Effects of manufacturing tolerances of permanent magnets in fractional slot permanent magnet synchronous machines

The fractional slot permanent magnet synchronous machines are well-known for their low torque ripple. However, in mass-production machines, not all the permanent magnets are identical due to the manufacturing tolerances and new torque harmonics could appear. In addition, the magnetic field is modified and unbalanced magnetic pull (UMP) could happen. Apart from that it is important to study the effect on vibrations since the magnetic field is no longer ideal. So, here, a study of the effect of unevenly magnetised permanent magnet distribution on torque ripples, unbalance magnetic pull, and vibrations is proposed. Apart from that, the tolerance grade sensitivity is studied. Finally, experimental tests show good agreement with finite element analysis

A Systematic Study on the Effects of Dimensional and Materials Tolerances on Permanent Magnet Synchronous Machines Based on the IEEE Std 1812

In the process of designing and manufacturing an electrical machine, a systematic study of dimensional and material tolerances is of the utmost importance. This paper proposes a systematic method by which the effect of design specification variations on permanent magnet (PM) synchronous machine performance may be identified and quantified. The method combines design of experiments techniques, open-circuit and short-circuit physical measurements, and virtual test simulations conducted based on the recently approved IEEE Std 1812 testing guide. Three case studies, two provided by a spoke-type PM radial field machine configuration, in two designs with different electromagnetic loading, and an axial flux PM machine are discussed. It is shown that based on the output performance, out of specification tolerances for magnet remanence, steel grade, as well as dimensional variables, and stator to rotor eccentricity, may be identified under certain conditions. It is also exemplified that the ratings, magnetic loading, and configuration of the machine play critical roles and should be thoroughly considered as part of the studies

Influence of manufacturing tolerances and eccentricities on the unbalanced magnetic pull in permanent magnet synchronous motors

Eccentricity is an inevitable fault in electric motors and hence its diagnosis is an important topic. Thus, the influence of static and dynamic eccentricities on the harmonics of the frequency spectra of the unbalanced magnetic pull is analyzed.In this study, dimensional tolerances of the rotor and the stator are also considered. All parts have dimensional tolerances in their designs and their real magnitudes vary to some extent from the theoretical values after the manufacturing process. Thanks to the finite element simulations, verified with experimental results, it is observed that the deviations originated by the manufacturing tolerances produce changes in the amplitudes of some harmonics and also additional and characteristic harmonics in the frequency spectra of the unbalanced magnetic pull. These are not negligible and must be taken into account when robust eccentricity detection procedures are defined. Otherwise, harmonics originated by tolerances and by eccentricities can be misidentified

Optimum Design of Axial Flux PM Machines based on Electromagnetic 3D FEA

Axial flux permanent magnet (AFPM) machines have recently attracted significant attention due to several reasons, such as their specific form factor, potentially higher torque density and lower losses, feasibility of increasing the number of poles, and facilitating innovative machine structures for emerging applications. One such machine design, which has promising, high efficiency particularly at higher speeds, is of the coreless AFPM type and has been studied in the dissertation together with more conventional AFPM topologies that employ a ferromagnetic core. A challenge in designing coreless AFPM machines is estimating the eddy current losses. This work proposes a new hybrid analytical and numerical finite element (FE) based method for calculating ac eddy current losses in windings and demonstrates its applicability for axial flux electric machines. The method takes into account 3D field effects in order to achieve accurate results and yet greatly reduce computational efforts. It is also shown that hybrid methods based on 2D FE models, which require semi-empirical correction factors, may over-estimate the eddy current losses. The new 3D FE-based method is advantageous as it employs minimum simplifications and considers the end turns in the eddy current path, the magnetic flux density variation along the effective length of coils, and the field fringing and leakage, which ultimately increases the accuracy of simulations. After exemplifying the practice and benefits of employing a combined design of experiments and response surface methodology for the comparative design of coreless and conventional AFPM machines with cores, an innovative approach is proposed for integrated design, prototyping, and testing efforts. It is shown that extensive sensitivity analysis can be utilized to systematically study the manufacturing tolerances and identify whether the causes for out of specification performance are detectable. The electromagnetic flux path in AFPM machines is substantially 3D and cannot be satisfactorily analyzed through simplified 2D simulations, requiring laborious 3D models for performance prediction. The use of computationally expensive 3D models becomes even more challenging for optimal design studies, in which case, thousands of candidate design evaluations are required, making the conventional approaches impractical. In this dissertation a new two-level surrogate assisted differential evolution multi-objective optimization algorithm (SAMODE) is developed in order to optimally and accurately design the electric machine with a minimum number of expensive 3D design evaluations. The developed surrogate assisted optimization algorithm is used to comparatively and systematically design several AFPM machines. The studies include exploring the effects of pole count on the machine performance and cost limits, and the systematic comparison of optimally designed single-sided and double-sided AFPM machines. For the case studies, the new optimization algorithm reduced the required number of FEA design evaluations from thousands to less than two hundred. The new methods, developed and presented in the dissertation, maybe directly applicable or extended to a wide class of electrical machines and in particular to those of the PM-excited synchronous type. The benefits of the new eddy current loss calculation and of the optimization method are mostly relevant and significant for electrical machines with a rather complicated magnetic flux path, such is the case of axial flux and of transvers flux topologies, which are a main subject of current research in the field worldwide

Design and Critical Evaluation of Permanent Magnet Wind Generator Technology for Small-scale Wind Energy Systems

Electrical and Electronic Engineerin

A fault tolerant motor drive for electric power steering systems

Ph. D. ThesisElectric machines are becoming increasingly prevalent in safety critical transport applications, whether as both the main drive components or in auxiliary systems. An automotive electric power steering system is an auxiliary drive system that replaces conventional hydraulic systems due to its high reliability, low size and cost, high security, good road feeling, control stability and operates when required. Permanent magnet AC motors are one of the most favourable choices for this application due to their high torque and power density, low torque ripple and low acoustic noise. The main challenge with PM machines in a fault situation is the drag torque resulting from short-circuit currents. These currents are induced by fluxes from the permanent magnets. This research investigates a 12 slot 8 pole interior permanent magnet motor. It investigates different winding arrangements and winding connections for a dual-lane system and compares them to a single-lane system. The baseline motor has 4 coils in parallel per phase for a single-lane system, and 2 coils per lane per phase for a dual lane system. In a dual-lane system, the stator coils can be connected in three different arrangements which are interleaved, half-half and quarter. The half-half arrangement is the best compromise for the baseline motor, as it produces the highest average torque and medium torque ripple under a symmetrical 3-phase short-circuit fault. A modular winding was implemented on the baseline motor’s stator to reduce drag torque and torque ripple under faulted conditions. However, the stator core saturates leading to higher torque ripple and a torque drop under normal conditions. Therefore, a new modular stator was developed to overcome saturation. This gave higher torque capability due to the wider wound teeth tooth arc used, and hence a higher winding factor. The fault-tolerance of the modular stator is significantly improved due to the higher coil inductance and lower drag torque. In the constant power region, the power is significantly compromised. The knee point speed is affected as the high q-axis inductance limits the availability of the supply voltage at a lower speed. Various approaches are presented that aim to reduce the overall motor inductance or only the q-axis inductance to recover the power drop. Firstly, the baseline motor’s rotor is shaped to reduce the q-axis flux. This is not feasible as the power cannot be fully recovered and the torque ripple becomes considerably high. Secondly, the number of turns is reduced, and the input current is increased to keep the MMF input unchanged. Using this approach, the power drop is fully recovered, but a thicker wire diameter should be used for winding, and higher input current means higher ECU losses. Finally, a novel SPM motor is presented. This overcomes the constant power region torque drop through reducing the q-axis inductance. Compared to the baseline motor, the power and torque density of the motor are considerably higher. The overall stator and rotor stack lengths are shorter. As the end-windings are bigger that affects the motors overall length which might also affect the motor size and packaging. The coils and motor lanes are segregated which helps in reducing the torque ripple under faulted condition. At lower speeds the average torque available within a short circuit or single MOSFET fault within the drive stage is similar to that of the baseline motor. At higher speeds the SPM offers greater average torque capability than the baseline motor.ZF Group, Newcastle Universit

Torque Performance Enhancement of Flux-Switching Permanent Magnet Machines With Dual Sets of Magnet Arrangements

Torque performance, especially torque density, is a critical performance index for the flux-switching permanent magnet (FSPM) machine that is attractive for the propulsion system. In this article, a novel FSPM machine with dual sets of magnet arrangements is proposed. With the novel topology, the torque density of the proposed machine is significantly improved due to much increased working harmonic contents of magnetomotive force (MMF). Moreover, the cogging torque is also inherently reduced, which makes the proposed machine a promising candidate in the FSPM machine family. The operating principle of the proposed FSPM machine is revealed based on the MMF-permeance model and the numerical finite element analysis (FEA). The effect of geometric parameters, such as magnet thickness, auxiliary tooth width, and rotor tooth width on the average torque and cogging torque, is also investigated. Finally, a prototype has been manufactured to validate the analysis conclusion. With experimental test results, it is demonstrated that the proposed topology can achieve 30.8% higher torque density, 79.4% lower cogging torque, and 15.6% higher power factor than the conventional counterpart