slide guide rails to enhance the stiffness of the moving plate. For the convenience of operation processes analysis, the coupling hybrid system is modeled mathematically and a full cycle simulation model is established. Top-level systemic control strategies including the starting, stable operating, fault recovering and stopping strategies are analyzed and discussed. The analysis results validate that the system can run stably and robustly with the proposed full cycle operation strategy. The effective electric output power can reach 26.36 kW with an overall system efficiency of 36.32%.

Research on the intermediate process of a free-piston linear generator from cold start-up to stable operation: Numerical model and experimental results

The free-piston linear generator (FPLG) has more merits than the traditional reciprocating engines (TRE), and has been under extensive investigation. Researchers mainly investigated on the starting process and the stable generating process of FPLG, while there has not been any report on the intermediate process from the engine cold start-up to stable operation process. Therefore, this paper investigated the intermediate process of the FPLG in terms of switching strategy and switching position based on simulation results and test results. Results showed that when the motor force of the linear electric machine (LEM) declined gradually from 100% to 0% with an interval of 50%, and then to a resistance force in the opposite direction of piston velocity (generator mode), the operation parameters of the FPLG showed minimal changes. Meanwhile, the engine operated more smoothly when the LEM switched its working mode from a motor to a generator at the piston dead center, compared with that at the middle stroke or a random switching time. More importantly, after the intermediate process, the operation parameters of FPLG were smaller than that before the intermediate process. As a result, a gradual motor/generator switching strategy was recommended and the LEM was suggested to switch its working mode when the piston arrived its dead center in order to achieve smooth engine operation.