Axial Flux Permanent Magnet Synchronous Machine Research Articles

Control strategy for permanent magnet synchronous motor with contra‐rotating rotors under unbalanced loads condition

This study presents an investigation into the control of an axial-flux permanent magnet synchronous machine (PMSM) with contra-rotating rotors fed by a single inverter, which corresponds to two PMSM connected in series. In this study, the mathematic model of the PMSM with contra-rotating rotors is developed and the instability of the system with unbalanced loads is analysed. Based on the analysis, a control strategy which can keep synchronism of the two rotors under varying load is implemented. In addition, a method for starting the motor reliably is proposed, which is applicable to ship or under water vehicle propulsions. The control strategy is implemented on a DSP 28335 processor featured hardware platform and is tested on a 1.2 kW prototype machine. Experimental results validate the correctness of the analysis and control strategy.

Development of a Yokeless and Segmented Armature Axial Flux Machine

This paper is about the mechanical design and analysis of a yokeless and segmented armature axial flux permanent-magnet (PM) synchronous machine, which consists of two external rotors and an inner stator. Although this new electrical machine has many advantages such as high torque density, shorter axial length, and high efficiency, its mechanical construction is yet challenging. This is mostly due to the high axial force between the stator and the two rotors, which can be further increased by the inevitable manufacturing and assembly tolerances. Based on the determined geometric dimensions of the electromagnetically relevant components, a reliable mechanical construction is developed in this paper. Subsequently, stress, deformation, thermal, and modal analyses are performed based on finite-element method (FEM). Finally, the first prototype is successfully manufactured and measured. It can be concluded that the proposed construction is reliable and the measured data match well with the results of the electromagnetic analysis.

Heat-transfer improvements in an axial-flux permanent-magnet synchronous machine

Axial-flux machines tend to have cooling difficulties since it is difficult to arrange continuous heat path between the stator stack and the frame. One important reason for this is that no shrink fitting of the stator is possible in an axial-flux machine. Using of liquid-cooled end shields does not alone solve this issue. Cooling of the rotor and the end windings may also be difficult at least in case of two-stator-single-rotor construction where air circulation in the rotor and in the end-winding areas may be difficult to arrange. If the rotor has significant losses air circulation via the rotor and behind the stator yokes should be arranged which, again, weakens the stator cooling. In this paper we study a novel way of using copper bars as extra heat transfer paths between the stator teeth and liquid cooling pools in the end shields. After this the end windings still suffer of low thermal conductivity and means for improving this by high-heat-conductance material was also studied. The design principle of each cooling system is presented in details. Thermal models based on Computational Fluid Dynamics (CFD) are used to analyse the temperature distribution in the machine. Measurement results are provided from different versions of the machine. The results show that significant improvements in the cooling can be gained by these steps.

Two-Dimensional Magnetostatic Finite-Element Simulation for Devices With a Radial Symmetry

This paper proposes a 2-D magnetostatic finite-element solver for radially symmetric devices, complementary to the standard Cartesian and axisymmetric solvers, which are typically used for translatory and cylindrically symmetric configurations. In contrast to the Cartesian and axisymmetric cases, a specific difficulty is encountered due to the particular dependence of the magnetic vector potential on the radial coordinate caused by the requirement for radial symmetry. Dedicated finite-element shape functions are developed such that the partition-of-unity property, consistency, and convergence of the formulation are guaranteed. Implementation aspects and modeling peculiarities are discussed. The new solver is validated for models in which analytical solutions exist. The modeling accuracy of the new 2-D solver is compared with a 3-D model for the calculation of the electromotive force and the electromagnetic torque of a twin-rotor axial-flux permanent magnet synchronous machine.

Comparison of single- and double-layer windings in an axial flux permanent-magnet synchronous machine

This paper deals with the comparison between single-layer and double-layer concentrated windings in an axial flux permanent magnet synchronous machine. The results are obtained using finite element method and present the differences in back electromotive force, cogging torque, detent force and static torque for both winding topologies. Streszczenie. W artykule przedstawiono porownanie miedzy pojedynczoi podwojnie-warstwowymi uzwojeniami koncentrycznymi maszynach synchronicznych z magnesami trwalymi i strumieniem osiowym. Do tego porownania wykorzystano metode elementow skonczonych, a wyniki analizy prezentują roznice we wstecznej sile elektromotorycznej, momentach pulsacyjnych, sile zaczepu i momencie statycznym dla obu topologii uzwojenia. (Porownanie pojedynczoi podwojnie-warstwowych uzwojen w maszynie synchronicznej z magnesami trwalymi i strumieniem osiowym)

Design and prototyping of an optimised axial‐flux permanent‐magnet synchronous machine

This study presents the design and performance analysis of a prototype axial-flux permanent-magnet (AFPM) synchronous machine. First, the design of AFPM machine is optimised by genetic algorithm based sizing equation and finite element analysis. The design objectives of this machine are maximum power density, minimum total harmonic distortion (THD) of the sinusoidal back-electromotive force (back-EMF) waveform and low cogging torque. Based on the optimised design of the machine a prototype 1 kW, three-phase, 50 Hz, four-pole AFPM synchronous machine is built. Then, the performance of the prototype machine is tested to see the cogging torque, torque–speed characteristic, efficiency and the THD of the induced voltage. It is found that the prototype machine validates the design in terms of high-power density, lowest possible THD of the back-EMF, low cogging torque while maintaining high efficiency.

Structure analysis of axial flux permanent magnet synchronous machine for wind generators

This paper presents the structure design and analysis of inner-rotor type axial-flux permanent magnet synchronous machines(AFPMs). The major sources of vibration in electric machines such as motors and generators are electric magnetic force distributed on both stators. The rotor suffers from unbalanced magnetic force when air gas are not symmetric on dual sides. In this paper, natural frequencies, mode shapes, and deformation were studied at normal and fault operations.

Electromagnetic Analysis of Axial-Flux Permanent Magnet Synchronous Machines With Fractional Windings With Experimental Validation

Axial-flux permanent magnet (PM) synchronous machines are suitable for direct drive applications. Axial-flux PM synchronous machines with fractional windings provide high power density, low torque ripple, and sinusoidal phase voltages, but suffer from high rotor losses due to stator field harmonics and slotting. This paper formulates an electromagnetic model based on the polar coordinate representation of the air-gap magnetic field. No-load and load tests are carried out on a soft-magnetic-composite stator axial-flux machine to validate the model. Rotor losses represent the largest loss figure in this machine. Most rotor losses are due to slotting harmonics.

A Combined Wye-Delta Connection to Increase the Performance of Axial-Flux PM Machines With Concentrated Windings

In this paper, a combined wye-delta connection is introduced and compared with a conventional wye-connection of a concentrated winding. Because the combined wye-delta connection has a higher fundamental winding factor, the output torque is higher for the same current density when a sinusoidal current is imposed. As the combined wye-delta connection has only a minor influence on the losses in the machine, the efficiency of the machine is also increased. The combined wye-delta connection is illustrated in detail for an axial-flux permanent-magnet synchronous machine with a rated power of 4 kW at a fixed speed of 2500 r/min, using finite element computation and measurements on a prototype machine.

Field weakening capability investigation of an axial flux permanent-magnet synchronous machine with radially sliding permanent magnets used for electric vehicles

Due to the advantage of high power density compared with the conventional radial flux machines, the axial flux permanent-magnet synchronous machines (PMSMs) are very suitable candidates for the power train of electric vehicles (EVs). In this paper, a new axial flux PMSM adopting radially sliding permanent magnets (PMs) to fulfill field-weakening control and to improve the operating speed range is investigated. The field-weakening structure and principle of the axial flux PMSM with radially sliding PMs are proposed and analyzed. The influence of radially sliding PMs on electromagnetic performances and parameters is analyzed based on FEM. The field-weakening method with radially sliding PMs, which is a mechanical method, is compared and combined with traditional electrical method. Due to the optimized combination of the two methods, the field-weakening capability of the machine is much improved and the maximum speed can reach up to six times of the base speed with constant power, which is very satisfying for EV drive application.

Evaluation of a Simple Lamination Stacking Method for the Teeth of an Axial Flux Permanent-Magnet Synchronous Machine With Concentrated Stator Windings

In this paper, a simple lamination stacking method for the teeth of an axial flux permanent-magnet synchronous machine with concentrated stator windings is proposed. In this simple lamination stacking method, only two lamination profiles are used and are stacked alternately. To evaluate the performance of this stacking method, a comparison is made between the proposed method with two profiles and a conventional stacking method that uses different profiles for each lamination layer, using a multilayer 2-D finite element model.

3-D Formal Resolution of Maxwell Equations for the Computation of the No-Load Flux in an Axial Flux Permanent-Magnet Synchronous Machine

This paper presents a 3-D analytical model of an axial flux permanent-magnet synchronous machine, based on formal resolution of Maxwell equations. This method requires much less computation time than conventional 3-D finite elements, and is therefore suitable for optimization purposes. In a first part, the mathematical procedure used to compute the machine no-load flux is described in detail. This method is 3-D, and then takes into account the radial edge effects of the machine, as well as the curvature effects by a resolution in cylindrical coordinates. Moreover, the originality of this method lies in the fact that it is totally analytical. The obtained results are verified using 3-D finite elements, and compared with simpler analytical models of axial flux machines, taken from the literature. This work puts in evidence the advantages of the proposed model. In particular, it is shown that the radial edge effects are important for a correct estimation of the no-load flux. On the contrary, the curvature effects are a second-order phenomenon.

Comparison of Nonoriented and Grain-Oriented Material in an Axial Flux Permanent-Magnet Machine

The performance and iron losses of an axial flux permanent-magnet synchronous machine (AFPMSM) using nonoriented (NO) steel are compared with the performance and iron losses of an AFPMSM using grain-oriented (GO) material. The machine is modeled by several 2-D finite element models in circumferential direction, at different radii. The material model for the GO material is an anhysteretic anisotropic model based on the magnetic energy. The magnetic energy is computed by using several measured quasi-static BH-loops on an Epstein frame in seven directions starting from the rolling direction to the transverse direction. The losses are calculated a posteriori, based on the principles of loss separation and dynamic loop measurements. A loss model was made for each of the seven directions, assuming unidirectional fields. In comparison with the more usual NO material, both the saturation induction and the torque are higher with GO material. The magnetic field in the GO material is lower than for NO material in the major part of the iron, but higher in the tooth tips where the field is not in the rolling direction. The stator iron losses are about 7 times lower for the considered GO compared to the NO material.

Axial-Flux Permanent-Magnet Generator for Induction Heating Gensets

This paper presents a single-phase slotless axial-flux permanent-magnet synchronous machine for induction heating gensets. A full-scale prototype of the machine (110 kVA, 400 Hz, 690 A) has been designed and subsequently analyzed through finite element analysis (FEA). Induced current distributions in the permanent magnets and in the rotors have also been calculated through FEA, showing that the resulting losses are kept at bay due to the low armature reaction. An effective way of achieving regulation of the power transferred to the load has been analytically derived. The prototype has also been built, and experimental tests confirm the aforesaid analyses.

Torque Analysis of an Axial Flux Permanent Magnet Synchronous Machine by Using Analytical Magnetic Field Calculation

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> This paper presents a torque analysis of an axial flux permanent magnet synchronous machine (AFPMSM) based on analytical analysis of magnetic field. The proposed method for torque analysis requires less computational time and is more flexible than finite element method (FEM). This is important advantage, especially for the early stages of the design process. Torque measurements on prototype AFPMSM confirm the validity of the torque analysis method. </para>

Structural mass in direct-drive permanent magnet electrical generators

Direct-drive permanent magnet (PM) synchronous machines for wind turbines are large, heavy and thus expensive to build, transport and install. The structure that is required to maintain the airgap clearance is often heavier than the electromagnetically active material. It is shown that the structural material can be estimated early on in the design process for axial-flux and radial-flux machines. By moving to a design without iron in the stator, the forces that the machine structure must deal with can be significantly reduced. The mass of iron-cored and air-cored axial-flux PM synchronous machines for a range of multi-MW wind turbines is compared. Scaling laws for PM generators in direct-drive wind turbines are also developed.

Preliminary Evaluation of a Megawatt-Class Low-Speed Axial Flux PMSM With Self-Magnetization Function of the Armature Coils

This paper presents an axial flux permanent magnet synchronous machine (PMSM) with a disk rotor between two stators, which are wound in a tooth-coil technology (a concentrate winding type with subunitary number of slots/pole/phase). This type of winding facilitates the magnetizing inductor function of stator coils. A potential magnetizing system can be envisioned by taking into account that the coil span and the slot pitch are practically identical, the number of tooth-coils and poles are almost the same, and that there are two rotor poles between four tooth-coils (two adjacent tooth-coils on each stator). Thus, a special magnetizing inductor is not required in order to magnetize, pair by pair, the permanent magnet (PM) rotor poles. The number of tooth-coils exceeds the number of rotor poles by one ( <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Ns</i> = 2 <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">p</i> + 1). Some aspects of the average performance obtained in both modes of operation-machine and magnetizing inductor-are highlighted by digital simulation. A scale-down demonstrating model also confirmed the feasibility of the magnetizing inductor function of the stator.

Quasi-3-D Analytical Modeling of the Magnetic Field of an Axial Flux Permanent-Magnet Synchronous Machine

A quasi-three-dimensional (3-D) analytical model of the magnetic field in an axial flux permanent-magnet synchronous machine is presented. This model is derived from an exact two-dimensional analytical solution of the magnetic field extended to the 3-D case by a simple and effective radial dependence modeling of the magnetic field. The obtained quasi-3-D solution allows rapid parametric studies of the air-gap magnetic field. Then, analytical modeling of the cogging torque is presented. It is based on the obtained quasi-3-D analytical solution. Results issued from the proposed model in the air gap are compared with those stemming from a 3-D finite-element method simulation as well as with prototype measured values.

Analysis of the cogging torque behavior of a two-phase axial flux permanent magnet synchronous machine

This paper deals with the study of the cogging torque of a small two phases axial flux permanent magnet synchronous machine (AFPMSM). Analytical models of airgap permeance and of magneto-motive force (MMF) are proposed. In order to validate the developed analytical model, the obtained results are compared with that of a three dimensional (3D) finite element method (FEM) simulation of the motor in a special case. The cogging torque due to the permanent magnets (PM) is then estimated based upon the virtual works method. The influence of both magnetization distribution and shape of stator auxiliary poles on the cogging torque is evaluated by means of the developed analytical model.