Air-gap Field Research Articles

A Pole-Changing Doubly Salient Permanent Magnet Motor

This article presents a new pole-changing (PC) doubly salient permanent magnet (DSPM) motor. According to the general air-gap field modulation theory (GAFMT), the permanent magnet excitation field can be modulated by the stator and rotor teeth to a series of harmonics with different pole-pair numbers and different slot pitch angles. Thus, when the armature winding is connected and reconnected according to the different excitation field harmonics, the PC operation can be realized to obtain a high torque density at the low-speed region and a widened speed regulation range with a simple motor structure. According to the GAFMT, the operating principle of the proposed motor is introduced, and the winding connection is discussed. Then, to verify the validity, the electromagnetic performance of the proposed motor is analyzed by the finite element analysis. Finally, a prototype is built and tested.

Analysis of the operation principle for tubular flux-switching permanent magnet linear machine

PurposeThis study aims to unveil the generation mechanism of the thrust force in a tubular flux-switching permanent magnet (PM) linear (TFSPML) machine; the operation principle of the TFSPML machine is analyzed.Design/methodology/approachFirst, the air-gap flux density harmonic characteristics excited by PMs and armature windings are investigated and summarized based on a simple magnetomotive force (MMF)-permeance model. Then, the air-gap field modulation theory is applied in analyzing the air-gap flux density harmonics that contribute to the electromagnetic force. In addition, a simple method for separating the end force of the TFSPML machine is proposed, which is a significant foundation for the comprehensive analysis of this type of machine. As a result, the operation principle of the TFSPML machine is thoroughly revealed.FindingsThe analysis shows that the average electromagnetic force is mainly contributed by the air-gap dominant harmonics, and the thrust force ripple is mainly caused by the end force.Originality/valueIn this paper, the operation principle of the TFSPML machine is analyzed from the perspective of air-gap field modulation.

Semi-Analytical Calculation of the Unsaturated Magnetic Field Distribution of a Slotted Spoke-Type Interior Permanent Magnet Machine With Conformal Mapping Method

The performances of an electric machine are closely related to magnetic field, and the magnetic field calculation is the basic work of machine design and optimization. In slotted spoke-type interior permanent magnet (IPM) machines, the analytical calculation of magnetic field is complicated because the boundary of machine is difficult to model. Therefore, this article introduces a conformal mapping (CM) method to calculate the magnetic field semi-analytically. First, the interior magnet is transformed into the surface-mounted magnet to reduce the complexity of boundary conditions. The air-gap magnetic field of the slotless spoke-type IPM machine is calculated by using the boundary conditions of the transformed surface-mounted permanent magnet machine. Then, based on the magnetic field of the slotless air gap, the magnetic fields of the slotted air gap and teeth are also calculated by the CM method. All the semi-analytically calculated magnetic fields are directly verified by the FEM simulation and indirectly verified by the comparison of torque and back EMF. In addition, the comparison of computation time between the semi-analytical method and the FEM shows that the semi-analytical method is time-saving.

A Novel Asymmetric Interior Permanent Magnet Synchronous Machine

This article proposes a novel asymmetric interior permanent magnet (AIPM) rotor topology that has a mixed V-shape and spoke-type PM configuration, designated as a 1.5-layer PM structure, and asymmetric arrangement of V-shape PMs and asymmetric flux barriers. The AIPM rotor topology is employed for IPM synchronous machines to utilize the magnetic-field-shifting (MFS) effect for increasing average torque by reducing the current angle difference between peak PM and reluctance torque components. Three machines, i.e., the proposed 1.5-layer AIPM, a symmetrical 1.5-layer IPM-I, and a conventional V-shape benchmark IPM-II are designed and optimized using the same stator, rotor diameter, and total PM usage by global parametric optimization considering the mechanical stress. The effect of asymmetric features of the 1.5-layer AIPM rotor design, i.e., asymmetry of V-shape cavity as well as position and dimensions of flux barrier, on maximum torque and open-circuit air-gap field are investigated. The electromagnetic performances of three machines are compared. It confirms that the proposed AIPM machine has the highest maximum torque and the smallest cogging torque simultaneously. It is also revealed that the torque enhancement is achieved by both the 1.5-layer structure and MFS effect. Finally, a prototype of an AIPM machine is manufactured and tested to validate the analyses.

Influences of low temperature on velocity of positive streamer under uniform field of air gap

Influences of low temperature on velocity of positive streamer under uniform field of air gap

Flux-Adjustable Permanent Magnet Machines in Traction Applications

This paper overviews the recent advances in flux-adjustable permanent magnet (PM) machines for traction applications. The flux-adjustable PM machines benefit from the synergies of the high torque density and high efficiency in conventional PM machines as well as the controllable air-gap field in wound-field machines, which are attractive for the traction applications requiring enhanced capabilities of speed regulation and uncontrolled voltage mitigation. In general, three solutions have been presented, namely the hybrid excited (HE), the mechanically regulated (MR), and the variable flux memory (VFM) machines. Numerous innovations were proposed on these topics during the last two decades, while each machine topology has its own merits and demerits. The purpose of this paper is to review the development history and trend of the flux-adjustable PM machines, with particular reference to their topologies, working mechanism, and electromagnetic performance.

Synthesis of Application-Optimized Air Gap Field Distributions in Synchronous Machines

This paper deals with the method of shaping the magnetic field distribution in the air gap of a synchronous machine. The main goal is to obtain a specific distribution of the magnetic field in the machine’s air gap to enable easier powering, to increase the torque and power density, or to limit the content of higher harmonics in the induced voltage and torque. This method can be applied for both the electrically excited machines and permanent magnets excited machines. The problem has been reduced to solving a modified system of algebraic equations obtained by the finite element method. The presented examples show the effectiveness of the proposed method both in shaping the contours of the air gap and the magnetization of permanent magnets placed on the rotor. The method can particularly be used in the design of the high-speed synchronous machines.

A novel spoke‐type PM motor with asymmetric rotor for low torque pulsation under MTPA control condition

This article presents a novel asymmetric spoke-type rotor structure. It is used to reduce rotor magnetic flux leakage, torque ripple and no-load back-EMF distortion rate while increasing the motor output torque. The traditional method of optimizing torque pulsation is the inverse cosine function, which gives a sinusoidal distribution of the air-gap magnetic field under no-load conditions of the motor. The fundamental wave magnetic field of the air-gap synthetic magnetic field under the load is offset from the no-load air-gap magnetic field. This paper presents the design theory and method of the new type of the asymmetric rotor. Asymmetric spatial harmonics are introduced to the rotor so that the motor has a sinusoidal distribution of the rated load air-gap field under MTPA control. In order to verify the superiority of the new rotor, the performance of four different spoke-type rotor structure motors under the same pole-slot combination was compared. Finally, an experimental prototype is built to verify the theoretical reliability against the design of this paper.

Cage Losses in Induction Motors Considering Harmonics: A New Finite Element Procedure and Comparison With the Time-Domain Approach

The cage losses in induction machines due to spatial or time harmonics in the air-gap field are a crucial parameter for a precise estimation of the machine efficiency. Using finite element, the common approach to consider the overall cage losses, by performing complete time-domain analyses. Alternatively, a rough estimation is achieved introducing equivalent parameters in the circuital model. In this article, a methodology for cage losses computation due to air-gap spatial field harmonics is described. The proposed finite element procedure is faster than the time-domain analysis since it is based on magneto-static and linear time-harmonic simulations to carefully consider the iron saturation and compute the induced currents in a specific working point. In the article, a detailed procedure description and comparison with respect to the time-domain approach is reported.

A Design Method of the Rotor Auxiliary Slot for the Water-filled Submersible Induction Motors

Aiming at the problem of electromagnetic vibration of the water-filled submersible induction motor (WSIM), a method of opening auxiliary slots on the rotor side is proposed to weaken the air-gap field harmonics caused by the rotor slot permeance harmonics. By analyzing the research status of electromagnetic vibration of the WSIM and the composition of the air-gap magnetic field of the motor, the idea that the auxiliary slots mainly affect the air-gap field harmonics by changing the air-gap permeance of the motor is put forward. The mathematical model of air-gap permeance of WSIM is established to simulate the influence of auxiliary slots on the air-gap permeance. Through the parametric analysis of the mathematical model, the change of auxiliary slot size is simulated. The air-gap permeance waveform is decomposed by the two-dimensional Fourier transform, and then the variation of the air-gap permeance harmonics with the size of the auxiliary slot is analyzed. Finally, the finite element simulation model of the WSIM with the auxiliary slots is established, and the waveform of the air-gap flux density of the motor is analyzed to verify the effectiveness of the mathematical model. Meanwhile, the results show that after opening the auxiliary slot, the radial electromagnetic force of the motor was reduced by 28.4%.

Mathematical Analysis Model of Double-Stator Field Modulation HTS Machine Based on General Airgap Field Modulation Theory

TheSuperconducting (SC) machines have the merits of higher power and torque density due to the fact of higher electrical or magnetic loading. Especially, the field modulation SC machine (FMSCM) recently attracts more and more attention from scholars, since this kind of SC machines successfully overcome the drawbacks of the traditional SC synchronous machines, such as the rotating seal of the coolant and the electrical brush-slip-ring for current transfer. However, the research method of the FMSCM mainly adopts the finite element analysis (FEA). Although the FEA can provide accurate results, the time consumption is huge, particularly at the initial design of the large capacity FMSCM, the time cost will be unacceptable. Aiming at solving this problem, this paper attempts to establish the mathematical analysis model of a double-stator field modulation HTS machine (DSFM-SCM) using a new theory of electrical machine, i.e., general airgap field modulation theory, for the first time. Both the FEA and experiments are carried out on the DSFM-SCM, to verify the effectiveness of the proposed mathematical analysis model. Although there is discrepancy between the proposed analytical method and FEA, it can basically meet the requirement at the initial design, and sufficiently exhibit the modulation effect of geometric structure, reflecting the essential mechanism of the DSFM-SCM. In addition, it is worth noting that the establishment process of analytical model of the DSFM-SCM can provide reference for the other FMSCMs.

Development of Dual Armature-Winding Four-Phase Variable Flux Reluctance Generator for Fault-Tolerant Capability Improvement

In this paper, a four-phase 8/10-pole dual armature-winding variable flux reluctance generator (DAW-VFRG) with double-redundant rectifier circuit is proposed for onboard generator. The proposed generator integrating four-phase VFRG and dual armature winding structure improves fault-tolerant capability and power density. Based on the airgap field harmonics analysis and winding function theory, the no-load back-EMF model and winding inductance model are deduced for determining the optimal stator/rotor pole combination. In addition, based on magnetic energy comparison, the influence of the connection categories between two sets of armature windings and rectifier circuits on output capability is also investigated when the saturation effect is considered. The key design parameters are globally optimized to improve the output capability and fault-tolerant capability simultaneously. Moreover, the electromagnetic performance of the proposed DAW-VFRG under both normal and fault condition is evaluated and compared with single armature-winding VFRG with half bridge rectifier. Finally, two prototypes are manufactured and tested to validate the finite-element predictions.

3D fast analytical computation of a permanent magnets axial coupler

PurposeThe purpose of this paper is to develop a new and fast three-dimensional (3D) analytical model to study a synchronous axial magnetic coupling with rectangular shaped magnets. This model takes into account edge and curvature 3D effects.Design/methodology/approachThis paper firstly introduces a 3D analytical model for an axial coupler with sector shaped permanent magnet (PM) based on magnetic scalar potential formulation in cylindrical coordinates. The magnetic field in PM, air gap and iron disks is computed by solving Laplace’s and Poisson’s partial differential equation. This solution is then used to compute the field in rectangular shaped magnets. To do so, the adopted approach consists to divide the rectangular magnet into sector radial slices each of which the 3D model allows the determination of the magnetic field distribution. The results obtained by the proposed 3D analytical model are validated through 3D finite element computations. Furthermore, a prototype axial magnetic coupler has been constructed so air gap flux density and static torque measurements are compared to the analytical predictions.FindingsThe results obtained by the analytical model show the effectiveness of the proposed geometry transformation approach. The developed model takes into account all the 3D effects without needing any correction factor.Research limitations/implicationsThe developed method provides an efficient and rapid tool for evaluating the influence of geometric and physical parameters of a synchronous magnetic coupling as part of a design optimization process.Practical implicationsThe developed method provides an efficient and rapid tool for evaluating the influence of geometric and physical parameters of a synchronous magnetic coupling as part of a design optimization process.Originality/valueA new and fast 3D analytical model, to improve the computation of the electromagnetic torque developed by a synchronous magnetic coupler with rectangular shaped magnets, has been developed. The proposed approach is really effective as it leads to consistent results when compared to 3D finite element method ones without any need for correction factor.

Analysis of Rotor Losses in Permanent Magnet Vernier Machines

This article investigates rotor losses in permanent magnet Vernier machines. By analyzing air-gap field, the harmonic distribution and frequency of modulated open-circuit flux density and armature reaction flux density are obtained. Then, rotor losses production principle is revealed and the influence of each harmonic on the loss is analyzed. Meanwhile, the influence of each harmonic on torque is also studied. It is found that some harmonics have no contribution to torque but produce additional losses. On this basis, a new rotor structure is proposed to suppress the rotor losses. Comparison results indicate that by suppressing nonworking harmonics, the rotor losses can be reduced effectively without affecting the effective torque. Finally, the theoretical and finite-element analysis results are verified by prototype tests.

The Diagnostics of the rotor misalignment of a permanent magnet motor

A mathematical model of the magnetic field in the working gap of a brushless motor is considered in a case of rotor misalignment arising during manufacture, for example, due to defects in end shields, or in operation due to bearing wear. a gap in a uniform (circular ring). The stator gearing is taken into account on average using the Carter coefficient, the magnetic field in the inhomogeneous air gap, created by the rotor magnets and the stator winding current, is assumed to be plane-parallel, having a two-dimensional character. It was found that the rotor misalignment associated with the rotational movement of the eccentricity causes nonsinusoidality of the idle EMF and pulsation of the electromagnetic moment with a frequency 3p times higher than the rotor speed. When the eccentricity is stationary, a variable EMF is induced along the rotor shaft, causing an alternating current in the circuit: shaft-bearings-bearing shields-stator housing. To clarify the nature of the defect in order to identify the actual misalignment of the rotor, it is recommended to control currents and voltages using specialized software and hardware complexes for spectrum analysis.

Analytical Investigation and Heuristic Optimization of Surface Mounted Permanent Magnet Machines With Hybrid Magnetic Structure

Cogging torque causes major operational setbacks for Permanent Magnet (PM) machine operation, particularly in applications where a quiet performance is desired. This paper presents a heuristic optimization framework to optimize the cogging torque in Surface Mounted Permanent Magnet (SPM) machines consisting of a hybrid magnetic structure (i.e., rare-earth and ferrite magnets). To avoid excessive computational time and volume associated with Finite Element (FE)-based optimization solutions, analytical approach is paired up with the optimization algorithm to determine the optimal design while FE is utilized for verification and validation purposes. First, analytical expressions are established for individual objective functions (i.e., airgap PM flux distribution, and cogging torque), and their corresponding spatial harmonics are identified using the air-gap field modulation theory. Next, the presented analytical model is utilized to optimize the system (i.e., minimize the cogging torque) to the desired target level via two different solutions (i.e., Genetic Algorithm (GA) and Particle Swarm Optimization (PSO)), and their respective performance are compared. To determine the efficacy of the presented solutions, the optimal hybrid machine response is compared against the baseline structure.

Model of implicit pole electric machine based on mathematical formulation of magnetic field in air gap

When modeling transient processes in electric power systems, the issue of designing reliable models of electrical machines is of great interest. The most difficult task is to design the models of synchronous and asynchronous machines. The purpose of the project is to develop mathematical models of synchronous and asynchronous machines based on a generalized model of an implicit-pole machine that considers the change of rotor speed, geometric shapes, and the type of rotor and stator windings. Methods of mathematical modeling of electromagnetic fields in the air gap based on Maxwell equations and methods of the theory of electrical circuits are applied. A system of nonlinear differential equations is considered as a mathematical model of a generalized implicit-pole machine. The key assumption made is the high magnetic permeability of the stator and rotor cores. Technical data of real electric machines have been used for verification. Mathematical models of an implicit pole synchronous machine and an asynchronous machine with a phase rotor have been developed. These models can be used to analyze transient processes in the aggregate without dividing them into electromagnetic and electromechanical ones. The novelty of the models is the fact of non-sinusoidal distribution of the magnetic field in the air gap, various types of multiphase AC windings and rejecting the principle of constant rotation frequency at the time step of the simulation. The proposed mathematical models can be used to solve design problems, to analyze the modes of electric power systems, the operation of relay protection and automation of electric power facilities, and emergency automation. In the future, the equations that consider the influence of the damper circuits and the peculiarities of the windings will be added to the developed mathematical models. The purpose of further research is to develop models of salient-pole synchronous machines and asynchronous machines with a squirrel-cage rotor.

Behaviour of Rotating Field in Slip-Ring Induction Motors with Chopper Control.(Dept.E)

Stator currents of slip-ring induction motor having chopper control in the rotor circuit, are no more sinusoidal. The analysis of the air-gap resultant field , due to such currents , results in a different expression for the established rotating field. The derived expression is examined in order to study the rotating field behavior under chopper control. In addition to some of the well known behaviours , a new one had been distinguished and proved by constructing the geometrical pattern of the air-gap resultant field. An effective winding factor is introduced to give the effect of nonsinusoidal stator currents on the magnitude of the resultant m.m.f. harmonics.

Analytical Modeling of High-Torque-Density Spoke-Type Permanent Magnet In-Wheel Motor Accounting for Rotor Slot and Eccentric Magnetic Pole

In this article, a hybrid analytical model combining the subdomain method and the lumped-parameter magnetic circuit method (LMCM) is presented for the analysis of a high-torque-density spoke-type permanent magnet in-wheel motor considering rotor slot and eccentric magnetic pole. In the proposed model, the air-gap field distribution is derived using the subdomain method, while the saturation and the axial flux leakage of the iron core are considered by LMCM. The rectangular-shaped spoke magnets, eccentric magnetic poles, and rotor slots are approximated by multiple layers of arc-shaped subdomains. The magnetic equivalent circuit (MEC) of the stator and rotor is established, and the nonlinearity of the iron core is taken into account. Based on the developed hybrid analytical model, electromagnetic performance, such as cogging torque, electromagnetic torque, flux linkage, and back electromotive force, are obtained. The analytical results are verified by the finite-element method, and its application in the optimization process is investigated. Finally, a 1800-Nm high-torque prototype machine is manufactured and tested, and the results validate the accuracy and efficiency of the analytical method.

Comparative Study of Torque Production Mechanisms in Stator and Rotor Consequent-Pole Permanent Magnet Machines

This article presents a comparative study of torque production mechanisms in stator and rotor consequent-pole permanent magnet (CPM) machines. Due to the CPM design, a unipolar PM magnetomotive force (MMF) distribution can be observed in both stator CPM (S-CPM) and rotor CPM (R-CPM) machines. As a consequence, more resultant air-gap field harmonics are responsible for torque production due to the double-side slotting effect. The PM MMF-permeance-based model is employed to identify and reveal the difference of sources and patterns of the working air-gap field harmonics in the two investigated CPM machines. The torque contributions of the main-order working harmonics of the two studied machines are analytically quantified by a unified torque equation and compared with the corresponding finite-element (FE) results. Finally, some experimental measurements are carried out on a 12-slot/14-pole R-CPM machine prototype to validate the theoretical and FE analyses.