Radial Magnet Research Articles

Limitation of the Allowed Coercive Force Caused by the Scattering Field of Magnet

It is shown that the shape of a sample determines the allowed lower-bound limit of the coercive force of material that can be used for fabrication of magnets with the given shape. Rings with radial magnetization are used as examples to show that such samples can be made of only rare-earth alloys with the coercive force that is sufficiently high to satisfy technical requirements at the maximum allowed working temperature of the magnet.

DESIGN OF A SINGLE-PHASE RADIAL FLUX PERMANENT MAGNET GENERATOR WITH VARIATION OF THE STATOR DIAMETER

This study aims to observe the influence of the changing stator dimension on the air gap magnetic flux density (Bg) in the design of a single-phase radial flux permanent magnet generator (RFPMG). The changes in stator dimension were carried out by using three different wire diameters as stator wire, namely, AWG 14 (d = 1.63 mm), AWG 15 (d = 1.45 mm) and AWG 16 (d = 1.29 mm). The dimension of the width of the stator teeth (Wts) was fixed such that a larger stator wire diameter will require a larger stator outside diameter (Dso). By fixing the dimensions of the rotor, permanent magnet, air gap (lg) and stator inner diameter, the magnitude of the magnetic flux density in the air gap (Bg) can be determined. This flux density was used to calculate the phase back electromotive force (Eph). The terminal phase voltage (V∅) was determined after calculating the stator wire impedance (Z) with a constant current of 3.63 A. The study method was conducted by determining the design parameters, calculating the design variables, designing the generator dimensions using AutoCad and determining the magnetic flux density using FEMM simulation. The results show that the magnetic flux density in the air gap and the phase back emf Eph slightly decrease with increasing stator dimension because of increasing reluctance. However, the voltage drop is more dominant when the stator coil wire diameter is smaller. Thus, a larger diameter of the stator wire would allow terminal phase voltage (V∅) to become slightly larger. With a stator wire diameter of 1.29, 1.45 and 1.63 mm, the impedance values of the stator wire (Z) were 9.52746, 9.23581 and 9.06421 Ω and the terminal phase voltages (V∅) were 220.73, 221.57 and 222.80 V, respectively. Increasing the power capacity (S) in the RFPMG design by increasing the diameter (d) of the stator wire will cause a significant increase in the percentage of the stator maximum current carrying capacity wire but the decrease in stator wire impedance is not significant. Thus, it will reduce the phase terminal voltage (V∅) from its nominal value.

General Analytical Modeling for Magnet Demagnetization in Surface Mounted Permanent Magnet Machines

This paper proposes an analytical model for the prediction of airgap magnetic field distribution for axial flux permanent magnet (AFPM) machine with partial magnet demagnetization. The AFPM machine geometry is first converted to a polar representation. Consequently, the subdomain model based on current sheet technique is developed. Then current sheet representation of PM is derived to consider the partial demagnetization using superposition principle. The back electromagnetic forces and cogging torque are obtained accordingly based on Maxwell's equations. The results show that the results of proposed approach agrees with that of finite-element method. The model is further validated by experiments under both healthy and demagnetized conditions, which can validate the proposed approach. Main contribution of the work is to consider the partial irreversible demagnetization. Moreover, the proposed method is applicable for both AFPM and radial flux permanent magnet machine.

Time‐domain computation of rotational iron losses considering the bulk conductivity for PMSMs

This study investigates an advanced finite-element (FE) technique for the evaluation of rotational iron losses based on a time-domain computation, where the bulk conductivity of the core materials is considered. The iron-loss characteristics are discussed for a radial flux permanent magnet synchronous machine (PMSM) for a wind generation application, with closed slots and outer rotor topology. The following factors are taken into account: (i) a real-time rotational iron-loss computation; (ii) the bulk conductivity of the steel laminations; and (iii) the influence of the controller harmonics on the system during transient conditions. The magnetic induction vector locus of each iron component is also discussed, where the magnetic induction is numerically modelled [three-dimensional (3D) finite element analysis (FEA)], computed using multiple magnetic antennae and is also experimentally verified. This comparative study shows a torque–frequency-loss computation that is presented from low to high frequencies (50–800) Hz. The FE model of the total iron losses for the PMSM using both pure sinusoidal and proportional–integral pulse-width modulation currents is studied and experimentally verified on a surface-mounted PMSM. The proposed method of iron losses prediction significantly reduced the rate of error between 3D FEA and experimental data to 1.7%.

Электрические генераторы мегаваттной мощности для ветроэнергетики: состояние и тенденции развития

The modern designs and development trends of electrical generators for wind turbines used in continental and offshore windmill farms are analyzed. Three types of generators for high-speed, low-speed, and gearless drives applied in the megawatt range of capacities are considered. The necessary electromagnetic loads of the generators and the change of their main sizes as a function of power capacity are determined. The trend toward using gearless permanent magnet synchronous generators for capacities up to 8 MW is pointed out. Synchronous generators with both radial magnetization and other versions (with an axial flux, without ferromagnetic cores, etc.) are considered. For capacities of 10 MW and higher, the wind turbine mass can be decreased essentially by using a superconducting generator. However, due to the cost of superconductor, the commercially profitable version has not unequivocally been determined. Different projects (with “warm” and “cold” superconductivity), as well as versions involving the use of both fully superconducting generators and generators containing only a superconducting excitation system are considered.

Ограничение допустимой коэрцитивной силы, вызванное полем рассеяния магнита

AbstractIt is shown that the shape of a sample determines the allowed lower-bound limit of the coercive force of material that can be used for fabrication of magnets with the given shape. Rings with radial magnetization are used as examples to show that such samples can be made of only rare-earth alloys with the coercive force that is sufficiently high to satisfy technical requirements at the maximum allowed working temperature of the magnet.

Effects of Manufacturing Imperfections on the Circulating Current in Ironless Brushless DC Motors

This paper deals with the effects of manufacturing imperfections on the circulating current characteristics of an ironless brushless dc (BLDC) motor through a two-dimensional finite-element method (2-D FEM). The circulating current caused by the manufacturing imperfections of the windings’ spatial location and the permanent magnets’ magnetization is analyzed. It is found that the former has a larger impact on the circulating current, and the parallel magnetization is smaller than radial magnetization. Then, the effect of the circulating current generated from the impedance unbalance in the parallel branches on winding losses and disturbance torque is further studied. The results show that the circulating current leads to large winding losses and different torque characteristics. The torque ripple becomes small when the circulating current exists in all the phase windings and the outer radius of the parallel branches has the same spatial location. Finally, the ironless BLDC motor is manufactured and tested. The experimental results are in good agreement with the 2-D FEM. It gives a significant guideline to the manufacturing process of such ironless BLDC motors.

Влияние текстуры на ширину доменов в спеченных магнитах (PrDy)(FeCo)B и (NdDy)(FeCo)B

AbstractWe analyzed magnetic force microscopy images of the surface of samples cut parallel and perpendicular to the texture created by applying a field in the process of pressing a magnet. The distribution of the linear size (width) of the domains correlates with the distribution of the projection area of grains onto the shear plane in sintered (PrDy)(FeCo)B magnets with radial magnetization and in prismatic (NdDy)(FeCo)B magnets. The effects of grain size and direction of the axes of easy magnetization of grains on the width of the domains, as the main factors controlling the domain structure, are discussed.

Comparison of Two Magnetizing Fixture Designs to Achieve Radial Magnetization Profile for Isotropic-Bonded Neo Magnets

The requirement of stringent environment and fuel efficiency target on automotive demand the use of lighter, smaller and efficient motors. The advantages like higher magnetic properties than ferrite, near net shape magnet production, and no use of heavy rare earth elements makes the use of bonded neo magnet in an automotive accessory motors very attractive. The isotropic nature of bonded neo magnets offers a feasibility to obtain wide range of magnetization profiles. The magnetization of the magnet influences the air-gap flux distribution and hence the motor performance 1. The performance of permanent magnet motors using isotropic bonded neo magnet can be improved by optimizing the air-gap flux profile using magnetization. Based on the magnet circumference touching the motor air-gap and the desired magnetization profile, an appropriate magnetization fixture needs to be designed. In an external rotor permanent magnet brushless (PMBL) DC motor and brushed DC motors, the inner circumference of the magnet touches the airgap and hence the magnet is magnetized using an inner-only fixture which has magnetization coils next to the inner circumference of the magnet. When the radial magnetization is desired, a double sided magnetization can also be used to magnetize bonded neo ring magnets 2. The use of double sided magnetization helps in enhancing the motor performance 3. To ensure that the bonded neo magnet is fully saturated a minimum magnetizing field of around 3T is to be generated throughout the magnet thickness 4. Considering this required magnetizing field, a fixture each offering inneronly and inner-outer magnetization (double sided magnetization) is designed using Finite element analysis (FEA) to achieve the radial magnetization. The simulation based results on the performance comparison of two fixtures is summarized. Figure 1 shows the simulated mid airgap flux density when magnet is magnetized with inner-only and inner-outer magnetizing fixtures. From this figure it is observed that, compared to inner-only fixture, the inner-outer fixture results in flat topped radial wave shape of flux density. Both designed fixtures are fabricated and the measured performance is used to validate the simulation results. It is observed that the peak magnetizing current and the energy required to achieve the magnet saturation in an innerouter fixture is 19.8% and 26.5% less respectively compared to inner only fixture. The reduction in peak magnetizing current and magnetization energy requirement will lead to reduced thermal stress and improved fixture reliability. It will also help in reducing the magnetization cycle time due to better heat dissipation in inner-outer fixture. During magnetization of the magnet, the effectiveness of the inner-outer fixture depends highly on the alignment of the inner and outer magnetization coils. The alignment and hence the performance of the fixture is influenced by the fabrication tolerances 5. Even a small misalignment between the coils on either side of the magnet will result in flux waveform distortion and reduction in magnetic flux. Considering the manufacturing tolerances, the effect of the various misalignment angles between the inner and outer magnetization coils on the magnet flux is arrived at using the FEA based simulation. The magnetization result is also measured by creating various misalignments between the inner and outer magnetization coils in the fabricated inner-outer magnetization fixture. Table I gives the measured magnet flux integral for various misalignment angles. It is observed from Table I that the $\pm 2 ^{circ}$ mechanical misalignment between the coils will not result in significant change in magnet flux and higher misalignment angles will result in considerable reduction of flux per pole. It is also observed that the misalignment effect is symmetric about the aligned position and higher degree of misalignment causes drooping wave shape on one side and a hump on the other side in any given pole depending on the direction of misalignment.

Influence of Soft Magnetic Material Type in Fixture Components on the Magnetization of Bonded Neo Magnet and Motor Performance

The advantages like higher magnetic properties than ferrite, near net shape magnet production, and no use of heavy rare earth elements makes the bonded neo magnet very attractive in motors used for automotive accessory, home appliance and office automation. The isotropic nature of bonded neo magnets offers a feasibility to obtain wide range of magnetization profiles. The magnetization of the magnet influences the air-gap flux distribution and hence the motor performance 1. Magnetizing fixture comprising of copper coils embedded in soft magnetic material is used to magnetize the magnet. When radial magnetization profile is desired, a back iron made up of soft magnetic material is also used to reduce the amount of magnetizing energy needed to saturate the magnet. Laminated steel is the preferred material for the magnetizing fixture as well as back iron. We have observed that at times solid steel is used in place of laminated steel. This paper presents the effects of using solid steel in place of laminated steel on the magnetization and motor performance. A magnetizing fixture is designed using the 2-D finite element analysis (FEA). The designed fixture is fabricated with fixture core and back iron made of laminated steel (LCLB). The magnetization performance of the designed fixture is evaluated using 2-D FEA and validated by performing magnetization of the magnets. To evaluate the influence of solid steel as a soft magnetic material two more combinations; (i) fixture core is of laminated steel but the back iron is of solid steel (LCSB) and (iii) both fixture core and back iron made of solid steel (SCSB) are simulated and evaluated. To ensure the full saturation in a bonded neo magnet a magnetizing field of 3T is desired through the thickness of the magnet 2. From the simulation it is observed that to achieve 3T magnetizing field for LCLB combination the magnetizing energy required is 5.44 kJ. The required energy increases to 34.02 kJ and 68.28 kJ for LCSB and SCSB combinations respectively. The increase in energy is due to the generation of eddy currents in solid steel components which counteract the applied field, reducing the available field for magnetization. The energy needed to achieve full magnet saturation in LCSB and SCSB combination exceeds the capability of most of the commercially available magnetizers also the increase in magnetization energy requirement will lead to higher thermal stress and reduced fixture reliability. Based on the capability of the available magnetizer we applied up to 6 kJ for LCSB and SCSB combinations. We have also measured the corresponding applied field at the back of the magnet as 1.6 T and 1.4 T respectively. The less than desired applied field led to the partial saturation of the magnet. Figure 1 shows the measured mid airgap closed circuit flux density waveforms for magnets magnetized using various combinations. From this figure it is observed that the use of solid steel component makes the airgap flux density waveform less radial. The flux integral is reduced by 11% and 6.6% for LCSB and SCSB combinations compared to LCLB combination. The magnets magnetized using various combinations are assembled in a motor for performance measurement, the results of which are summarized in Table I. The presence of solid steel component during magnetization leads to partial magnetization of the magnet, resulting in higher no-load speed and lower stall torque. It is also observed that the motor with the magnets magnetized using LCSB and SCSB combinations has 64% and 60% lower cogging torque compared to LCLB combination. This is due to partial magnetization and hence lower flux offered by the magnets from these combinations.

Design and Analysis of Halbach Ironless Flywheel BLDC Motor/Generators

Halbach array-based permanent magnet (PM) brushless machines exhibit attractive features when compared with conventional PM brushless machines with normal segmented magnets, including the flux-concentration effect, essentially sinusoidal air-gap flux density due to PMs, and negligible cogging torque. However, as for a motor/generator used in flywheel, an external rotor composed of Halbach array and rotor back-iron is necessary to increase the inertia of flywheel system, which is different from the conventional external-rotor machines. In this paper, a Halbach array PM motor is studied compared with the radial magnet array. The relationship between the air-gap field distribution and the magnet array is discussed. Finally, the eddy current loss in PMs of the external-rotor ironless brushless dc motor is analyzed between the Halbach PM motor and the conventional PM motor.

A Novel Linear Stator-PM Vernier Machine With Spoke-Type Magnets

Linear-machine-based direct-drive systems are attracting extensive attention for many industrial applications including wave energy conversion and railway traction. By using a linear machine, the complexity of linear motion systems is considerably reduced due to the elimination of the rotary to linear conversion mechanism. In this paper, a novel linear stator spoke-type permanent magnet (PM) vernier (LSSPMV) machine is presented in which the surface-mounted radial magnets used in conventional linear stator-PM vernier machines are replaced by spoke-type magnets that are magnetized parallel to the direction of motion. The proposed machine adopts a non-magnetic space between PMs and stator pole shoes decreasing the leakage flux and leading to higher flux linkage of armature winding. After designing the proposed LSSPMV machine and comparing it with its existing counterpart, the finite-element method results reveal that the thrust force density and the efficiency of the proposed machine are higher than that of the existing one as a result of utilizing spoke-type PMs.

Moving magnet linear actuator with self‐holding functionality

Electromagnetic design of a moving magnet frictionless linear actuator with self-holding functionality for mechanical gear shifting in a transmission application is presented. The actuator consists of a tubular magnetic circuit with stationary armature and axially moving rotating permanent magnet assembly with radial magnetisation in the air gap. The armature contains a two-piece magnetic core with a space for a winding. The effects of armature slot opening and magnetic circuit materials on the self-holding force and the acting force are studied in the whole moving range. The designed moving magnet linear actuator improves the performance of the vehicle by enabling shift times below 50 ms with no energy consumption between the shifts.

Influence of magnetization pattern in three-phase permanent magnet synchronous machines

This paper investigates the influence of different magnetization patterns in the performance of three-phase permanent magnet synchronous motors (PMSMs). Three magnetization patterns widely used in surface-mounted PMSMs are radial magnetization, parallel magnetization and Halbach magnetization. In this paper, these magnetization patterns are applied to 9-slot/10-pole and 12-slot/10-pole surface-mounted PMSMs. The electromagnetic characteristics and performance of the motors such as air gap flux density distributions, phase back-emf, cogging torque, unbalance magnetic force and output torque are intensively evaluated in 2D finite element method. Important findings indicate that PMSMs with ideal Halbach magnetization exhibit almost zero cogging torque and sinusoidal phase back-emf waveform. Parallel magnetization and radial magnetization yield more trapezoidal phase back-emf waveform which results in higher torque ripples. One way to minimize the torque ripples is by reducing the higher-order harmonic components in the phase back-emf waveform which can be achieved with the best value of magnet pole arc. Additionally, a potential cost saving can be realized since the motor has smaller magnet volume as being demonstrated in the case of 9-slot/10-pole, three-phase surface-mounted PMSM in this paper.

Comparative Analysis and Experimental Verification of a Linear Tubular Generator for Wave Energy Conversion

To improve the power density and reliability of radial (RS-TLPMG) and quasi-Halbach (QH-TLPMG) magnetization tubular linear surface-mounted permanent magnet generators under realistic sea-state conditions, a novel tubular linear generator with multilayer interior permanent magnets (MI-TLPMG) for wave energy conversion is proposed and analyzed in this paper. Using finite element analysis (FEA), a comprehensive comparison of air gap flux density, flux linkage, back electromotive force (back-EMF), load, and no-load performance is investigated to verify the advantages of the proposed machine. The FEA results indicate that MI-LTPMG with a flux-concentrating effect has higher back-EMF, air gap magnetic flux density, flux linkage, and output power than do the other two machines. Finally, a prototype is manufactured and measured on the test platform and wave tank. The experiment and simulation results show a great agreement with each other.

Radially-Magnetized Transverse Flux Permanent Magnet Generator

The transverse flux permanent magnet motor attracts wide attention in the field of low-speed direct-drive wind power due to its high torque density, good low-speed performance and other advantages. Aiming at the problem of high leakage coefficient of permanent magnet in the attached transverse flux permanent magnet wind power generator, a radially-magnetized transverse flux permanent magnet generator (RM-TFPMG) is proposed in this paper. Details of the structure and principle of the proposed machine are introduced firstly, and then the main parameters of the RM-TFPMG are calculated by the magnetic circuit method. Finally, the three-dimensional finite element analysis of the radial load flux transverse flux permanent magnet generator is carried out by ANSYS Maxwell software, and the parameters of machine and Back Electromotive Force (EMF) are obtained under the condition of no-load, and the rationality of the designed motor is verified.

Design of a novel dual flux modulation machine with consequent-pole spoke-array permanent magnets in both stator and rotor

In this paper, a novel dual flux modulation (DFM) machine with consequent-pole spoke-array permanent magnets (PMs) in both stator and rotor is proposed to achieve a larger torque density than regular flux modulation machines. The stator has two portions of PMs, one is set next to the stator slot with circumferential magnetization, and the other is inserted between the stator yoke and flux bridge with radial magnetization. The rotor has consequent-pole spoke-array PMs and alternative flux bridges. The proposed DFMPM machine can be regarded as the combination of the flux switching PM (FSPM) machine and vernier PM (VPM) machine. First, the structure and operational principle are introduced. Then, the proposed 12-stator-slot/10-rotor-tooth dual flux modulation permanent magnet (DFMPM) machine is compared with the FSPM machine and VPM machine in terms of flux lines distribution, back-electromotive force (EMF), cogging torque and average torque. Finally, the four proposed DFMPM machines are optimized for maximum average torque and minimum ripple torque considering the effects of split ratio, slot opening ratio, PM depth, PM angle and iron angle.

High power density multiple output permanent magnet alternator

The power electronic equipment is very much required when the alternator, capable of providing certain voltage level with a certain frequency, has to supply power to the load that needs voltage level or frequency other than the rated voltage and rated frequency of the alternator. At the same time, the power electronic circuit will also become complex if the alternator is to be provided with multiple outputs of different voltages with different frequencies. Therefore, a hybrid flux permanent magnet alternator (PMA) with multiple outputs is proposed in this study by utilising both radial flux permanent magnet (PM) technology and axial flux PM technology. The utilisation of both radial and axial flux PM technologies in the proposed hybrid flux multiple output PMA increases the power density also compared with that of conventional multiple output PMA. A comparison study is carried out between the conventional PMA and proposed hybrid flux multiple output PMA for the same volume and the same is also validated with finite-element method software and prototype testing.

Design, analysis, and realization of a magnetic force transmission PM brushless motor

This paper proposes a novel direct‐driving motor that successfully embeds a permanent magnet brushless motor (PMBM) into a concentric magnetic gear (CMG). The PMBM and CMG share a common high‐speed rotor. In order to reduce the flux harmonics and get high torque density, Halbach arrays are employed to arrange the PM's poles on each side of the rotor, which completely makes the internal PMBM field and the external CMG field decoupled and improves the magnetic field characteristic. The results of finite element computation show that the proposed motor with Halbach arrays has over 7.5% higher torque density and that the cogging torque reduces to 1/20 that of a conventional radial magnetization motor; moreover, in the test, the back‐EMF waveform is closer to a sine wave and the three‐phase current waveforms show good agreement between the simulated and experimented results under the same load. The torque ripple is small and the dynamic performance is good. At the same time, the efficiency at the rated point is very close to the experimental one, which validates the effectiveness of the design method for the PMBM in the CMG. © 2018 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.

Авиационные генераторы с постоянными магнитами

The capacity of aircraft generators reaches several hundred kV×A. For decreasing their specific mass, the generators are designed for limiting rotation frequencies in terms of strength considerations. With increasing the capacity of aircraft generators, their excitation system on the basis of high-energy permanent magnets can be regarded as an alternative to the electromagnetic excitation system. This is because the volume of magnets is several times smaller than that of ferromagnetic poles with windings. Therefore, permanent-magnet generators can be made with a rotor having a smaller mass and a shaft diameter several times larger than in a generator with electromagnetic excitation, due to which it becomes possible to increase their critical frequency and limit power capacity. In addition a rotor with permanent magnets has a higher strength and can be designed for higher rotation frequencies and for a larger number of poles than in case of using electromagnetic excitation. Permanent-magnet generators have higher efficiency values and higher overloading ratios. Rotors fitted with tangential magnets produce higher magnetic induction values in the air gap than the rotors fitted with radial magnets, due to which it becomes possible to make a high-frequency generator with a shorter length while keeping its mass almost unchanged. On the other hand, generators fitted with radial magnets have a stronger rotor and can be designed for higher capacities and rotation frequencies than generators with tangential magnets. Among many design arrangements of rotors, those with nonmagnetic yokes are the simplest ones, whereas arrangements with yokes made of gradient material are the most efficient ones. The most rational application of multipole permanent-magnet generators is in DC systems involving electronic converters.