Tubular Permanent Magnet Machine Research Articles

An Energy-Harvesting System Using MPPT at Shock Absorber for Electric Vehicles

This paper investigates an energy-harvesting system that uses of vibration energy at a shock absorber for electric vehicles. This system mainly comprises a linear electromagnetic generator and synchronous buck converter. To obtain the electrical energy through a linear electromagnetic generator, the perturb and observe maximum power point tracking (P&O MPPT) scheme is applied at the converter. The power converter circuit is designed with a diode rectifier and synchronous buck converter. The generated electric power is able to transmit to the battery and the damping force of the shock absorber is adjusted by the controlled current of generator. The linear electromagnetic generator was designed as a single phase eight-slot eight-pole tubular permanent magnet machine. The performance of the proposed energy-harvesting system was verified through simulations and experiments.

Optimal Design of a Novel Exterior Permanent Magnet Tubular Machine for Energy Harvesting From Vehicle Suspension System

This article presents an optimal design of an exterior permanent magnet (PM) tubular machine for energy harvesting from vehicles suspension systems. By using a hybrid structure, in which, the mechanical damper remains and electrical components are attached to the damper's frame, the damping force and damping coefficient are still ensured while wasted kinetic energy is recovered. To maintain driving comfort and safety, the detent force and electromagnetic force must be minimized. The constraints for dimensions of the exterior PM tubular machine were obtained from an actual shock absorber from a school bus, a large-size vehicle. Different topologies were investigated to maximize the output power and power density, while the electromagnetic force was minimized and negative effects of the initial positions to the machine's performance were mitigated. Finite element analysis showed that maximum and average power from an optimal design were about 210.2 and 103.7 W, respectively. To verify the validity of the design, a prototype was fabricated and tested.

Consideration on Eddy Current Reduction Techniques for Solid Materials Used in Unconventional Magnetic Circuits

The use of solid materials in tubular, linear machines has significant manufacturing benefits, as this removes the need for an axially laminated stator. However, this comes at the cost of extra eddy current losses. In this paper, a detailed analysis of the behavior of eddy currents in a tubular permanent magnet machine that comprises unconventional magnetic circuit is given, highlighting the importance of the field distribution and current directions, when compared with conventional magnetic circuits. This analysis is then used to identify and investigate the appropriate eddy current reduction methods for tubular machines, when a solid material stator is being used. Accurate three-dimensional models have been built and then validated on previously built testing setups. Different winding configurations have been accounted. Finally, considerations of these techniques are given when being implemented into an actual, tubular machine design, highlighting the improved performance and losses.

Optimal Design of a Novel Single-Phase 8-Slot 8-Pole Tubular Electromagnetic Shock Absorber to Harvest Energy

Based on the dimensions and thresholds of power and power density of a commercial shock absorber in the Korando car (an SUV branch), a single-phase 8-slot 8-pole tubular permanent magnet (PM) machine was designed and fabricated. The main focus of this paper is on the increase of output power in a given space and reduction of the volume. This is successfully solved using a novel structure for PM and considering a number of slot-pole combinations. Furthermore, a multiobjective optimization is performed to find the tradeoff between various objectives. The experimental results reveal that the proposed machine can possibly regenerate a maximum of 339.95 W and an average of 96.67 W in terms of its output power. Considering the active volume, which includes PM, iron core, pure copper, and an electromagnetic air gap, the estimated maximum and average power density are 0.311 and 0.087 W/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> , respectively. Despite these satisfactory results, one of the major drawbacks to exploiting this machine is its high electromagnetic force, which causes negative effects on safety and driving comfort. With the aim to commercialize the proposed device, further work will be concentrated on the reduction of electromagnetic force and on performance of the road tests.

Comparative Study of Novel Tubular Partitioned Stator Permanent Magnet Machines

Permanent magnet (PM) tubular machines have been applied to many applications, since they offer high force density with no end winding and no net attractive force between the mover and the stator. In this paper, two novel PM tubular machines named partitioned stator interior PM (PS-IPM) and partitioned stator surface-mounted PM (PS-SPM) tubular machines are proposed. Both the proposed machines are optimized for maximum thrust force, and their electromagnetic performance has been compared. It is found that the PS-IPM tubular machine has higher thrust force capability and better magnet utilization compared with the PS-SPM tubular machine. However, due to lower cogging force, the PS-SPM tubular machine has fewer thrust force ripples. Furthermore, the effect of the cross-coupling magnetic saturation on PM flux linkage, and the $d$ -axis and $q$ -axis inductances for both the proposed machines is investigated. It is demonstrated that the PM flux linkage and the $d$ -axis and $q$ -axis inductances are affected by the cross-coupling magnetic saturation. Moreover, the force–velocity characteristics of the proposed machines are calculated and compared. It is observed that both the machines have good constant power operation capability.

Performance evaluation of fractional-slot tubular permanent magnet machines with low space harmonics

Abstract This paper evaluates the perforamnce of fractional-slot per pole winding configurations for tubular permanent magnet (PM) machines that can effectively eliminate the most undesirable space harmonics in a simple and cost-effective manner. The benefits of the proposed machine topology winding configurations are illustrated through comparison with 9-slot, 10-pole tubular PM machine developed for a free piston energy converter under the same specification and volumetric constraints. It has been shown that the proposed machine topology results in more than 7 times reduction in the eddy current loss in the mover magnets and supporting tube, and hence avoids potential problem of excessive mover temperature and risk of demagnetization.

Inductances and Phase Coupling Analysis of Tubular Permanent Magnet Machines With Transverse Flux Configuration

This paper performs the inductance and phase coupling analysis of a tubular permanent magnet machine with transverse flux configuration for electromagnetic launch (EML) applications. Self-inductance and mutual inductance are calculated by employing a frozen permeability finite element technique, and the results are validated by measurement on a prototype machine. The analysis of phase coupling problem is then undertaken, and it has been shown that the tubular transverse flux machine has the merit of magnetic decoupling between phases, and thus shows a brilliant prospect in EML applications. The influence of armature reaction on the inductance is also investigated, and it indicates that at higher current, the inductance fluctuates once instead of twice within one electrical cycle, which is quite different from that in conventional machines.

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.

Analysis of Eccentricity in Permanent-Magnet Tubular Machines

Translator eccentricity in permanent-magnet tubular machines is practically inevitable. It results in nonuniform air-gap magnetic-field distribution, which is inherently a 3-D problem. In this paper, the effects of eccentricity on field distribution and on the resulting unbalanced radial force are analyzed. Both no-load and load conditions are presented. The proposed mathematical model only requires the knowledge of the 2-D air-gap flux density distribution when the translator is at the balanced position. The analytical expression of the unbalanced radial force is derived together with the translator deformation analysis. The information of unbalanced radial forces acting on the translator and the translator displacement can be used for machine and bearing sizing. Analytically predicted and 3-D finite-element-calculated results are compared and validated by the experimental results obtained on a specially set-up test rig.

Prediction and Measurement of Iron Loss in a Short-Stroke, Single-Phase, Tubular Permanent Magnet Machine

This paper describes analytical prediction and measurement techniques for quantifying iron loss in a short-stroke, single-phase, tubular permanent-magnet machine which is developed for direct-drive linear compressors in domestic refrigeration applications. The technique is based on the analytically derived magnetic field distributions in three discrete regions of the stator core and a well established iron loss model, and is validated by finite element analysis. An iron loss measurement technique is developed in which the moving magnet armature is supported by flexural springs that eliminate frictional loss and facilitate resonant operation during measurement. A good agreement between the prediction and measurement is observed.

Demagnetization Assessment for Three-Phase Tubular Brushless Permanent-Magnet Machines

We describe an analytical technique for assessing the risk of partial demagnetization in tubular permanent-magnet (PM) machines. The technique establishes analytical expressions for the open-circuit and armature reaction fields in the cylindrical coordinate system and superposes the fields in the permanent-magnet regions to determine the extent to which the magnets may be partially irreversibly demagnetized. We have applied the technique to a quasi-Halbach magnetized tubular PM machine equipped with a modular stator winding, and have validated the predictions by finite-element analysis. We found that partial demagnetization may occur even under an open-circuit operating condition when the machine is operating at high temperature. We propose alternative Halbach magnetization distributions that improve the demagnetization withstand capability. The analytical technique provides a computationally efficient tool for identifying regions that are prone to partial demagnetization and for assessing the consequences. It enables the risk of demagnetization to be fully assessed at the design stage so as to achieve a robust machine, particularly when operating in harsh environments.

Mitigation of Cogging Force in Axially Magnetized Tubular Permanent-Magnet Machines Using Iron Pole-Piece Slotting

We present a method for reducing the cogging force in axially magnetized tubular permanent-magnet machines. The method consists of placing several similar slots on the iron pole-pieces. By choosing the appropriate positions of the pole-piece slots, specific harmonics of the cogging force can be eliminated, and, as a result, the cogging force may be greatly reduced. To obtain a general expression for the positions of the iron pole-piece slots, we use harmonic analysis with Fourier series. We demonstrate the validity of the cogging force reduction technique by finite-element analysis.

Indirect and Direct Force Control of a Two-Phase Tubular Permanent Magnet Machine

Vector control and direct torque control are employed extensively in variable-speed ac drive systems, particularly for induction machine drives. This paper describes the application of both control techniques to a two-phase tubular permanent magnet machine supplied from a full H-bridge inverter, with space vector modulation being employed to derive variable-amplitude, variable-frequency, two-phase sinusoidal voltages. The direct force control (DFC) strategy controls the flux and thrust force by means of an active and reactive force estimator, utilizing the concept of active and reactive power. It is demonstrated that compared to vector control, DFC is more effective in compensating for the influence of the cogging force

Three-dimensional analysis of tubular permanent magnet machines

This paper presents results from a three-dimensional finite element analysis of a tubular permanent magnet machine, and quantifies the influence of the laminated modules from which the stator core is assembled on the flux linkage and thrust force capability as well as on the self- and mutual inductances. The three-dimensional finite element (FE) model accounts for the nonlinear, anisotropic magnetization characteristic of the laminated stator structure, and for the voids which exist between the laminated modules. Predicted results are compared with those deduced from an axisymmetric FE model. It is shown that the emf and thrust force deduced from the three-dimensional model are significantly lower than those which are predicted from an axisymmetric field analysis, primarily as a consequence of the teeth and yoke being more highly saturated due to the presence of the voids in the laminated stator core.

Finite element analysis of saturation effects in a tubular linear permanent magnet machine

PurposeThe aim of the paper is to find the simple and accurate model for the analysis of a drive with a tubular linear permanent magnet machine (TLPMM). Attention is paid to the models that take into account the saturation effects and is useful in the calculations of electromagnetic forces.Design/methodology/approachA circuit model and a field‐circuit model (FCM) are considered. The FCM includes finite element (FE) formulation for the axisymmetric electromagnetic field, equations which define the connections of windings and converter elements, and expressions that describe the control system. The FE method is used to determine the parameters of the circuit model. In order to simplify the circuit model, saturation effects caused by armature reaction are ignored. The electromagnetic force calculation is based on the virtual work principle and uses an approximate expression for the derivative of system co‐energy. The results obtained for the proposed models have been compared.FindingsThe proposed FE method of force calculation conforms with the applied method of movement simulation. For the rotor position when the cogging force is equal to zero the calculated cogging force is “almost” zero within seven‐decimal‐place accuracy. The effects of armature reaction on the performance of a TLPMM machine are similar to those which occur in a classical DC machine; in particular the demagnetising effect caused by saturation is observed.Originality/valueThe paper shows the influence of the saturation effects on the electromagnetic force of a TLPMM. In the case of “strong saturation”, the classical circuit model may be inappropriate for engineering calculations.

Analytical Prediction of Eddy-Current Loss in Modular Tubular Permanent-Magnet Machines

The paper describes an analytical technique for predicting the eddy-current loss in the moving armature of a tubular permanent magnet machine. This loss component is usually neglected in conventional tubular permanent magnet machines since high-order time harmonics in the stator current waveform and space harmonics in the winding magnetomotive force (MMF) distribution are generally considered to be insignificant. However, a relatively new topology of tubular permanent magnet machine, sometimes referred to as "modular", has emerged in which the fundamental component of the stator MMF has fewer poles than that of the permanent-magnet armature, the thrust force being developed by the interaction between a higher order MMF harmonic and the permanent magnet field. Thus, the presence of lower and higher order space harmonics in the winding MMF distribution of a modular machine may gives rise to a significant eddy-current loss in the moving-magnet armature. An analytical model is developed to predict the eddy currents which are induced in the magnets, as well as in any electrically conducting supporting tube which may be employed, and to quantify the effectiveness of axially segmenting the magnets in reducing the eddy-current loss. The validity of the developed model, which is also applicable to conventional designs of tubular permanent-magnet machine, is verified by time-stepped transient finite-element analysis (FEA).

Influence of soft magnetic materials on the design and performance of tubular permanent magnet machines

This paper investigates the influence of the choice of soft magnetic material on the performance of a tubular permanent magnet machine, and quantifies the relative merits of silicon iron laminations and soft magnetic composites (SMCs). The machine is equipped with a modular stator winding and employs a quasi-Halbach magnetized moving-magnet armature. It is shown that, despite its poorer space utilization, a machine whose stator is fabricated from silicon iron laminations has the highest force capability, efficiency and power factor. A machine with a SMC stator, on the other hand, has potential advantages in terms of ease of manufacture and lower cost.

Stator Iron Loss of Tubular Permanent-Magnet Machines

Whilst methods of determining the iron loss in rotating permanent magnet machines have been investigated extensively, the study of iron loss in linear machines is relatively poorly documented. This paper describes a simple analytical method to predict flux density waveforms in discrete regions of the laminated stator of a tubular permanent magnet machine, and employs an established iron loss model to determine the iron loss components, on both no-load and on-load. Analytical predictions are compared with the iron loss deduced from finite element analyses for two tubular permanent magnet machine designs, and it is shown that if a machine has a relatively high electrical loading, the on-load iron loss can be significantly higher than the no-load value.

Direct drive electrical power take-off for offshore marine energy converters

This paper investigates the issues associated with converting the energy produced by marine renewable energy converters, namely wave and tidal stream devices, into electricity using direct drive electrical power take-off, without use of complex pneumatic, hydraulic or other mechanical linkages. In order to demonstrate the issues, two alternative topologies of linear electrical machines are investigated: the linear vernier hybrid permanent magnet machine and the air-cored tubular permanent magnet machine. The electrical characteristics of these machines are described and compared in the context of mechanical integration. Potential solutions to the issues of sealing, corrosion and lubrication are discussed taking into account the electrical properties of the two topologies.

Cogging-force-reduction techniques for linear permanent-magnet machines

The paper describes two techniques for reducing the cogging force associated with the finite length of the ferromagnetic armature core of a tubular permanent-magnet machine. Analytical expressions for predicting the cogging force which is associated with armature cores having skewed or stepped end faces are established, and a cogging-force-minimisation technique is employed to determine an optimal armature-core length. The validity of the analytical cogging-force predictions and the effectiveness of the cogging-force-reduction techniques are verified by three-dimensional finite-element analyses and experimental measurements.