Axial Flux Permanent Magnet Synchronous Machine Research Articles

An analytical formula for back EMF waveform of a counter‐rotating dual‐rotor non‐slotted axial flux permanent magnet synchronous machine

AbstractFractional slot concentrated winding (FSCW) permanent magnet (PM) machines are gaining more attention for low‐speed applications because of their great advantages. This paper proposes a new structure for an FSCW double rotor axial flux PM synchronous machine (DRAFPMSM) with counter‐rotating rotors. In addition, an analytical formula for the Back EMF waveform of the proposed structure is presented. The accuracy of the formula is investigated by designing a 40 W sample machine. The rotors of this machine rotate with speeds of 600 and 428 rpm in opposite directions. The Back EMF of the sample machine is calculated using the suggested formula. In addition, the finite element method (FEM) is used to determine the Back EMF waveform. Comparison of the obtained results confirms that the new formula yields the Back EMF precisely such that the results well match to those of FEM. The effect of the relative position of two rotors on the Back EMF waveform is investigated using the proposed formula. In addition, the optimum relative position of two rotors for minimizing the Back EMF THD of the case study machine is determined.

FEM and CFD thermal modeling of an axial-flux induction machine with experimental validation

Axial-flux electrical machines are ideal candidates as in-wheel motors for electrical vehicles (EVs). Due to their characteristics of high power density and compact structure, thermal management is vital for them. Lowering the temperature of the stator windings can protect the insulation material from rapid degradation and reduce the extra copper losses by decreasing their electrical resistance. Contrary to the widely reported axial-flux permanent magnet synchronous machines, thermal modeling methods of axial-flux induction machines are rarely seen in previous literature. Hence, the present work aims at investigating their thermal response based on both the finite element method (FEM) and computational fluid dynamics (CFD) techniques. In addition, a test rig is built to validate the computed results of these two thermal models with the experimental measurement. The CFD conjugate heat transfer analysis is found to be more accurate than the FEM thermal analysis in predicting the temperature distribution of different components in the machine and the temperature rise of the airflow, with lower than 5 ∘C average errors deviating from the corresponding measured data at three rotation speeds. Additionally, the CFD simulation is able to capture the backflow occurring near the outlets of the casing that has been found during the experiments.

The Neutral Voltage Difference Signal as a Means of Investigating Eccentricity and Demagnetization Faults in an AFPM Synchronous Generator

This article investigates the neutral voltage difference signal, VNO signal, for fault diagnosis. The aforementioned signal is the signal of the voltage between the common star point of the stator and the common star point of the load. The under-study faults are demagnetization and static eccentricity faults, while the machine in which the faults are investigated is an axial flux permanent magnet (AFPM) synchronous generator, suitable for wind power applications. This study was conducted using a 3D finite element method (3D-FEM), and the machine’s FEM model was validated through experiments. This method is one of the most accurate methods for electrical machine computation, allowing for a detailed study of electromagnetic behavior. The components that constitute the VNO signal were determined using a 3D-FEM software program (Opera 18R2). Subsequently, further analysis was performed using MATLAB R2022b software, and a fast Fourier transform (FFT) was applied to this signal. In all the investigated faulty cases, new harmonics appeared, and the healthy amplitudes of most of the already existing harmonics increased. These findings can be used for fault identification. The analysis revealed that the harmonic frequency of 1.5fs was the most dominant in the case of demagnetization, while in the case of static eccentricity, the most dominant harmonic was a frequency equal to the machine’s operating frequency, fs. The novelty of this study is that this signal has not previously been used for fault identification, especially in AFPM synchronous machines. This signal depends on EMF voltage and stator phase currents but is less sinusoidal. Consequently, it can detect faults in cases where the aforementioned signals cannot be used for detection.

Mechanical and Thermal Design of an Optimized PCB Motor for an Integrated Motor Drive System With GaNFETs

PCB motors are a viable alternative to commercial electric motors as they are easy to mass-produce and have low manufacturing costs. Moreover, integrated motor drives have gained popularity thanks to their high power density and ease of installation. However, the mechanical and thermal aspects are challenging due to reduced volume. In this paper, a GaNFET switched integrated motor drive axial-flux permanent magnet synchronous machine with a printed circuit board (PCB) stator is designed. The machine is optimized using a multi-objective, non-dominated sorting genetic algorithm (NSGA-II). Two stator types are designed and manufactured to compare the effect of eddy losses. A method for measuring eddy current losses on PCB stators is proposed and used. A mechanical and thermal solution is proposed and tested. Integrated motor drive with sinusoidal pulsed width modulation (SPWM) is used to drive the motor up to 5000 RPM at a 1 MHz switching frequency. The overall system‘s efficiency is 82% at 0.36 N.m and 90% at 0.18 N.m torque. The system has an output power of 270 W of output power with 59 °C stator temperature that proved the effectiveness of the proposed passive cooling method.

Multiagent Control in Modular Motor Drives by Means of Deterministic Consensus

A modular motor drive (MMD) is composed of multiple identical pole drive units (PDUs), which consist of a concentrated stator winding fed by a dedicated power electronic converter module. To exploit the full flexibility and reliability of these MMDs, they are equipped with multiple controllers, operating as synchronized peers, and each driving only a limited set of PDUs. An MMD can hence be considered as a multiagent system, in which all the agents cooperate to deliver a certain torque demand. The goal of this work is to distribute this torque demand among the agents of a modular drive in a completely decentralized way, in order to comply with the flexibility and reliability of the hardware, while still taking into account the present condition of each agent. A deterministic consensus algorithm is used for this purpose, in combination with decentralized proportional integral (PI) current controllers. Communication is only allowed between neighboring agents, to limit the communication load. It is shown in this article that the total torque demand is delivered, under healthy conditions as well as during agent malfunctions and with nonidentical agents. The concept is demonstrated on a modular axial-flux permanent magnet synchronous machine with 15 PDUs that are assigned to five agents.

Design and Analysis of a Self-Circulated Oil Cooling System Enclosed in Hollow Shafts for Axial-Flux PMSMs

With the every-increasing demands for miniaturization, the efficient cooling techniques for axial-flux permanent magnet synchronous machines (AF-PMSMs) have attracted great attentions. In this paper, the thermofluid behaviors of a 7-kW 4000-rpm AF-PMSM are numerically investigated based on a coupled model, and the computations are validated by thermal tests performed on the AF-PMSM prototype. To enhance the heat dissipation capacity of the rotor components, it is proposed a self-circulated oil hermetically cooling system, for which the cooling liquid circulates in a hollow shaft. During operation, the oil flow driven by the centrifugal blades forms a closed oil circuit, and can create an effective heat flow path from the hot areas to the cold regions. The oil cooling system is employed in the AF-PMSM to effectively maintain, and evenly distribute, the motor temperature rise. The cooling effectiveness is proved by fluid flow and thermal coupled analyses. To further improve the heat dissipation capacity, the self-circulated oil-cooling structure is modified based on the calculated fluid flow distribution. Finally, the structural parameters are optimized by Taguchi method to minimize both the temperature rise and the rotor weight increment, and the optimization results are numerically and experimentally validated.

Analysis of winding MMF of PMSM with multi-phase and multi-layer layout using holospectrum method

A holographic spectrum method (HSM) of multi-phase and multi-layer winding magnetomotive force (MMF) for 12-slot/10-pole combination with fractional-slot concentrated-winding (FSCW) is proposed in this paper. Taking two kinds of traditional winding layout as example, amplitude and phase distribution of the vth harmonic are calculated. The holospectrum of the multi-phase and multi-layer winding are derived based on three-phase single-layer (TP-SL) and three-phase double-layer (TP-DL) holospectrum theory. Then the finite element method (FEM) is used to validate the resultant winding MMF calculated by the holospectrum method. Results show that two methods have a good consistency in changing tendency. Prototype manufacture and test of axial-flux PMSM with a double-three-phase four-layer (DTP-FL) winding have been completed. A load experiment with two groups of resistance is established to validate its output power of axial-flux permanent magnet synchronous machine (PMSM). The temperature distribution of axial-flux PMSM is evaluated with PT100 and infrared thermal imager. The experiments shows that the axial-flux PMSM runs at a relative temperature rise.

Multi-Agent Position Estimation in Modular Motor Drives Using Low-Resolution Sensors

Because a modular motor drive is composed of multiple identical pole drive units and controllers, it can be considered as a multi-agent system. This makes modular motor drives particularly interesting during faults, when the remaining healthy agents assure the continuity of operation. Nevertheless, for some crucial information such as the rotor position, these drives still rely on a single sensor which introduces a single point of failure. In this work, this single high-resolution position sensor is replaced by multiple binary, low-resolution position sensors. A vector tracking observer algorithm, implemented in the different controllers, processes this low-resolution sensor data into a position estimate. Subsequently, these position estimates of the different agents are exchanged with other agents using a distributed averaging algorithm. For this latter approach, it is shown in this work that it improves the position estimation under healthy conditions as well as during agent malfunctions. The concept is demonstrated on a modular axial-flux permanent magnet synchronous machine with fifteen pole drive units, each equipped with a low-resolution position sensor, that are assigned to five agents.

A Rotor Cooling Enhanced Method for Axial Flux Permanent Magnet Synchronous Machine With Housing-Cooling

This paper focuses on the rotor thermal design of the axial flux permanent magnet synchronous machine (AFPMSM) with housing water-cooling. By adapting the effect of disc rotor self-ventilation, an enhanced cooling method has been innovated to optimize the temperature distribution of the rotor in AFPMSM. Two scenarios will be created to compare their influence on cooling enhancement of rotor with the diverse types of fins, the annular fin and the rectangular fin, each will be installed separately to the internal surface of housing. There is a significant effect by using this method in all types of AFPMSM with housing cooling, especially for multi-rotor ones. The results of CFD show that the effectiveness of annular fins is essentially enhanced, with 25% more heat dissipation achieved.

A Novel Multi-Layer Structure for Axial Flux Machines

In an axial flux machine (AFM), an effective air-gap area is a ring form of which the size is decided by the outer and inner diameter of the stator. The inner diameter is usually not small, which leaves a relatively large inner space unused because the slot size has to keep uniform radially for current density. This article proposes a multi-layer structure that adopts concentric rings consisting of multiple layers instead of the conventional single layer. A conventional single-sided axial flux permanent magnet synchronous machine (AFPMSM) is introduced as a baseline design with 110 kW output power and 350 mm outer diameter. Through transforming this baseline design, the benefit of this new structure has been revealed with three different investigations in this article: 1) the output power per mass can be improved; 2) the compactness (output power in a limited space) can be increased; and 3) the constant power speed range could be significantly extended with a new type inverter (only design specification of the new inverter given), in which no additional technique of flux modulation required. The proposed multi-layer structure bypasses the issue that the area of a single ring cannot be enlarged due to the limitation of inner diameter. The air-gap area can be extended to the complete inner radial space for generating electromagnetic force. Besides, two factors, i.e., the number of layers and the coordination between multiple layers, make the performance of AFM highly customizable for different design objectives.

Design and Analysis for Multi-Disc Coreless Axial-Flux Permanent-Magnet Synchronous Machine

The multi-disc axial flux permanent magnet synchronous machine (AFPMSM) can provide high fault tolerance capability in applications requiring high reliability. Firstly, a topological structure of coreless AFPMSM is proposed, the magnetic circuit and the air-gap flux intensity are discussed in detail to improve stable operation of machine in this paper. The relationship between air-gap flux density and the parameters of rotor are clarified respectively based on finite element algorithm (FEA). Furthermore, the winding structure and power driver topology with fault tolerance are presented. And the 3D FEA analyzes the operation characteristics and stress of the machine. Finally, the prototype is manufactured. The results are demonstrated that the torque output and fault tolerance performance is better.

Multi-layer quasi three-dimensional equivalent model of axial-flux permanent magnet synchronous machine

Axial-flux permanent magnet synchronous machine (AFPMSM) enjoys the merits of high torque density and high efficiency, which make it one good candidate in the direct-drive application. The AFPMSM is usually analyzed based on the three-dimensional finite element method (3D FEM) due to its three-dimensional magnetic field distribution. However, the 3D FEM suffers large amount of calculation, time-consuming and is not suitable for the optimization of AFPMSM. Addressing this issue, a multi-layer quasi three-dimensional equivalent model of the AFPMSM is investigated in this paper, which could take the end leakage into consideration. Firstly, the multi-layer quasi three-dimensional equivalent model of the AFPMSM with single stator and single rotor is derived in details, including the equivalent processes and conversions of structure dimensions, motion conditions and electromagnetic parameters. Then, to consider the influence of end leakage on the performance, a correction factor is introduced in the multi-layer quasi three-dimensional equivalent model. Finally, the proposed multi-layer quasi three-dimensional equivalent model is verified by the 3D FEM based on an AFPMSM under different structure parameters. It demonstrates that the errors of flux linkage and average torque obtained by the multi-layer quasi three-dimensional equivalent model and 3D FEM are only around 2% although the structure parameters of the AFPMSM are varied. Besides, the computation time of one case based on the multi-layer quasi three-dimensional equivalent model is only 6 min, which is much less than that of the 3D FEM, 1.8 h, under the same conditions. Thus, the proposed multi-layer quasi three-dimensional equivalent model could be used to optimize the AFPMSM and much time could be saved by this method compared with the 3D FEM.

Comparison of PCB winding topologies for axial‐flux permanent magnet synchronous machines

Although axial-flux permanent magnet machines have high torque densities, challenges regarding mass production of stators make them a less appealing choice. Printed circuit board (PCB) axial-flux machine is a type of machine with a stator that is made of layers of PCB. Given the precise, fast, and cheap mass production capabilities of PCB manufacturers, PCB axial-flux machines stand as a viable alternative for conventional round-wire winding machines. In this study, five different winding topologies are compared. Their induced phase voltages and torque are calculated using the developed magnetic scalar potential method and finite element analysis (FEA). Proposed windings are tested on a 16-pole, 2000-RPM, double rotor-single stator axial-flux permanent magnet synchronous machine. Results showed that the parallel winding had the smallest resistance and loss. Moreover, radial and concentric winding had the highest induced voltage and torque while the radial winding had 20% less phase resistance than concentric. Also, the induced voltage of radial winding had the smallest total harmonic distortion in comparison with other winding types. A novel unequal width parallel winding is proposed and it is compared with parallel winding separately. It is found that by simply increasing the cross-section area of wave windings, it is possible to decrease copper loss by 17%.

Performance Analysis of a Coreless Axial-Flux PMSM by an Improved Magnetic Equivalent Circuit Model

In this paper, an improved magnetic equivalent circuit (MEC) model of a coreless axial-flux permanent-magnet synchronous machine (AFPMSM) with printed circuit board (PCB) winding is presented, which considers the curvature effect, fringing effect, leakage flux, and rotor yoke's magnetic saturation. The axial air-gap flux density distribution, no-load back electromotive force, winding inductances, and electromagnetic torque are predicted by the proposed MEC model. The steady-state performances of the machine under heavy load, as well as the dynamic response of the machine with a connected voltage source and mechanical load, are calculated. All the results obtained from the proposed model are in good agreement with those issued from the 3D finite element method (FEM). The computation time of the proposed model is greatly reduced compared to 3D FEM. Finally, a coreless AFPMSM prototype with a PCB stator is manufactured, and experiments are carried out to verify the predicted results.

Coupled electromagnetic and thermal analysis of an axial flux permanent magnet synchronous machines with printed circuit board winding

Axial flux permanent magnet synchronous machines (AFPMSMs) with printed circuit board (PCB) windings take on different electromagnetic and thermal performance characteristics when compared to the traditional radial flux permanent magnet synchronous m

Active Demagnetization Fault Compensation for Axial Flux Permanent-Magnet Synchronous Machines Using an Analytical Inverse Model

In this article, a fault tolerant deadbeat controller is proposed, which is able to compensate for both partial and uniform demagnetization faults in axial flux permanent-magnet synchronous machines with double rotor, even with asymmetric defects in both rotors. For this purpose, the system model used by the deadbeat controller is equipped with a look-up table of the back-emf, in function of the mechanical rotor position. This look-up table is generated by means of an analytical model based on the magnetic vector potential. With this information, the deadbeat controller can eliminate both the additional bias and ripple in the stator current components caused by demagnetization faults. The proposed control strategy is validated on a 4 kW test set-up of an axial flux permanent-magnet synchronous machine with yokeless and segmented armature topology fed by a three-phase two-level voltage source inverter, both by simulations and experiments. Key performance indicators concerning bias and ripple of the current components illustrate the superior performance of the proposed control strategy in comparison to a standard deadbeat controller without an up-to-date look-up table of the back-emf.

New finite element based method for thermal analysis of axial flux interior rotor permanent magnet synchronous machine

A new method for computing convection heat transfer coefficients in axial flux permanent magnet synchronous machines (AF-PMSMs) is introduced. The method is based on a two-step finite element analysis (FEA). In the first step, a simple 2D model is introduced and fluid flow and temperature fields are studied using computational fluid dynamic (CFD) technique. Convective heat transfer coefficients for all heat transfer surfaces of the model are extracted from CFD analysis and are then used as inputs to a 3D thermal finite element model. Two case studies are considered to check the accuracy of the abovementioned two-step method. One of the case studies is an air-cooled AF-PMSM that has air inlet and outlet on its enclosure and the other case is a totally enclosed AF-PMSM. The correctness of the mentioned method is verified by practical measurements on the second case. In addition, an experimental set-up is conducted to measure the temperature of different parts of the second case. It is observed that the results of the two-step CFD-FEA model are in good agreement with the experimental data. In the other case, the effect of inlet air velocity on heat transfer coefficients, as well as the thermal behaviour of the machine are explored.

Dual-Skew Magnet for Cogging Torque Minimization of Axial Flux PMSM With Segmented Stator

Reduction of the cogging torque is preferred during the design process of a permanent magnet (PM) synchronous machine (PMSM), especially in the low-speed traction applications. This article presents a dual-skew magnet technique to minimize the cogging torque in the axial-flux permanent-magnet (AFPM) motor with a yokeless and segmented armature (YASA). The theoretical expression of the cogging torque of the AFPM machine is deduced, and the advantages of the dual-skew magnet are verified by comparing with a sector shape magnet and a conventional skew magnet using the 3-D finite-element method (FEM). The simulation results show that, compared with the other two magnet topologies, the machine equipped with the dual-skew magnet shows its advantages in the reduction of cogging torque, torque ripple, and magnet eddy current loss.

2-D Analytical Model for Predicting Magnetic Flux Distribution in Slotless Single-sided Axial Flux Permanent-Magnet Synchronous Machines

This paper estimates the magnetic flux density components in the slotless single-sided axial flux permanent-magnet synchronous machines (SAFPMSMs). For this purpose, a 2-D analytical model based on the sub-domain method is utilized in which the cross-section of the presented machine is divided into the seven sub-regions such as stator side exterior, stator, winding, air-gap, permanent-magnets (PMs), mover and mover side exterior. Based on the Maxwell equations, the related partial differential equations (PDEs) of magnetic flux density components are formed in each sub-region which are identified as the essential step for obtaining the machines quantities. According to the superposition theorem, two separate steps are implemented for calculating the magnetic flux density components. In the first step, open circuit analysis includes various type of magnetization patterns, i.e. parallel, ideal Halbach, 2-segment Halbach and bar magnet in shifting direction is investigated and armature currents are zero and in the second step PMs are inactive and the magnetic flux density components are originated due to only armature reaction. Eventually, 2-D finite element method (FEM) is determined to confirm the accuracy of the presented analytical approach and an acceptable agreement between the analytical and FEM models can be observed.

Design, Analysis, and Prototyping of a Water-Cooled Axial-Flux Permanent-Magnet Machine for Large-Power Direct-Driven Applications

This paper presents a design process and a detailed multiphysics analysis of an axial-flux permanent-magnet synchronous machine for large-power direct-drive applications. The machine in this paper is 130 kW at 26 r/min with a dual-stator inner-rotor structure. A stator core that is assembled with segmented and prewound teeth is first proposed and applied in a large-power axial-flux permanent-magnet machines (AFPMs). Through this method, the challenges of manufacturing large-diameter AFPMs can be solved. A novel water-cooling system is embedded in the machine to transfer the heat. In addition, the assembling procedure and the manufacture process are also proposed, and a novel distributed follower bearing is used to reduce the deformation and stress of the rotor disk. Finally, based on the multiphysics design, a prototype machine is manufactured and tested. The experiment results match well with the finite-element analysis.