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Most permanent magnet (PM) loss studies consider only eddy current loss and neglect hysteresis. In this article, the hysteresis behavior of two NdFeB PM grades with different magnetic properties is assessed when applied in a PMSM. Data from vibrating sample magnetometer measurements and hysteresis modeling are used as a base. In addition to the main magnetic phase, the samples contained magnetic phases with reduced coercivity. Such phases may contribute to hysteresis losses in a PM material. A new model is introduced to simulate the hysteresis of rare-earth magnets of any geometric shape in the second and first quadrants of the intrinsic BH -plane. The magnetic field strength distribution in the PM material of an electrical motor is analyzed by two-dimensional finite-element method. The results are used as the input data for an analytical hysteresis model. The results indicate that the hysteresis loss resulting from the structural imperfections and geometry of the magnet may introduce a considerable loss in NdFeB PMs applied in rotating electrical machines.

The impact of the current vector angle on the performance of a synchronous reluctance motor (SynRM) with an axially laminated anisotropic (ALA) rotor intended for high-speed applications is studied. The paper shows that the current vector angle not only impacts on the stator winding Joule losses and iron losses, but also strongly influences eddy current losses in the rotor (when the rated torque is produced). The rotor eddy current losses are determined by the flux density harmonics produced by the stator. The air gap flux is affected both by the magnitude and angle of the current vector. However, the rotor surface harmonics are not directly related to the overall flux level (which can be quite low at a high current angle) but depend more on the values of the individual slot current linkages. Considering the dependence of rotor losses on the current vector angle, the most efficient operating point is suggested to be close to 45° or slightly larger, up to 65°, where a compromise between increasing rotor eddy current losses and winding Joule losses and decreasing stator iron losses is found. The eddy current loss distribution between the layers of an ALA rotor is analyzed in detail. With a larger current vector angle, the eddy current losses increase most in the layers close to the d-axis. This is explained by the largest current linkage in the stator slots that are in front of the layers close to the d-axis. The dependence of torque ripple on the current vector angle is also observed. An analysis of rotor and stator high-order harmonics, which determine the torque ripple, is performed.

A fully submersible permanent magnet synchronous machine (PMSM) is introduced in this paper. Underwater operation requires that the machine parts have to be resistant to corrosion, and the electrically conducting parts need to be properly insulated from water. The machine under study consists of a rotor-surface permanent magnet rotor, protected by a glass-fiber-reinforced plastic (GFRP) cover, and a fully stainless stator. The novelty of the machine is found in the design and special materials used in the manufacture. The stator core is made of ferritic stainless steel laminations, and the selected winding material is polyvinyl chloride (PVC) insulated copper wire. An extra low-speed, stainless-core submersible PMSM was constructed and tested. To simplify the machine construction, a tooth-coil single-layer winding was adopted. An asymmetric stator design was selected to enhance the machine performance. The performance of the 1.7 kW, 80 r/min machine was analyzed by finite element analysis (FEA) and validated by experimental tests, where despite a very low rotational speed, 74% efficiency was reached at the target load point. The PMSM was found to be fully functional for the application.

The research was focused on the development of technologies for long-term storage of probiotics immobilized in gel carriers at various low temperatures. The aim of the research was to investigate the viability and adhesive activity of probiotic microorganisms after immobilization in an alginate gel and storage at various low temperatures for 24 months (observation period). Probiotic strains of the microorganisms Escherichia coli M–17 (E. coli M–17), Lactobacillus acidophilus IMB B-2637 (L. acidophilus), Saccharomyces cerevisiae IMB Y-505 (S. cerevisiae) were immobilized in 1% alginate gel either without additives or supplemented with lactose, sucrose, skimmed milk and protective SML medium. The samples were frozen at uncontrolled cooling rates in refrigerators with temperatures of –20, –40, –75 °C. When then frozen down to –196 °C, the samples were cooled to –40 °C at the rate of 1 deg / min, and then immersed in liquid nitrogen. The samples were stored at the indicated temperatures for 24 months (observation period). During storage, part of the granules was thawed in a water bath and dissolved in 4% EDTA solution. The viability of microorganisms was determined by the ability of microbial cells to form colonies on agar media. Adhesive activity was determined by the ability to adhere to human erythrocytes 0(I) group, Rh (+). When cooled down to –20, –40, –75, –196 °C and after subsequent storage at –196 °C, immobilized microorganisms in all varieties of gel did not die. In most samples cells died between 1 and 3 months of storage at –20, –40, –75 °C. After 24 months, it was found that the most resistant to storage at these temperatures were the bacteria L. acidophilus, and the most sensitive were the yeast S. cerevisiae. The viability of all microorganisms immobilized in 1% gel without additives was minimal. The addition of disaccharides of lactose, sucrose, SML medium and milk to the alginate gel increased the cryoprotective properties of the alginate gel. The highest viability of immobilized microorganisms was observed in the gel with SML medium. As the storage temperature decreased from –20 to –75 °C, the number of viable microbial cells in all gel samples increased. Immobilization in gel carriers and storage for 24 months did not affect the adhesive properties of probiotics E. coli M–17, L. acidophilus, S. cerevisiae. The changes in the viability of microbial cells are associated with the severity of damaging physicochemical factors that accompany the processes of water crystallization at temperatures of –20, –40, –75 °C. The protective effect of additives in the composition of the alginate gel is analyzed. The viability of microorganisms immobilized in gel granules during long-term storage at different low temperatures is influenced by the species cryoresistance of microorganisms, temperature regimens and storage duration, the composition of the gel carrier. The processes of immobilization and storage at low temperatures did not change the adhesive activity of immobilized microorganisms.

This paper studies a modular permanent magnet synchronous machine (MPMSM) converted from a traditional single-core 24-slot 20-pole tooth-coil winding permanent magnet synchronous machine (TCW PMSM). The performance of the TCW PMSM is compared with the performance of the MPMSM (rearranged from this TCW PMSM) by the finite element method (FEM). It is found that if an electrical machine with a modular structure is designed, the efficiency of the MPMSM may be degraded compared with the conventional structure, especially, if a solid rotor yoke is used. However, the advantages of the proposed MPMSM are related to an option to scale up the machine by applying a certain number of modules, or removing faulty modules (while keeping the healthy modules working). This makes this type of a motor design applicable in certain life-critical applications, or in applications where scaling of the motor power might be needed. As a prototype, a low-power, low-speed generator added to a hook block of a hoist to supply power to measurement electronics is studied.

This paper presents an analysis of a permanent magnet synchronous machine (PMSM) with an unequal teeth width. The use of the concentrated nonoverlapping winding, also known as tooth-coil winding, has certain advantages. In addition, asymmetric features can be exploited by adopting this winding configuration to improve the machine performance. The procedure involves increasing the stator tooth width and, to the same extent, reducing the adjacent stator tooth width. In this paper, an analytical method was employed to predict and understand the back-EMF behavior, and a finite element analysis (FEA) was performed to verify the analytical method. Furthermore, a detailed study of the machine performance (electromagnetic torque, flux density, and losses) was carried out by the FEA. The machine was built and tested to validate the performance of the machine.

The phenomenon of third-order harmonics in synchronous reluctance motors (SynRMs) with an axially laminated anisotropic (ALA) rotor is studied. The paper shows that a specific rotor geometry can distort the air-gap flux density and induce third-order currents in the stator winding if it is delta connected. The third-order currents in different phases have the same phase shift angle and cannot, therefore, produce a rotating air-gap flux. Instead, they produce an air-gap flux that pulsates in time, and thus, causes eddy current losses in the rotor conducting parts. The phenomenon was described and proven with finite element simulation (FEM) of a 12 kW high-speed ALASynRM. The research underlines the importance of considering the third harmonic phenomenon in the design of a two-pole ALASynRM intended for high-speed applications, where a specific rotor geometry with a wide magnetically conducting middle layer is required.

The impact of the thickness of the magnetic and nonmagnetic layers in an axially laminated anisotropic (ALA) rotor in a synchronous reluctance motor (SynRM) aimed at high-speed applications was studied. Considering possible manufacturing issues, the layers are desired to be thick rather than thin. At the same time, the layer thickness is related to the electromagnetic capabilities of the ALASynRM. In the study, a 12 kW ALASynRM was considered. As a reference, a 12 kW IM with a smooth solid rotor with copper end rings was used for comparison with the designed ALASynRM. The manufacturing procedures of an ALA rotor with certain materials were verified in practice. Strength tests of the samples were implemented showing the suitability of the selected materials for application in the prototype. In the electromagnetic design, the thickness of the ALA rotor layers has shown to have a significant impact on the rotor eddy current losses. The stator iron losses and the winding Joule losses also depend on the rotor design. Torque ripple is considerably affected by the thickness of the rotor layers and their position in relation to the stator slots.

The performance capabilities of an axially laminated anisotropic rotor (ALA) in a high-speed synchronous reluctance motor (SynRM) were studied. A 12 kW ALASynRM was designed as an alternative to a high-speed induction motor (IM) with a solid rotor. The electromagnetic design was implemented taking into account possible issues related to the new manufacturing methods, which require thicker rotor layers than in a typical ALA. The ALASynRM shows a higher efficiency than the corresponding IM with a smooth or slitted solid rotor equipped with copper end rings. To verify the design method, a prototype IM with a smooth solid rotor was built and tested. In the analysis, it was found that, similar to IMs, in an ALASynRM a considerable part of losses takes place in the rotor despite the absence of slip-related losses in the SynRM. The distribution of eddy current losses in the ALA rotor is significantly uneven. The torque ripple in the ALASynRM is considerably larger than the corresponding ripple in IMs.

This paper presents a comparison of optimized tooth-coil-winding modular permanent-magnet synchronous machines (TCW-MPMSMs) with H-shape and U-shape stator segments for fault-tolerance applications. Four optimization objectives are studied by means of a Particle-Swarm-Optimization (PSO) based routine: maximum rated torque, minimum torque ripple, maximum efficiency, and minimum mutual inductance. Two 22-pole and 24-slot TCW-MPMSMs with H-shape and U-shape stator cores were evaluated and optimized aiming to optimize each objective. From the results, it was found that U-shape machines are suitable for high-torque and low-mutual-inductance requirements, crucial in fault-tolerant applications, whilst H-shape machines offer low ripple torque and slightly lower performance indices for fault-tolerance applications.

A direct-on-line synchronous reluctance motor (DOLSynRM) with a rotor topology defined by the natural flux line curve approach is studied as an alternative to an induction motor (IM) using a 1.5 kW, 50 Hz synchronous reluctance motor as an example. If an IM stator is used, a direct squirrel cage rotor replacement by a synchronous reluctance rotor equipped with a damper winding might not guarantee a sufficient starting performance. To ensure the strong enough starting capability, the natural flux line curve approach was modified in the motor design. The proposed approach was applied in the optimization of the DOLSynRM using a genetic algorithm. The optimization results show a clear tendency in the rotor design when trying to achieve a reliable starting capability and to maximize the steady-state performance of the DOLSynRM. Both the stator design and the rotor diameter of the optimized DOLSynRM are different from those of a corresponding squirrel cage IM. A DOLSynRM prototype was built and tested. It showed about a 5.4%-unit higher efficiency than the original prototype IM and acceptable starting performance under the rated load condition. The starting capabilities of the motors under study are considered with different loads. The locked rotor apparent power and current stay below permissible limits in both the IM and the DOLSynRM. Post-print / Final draft

High-speed solid-rotor induction machines (IMs) suffer from higher rotor eddy-current losses (as a result of the air-gap flux density harmonics) more than any other type of high-speed machines because of the solid-rotor steel. A double-layer short-pitch asymmetric winding arrangement with prefabricated coils is proposed in this paper to mitigate the solid-rotor losses and enable easier assembly. However, the asymmetric winding also brings some current unbalance because of three-phase asymmetric stator inductances, especially the winding leakage inductances. Current unbalance can cause harmful effects for both the machine and supply, e.g., torque ripples, unbalanced magnetic pulls, and unbalanced thermal loads of the supply network and supply power electronics. Additionally, a three-phase unbalanced current can cause an extra source of electromagnetic emission to the environment, which can be harmful to surrounding electronics and can cause extra eddy-current losses in surrounding solid surfaces (e.g., a metal terminal box). To mitigate the current unbalance, two methods are compared in this paper. The first method is a slight increase of the stator slot height and placing of the coil sides at the top or bottom within the slot height for different phases. The stator slot height is optimized based on the 2-D finite-element method (FEM) to achieve the best solution for mitigation of the current unbalance. The other method is based on the results of the first method, and the coil side position for a specific phase is further adjusted. Unlike conventional methods of mitigating the current unbalance by power electronics, the proposed method suppresses the current unbalance solely by adjusting the machine design, which avoids extra investments for power electronics devices. In addition, the machine control strategy remains unchanged compared with the traditional one.

Finite-element analysis (FEA) is one of the most significant tools in the designing and analyzing of electrical machines, which mainly includes the transient-magnetic (TM), magnet-static (MS) and time-harmonic (TH) solutions. The transient-magnetic (TM) solution in FEA is capable of considering both the harmonic effects and eddy-current effects accurately, which makes it suitable for the induction machine (IM) simulation. However, the drawback of the TM FEA for the IM modelling is that it takes some time before reaching the steady state because of the longer numerical transient, which is affected by the inherent electromagnetic time constants of the IM directly. In this paper, a new approach is proposed to reduce the duration of the numerical transient of the IM in the time-stepping FEA so that the steady state can be achieved in a short time. The proposed approach is capable of creating an initial condition close to the final steady state for the simulation by eliminating or reducing the stator and rotor electromagnetic time constants separately. The stator electromagnetic time constant is eliminated by an initial current excitation (obtained by the TH solution or the analytical method) and the rotor electromagnetic time constant is reduced by a locked rotor model at the beginning of the simulation. Then the initial current excitation is switched to a voltage excitation and the locked rotor is turned to the rotating state at proper time. Finally, the proposed approach is tested and proved efficient to reduce the transients by two typical cases (a solid-rotor IM and a squirrel-cage IM) with 2D FEA.

High-speed induction machines equipped with a solid steel rotor are capable of achieving a high-rotating speed than other types of machines, because of their simpler and more robust rotor structure. At the same time, however, the eddy-current losses in the solid rotor may be critical, because of the high conductivity of the rotor material, which makes it easy for axial eddy currents to travel in the solid rotor. To efficiently mitigate the rotor eddy-current losses, it is important to accurately determine the rotor eddy-current losses induced by a particular harmonic in advance. In this paper, an improved virtual permanent magnet harmonic machine (VPMHM) model equipped with a sinusoidally magnetized virtual magnet based on the finite element method (FEM) is proposed for determination of the rotor eddy-current harmonic losses. The 2-D fast Fourier transform was used to accurately analyze the time-spatial air-gap flux density harmonics. The VPMHM model was enhanced to ensure that it was able to exactly produce the required flux density harmonics in the air gap. Two algorithms for the improved VPMHM models with different hybrid excitations were proposed to determine the harmonic losses together with the other important harmonic behavior. The model was further investigated to separate the electromagnetic transients from different harmonics. Finally, the simulation time for the harmonic losses required by the enhanced VPMHM model was significantly reduced by separating the harmonic transients. All the results and conclusions presented in this paper are based on the FEM analysis.

High-torque-density electrical machines applied to traction operate at a varying torque and speed, and therefore, when minimizing the torque ripple, a wide variety of possible loads must be considered. Different loads need different current vectors, which affect the magnetic state of the machine. Thus, the torque ripple must be analyzed by using appropriate current vectors as it is proposed and described in this paper. The torque ripple values were estimated also for a skewed machine using a new method of torque ripple estimation of a skewed machine. The method is computationally less heavy and allows computing the torque ripple at all possible load points. Using the proposed method in the analysis of the machine parameters, the rotor pole geometry was adjusted to minimize the torque ripple without degradation of the other properties of the machine. Finally, the best machine performance was compared with the measured results.

High-speed solid-rotor induction machines (HSIMs) are popular within high-speed applications because of their high rotor structural integrity and their fairly well-established manufacturing process guaranteeing high quality series products. Designing a new high-speed electric machine requires a multidisciplinary team to accomplish the machine performance desired. In case of a high-speed machine design, the components and applied materials often reach their physical limits at the rated operating condition. Therefore, the design process is highly iterative and thus a systematic approach has a high potential to reduce the time of the design phase significantly. In this study, a systematic design process is proposed for a modular, multimegawatt (MMW) HSIM with three radial active magnetic bearings. The process includes a traditional multidisciplinary design flow with extra critical aspects of MMW high-speed machines: manufacturability, bearing system, housing and operating unit. In addition, the manufactured machine is reported. The proposed systematic design process is described, including several multidisciplinary critical design aspects of high-speed machinery. Post-print / Final draft

The paper considers an idea of novel modular approach to a resolver design. The rotor of the resolver represents a simple piece of laminated steel with variable reluctance as in commercial resolvers. The stator part, however, is different. Each stator tooth is replaced with an H-core segment, which contains two windings: excitation and sensing. The design is unified in such a way that the same cores can be used in manufacturing resolvers of different sizes with different pole numbers. The size of the stator is significantly reduced because it can cover only about one saliency pitch of resolver. The solution is especially advantageous in electrical drives where the motor has a large number of poles. However, the side effect, which has a negative impact on the accuracy of a resolver with the proposed stator, should be considered during the design. The side effect is analyzed in the proposed resolver structure using the finite element method (FEM) and some approaches to reduce its negative effect are proposed.

The efficiency improvement is one of the main research topics in the field of electrical machines nowadays. The three-phase induction machines represent the majority of the machines used in the industry. Due to that, there is a motivation for researches to investigate how to improve induction machines efficiency or how to replace them with other types of machines having higher efficiency level. The benefits of the induction machine are line-start capability, robustness, and price. Line-start capability means that there is no need for extra power electronics. Avoiding of the power electronics in the case when only one certain load point is needed can lead to a price decrease, smaller environmental footprint, and more compact solution. An alternative machine type that can provide potentially higher efficiency compared to the induction machine, especially for small powers is a line-start permanent magnet machine.The topic of this paper is dedicated to the design and analysis of a 1.5 kW four-pole line-start permanent magnet machine for general usage. The designed line-start permanent machine is using the same stator lamination as the standard commercial 1.5 kW totally enclosed fan-cooled induction machine. The results of the design, together with electromagnetic analysis of the line-start permanent magnet machine performance, are illustrated. It is demonstrated that a simple replacement of the induction machine rotor by the line start permanent magnet rotor using the presented approach can facilitate in achieving of the higher efficiency level of the machine.

The impact of the air-gap length and usage of semimagnetic wedges on the performance of a synchronous reluctance motor (SynRM) with an axially laminated anisotropic (ALA) rotor aimed at high-speed applications was studied. The studied ALA rotor is intended to be manufactured by heat treatment of a stack of special magnetic and nonmagnetic materials. A 12 kW, 24000 rpm motor is considered in the study. An increase in the air-gap length can significantly reduce the rotor eddy current losses and stator iron losses. However, because of a lower magnetizing inductance with a larger air gap, the supply current is higher resulting in higher stator winding Joule losses. Along with the air-gap length analysis, the semimagnetic wedges were considered as a way of mitigation of high-order harmonics of the air-gap flux density. Wedges with a high relative permeability allowed to improve the efficiency of the machine. The case with realistic semimagnetic wedges was studied (having standard relative permeability values). Both air-gap length adjustment and semimagnetic wedges can be used to improve the efficiency of an ALASynRM.

The paper presents a design and mechanical analysis of a 100-kW high-speed electric motor with a stator core made of amorphous magnetic material for the air conditioning system of more electric aircrafts. Evaluation of critical rotational speeds and rotor mechanical stresses are reported. Experimental research has been made at a rotational speed range of 10000-60000 rpm accompanied with the rotor vibration measurements. During the tests, special attention was paid on assessing the critical rotational speeds. The paper will be useful to researchers and engineers working in the field of aircraft electrical machines and high-speed electric machines.

Torque ripple and cogging torque are thoroughly studied phenomena that occur in permanent magnet synchronous machines (PMSM). They are highly unwanted because they cause extra vibrations and noise in the system. Therefore, special design methods of PMSMs are used to reduce the torque ripple and cogging torque. This paper introduces and discovers a new method for the reduction of the torque ripple and cogging torque by applying so called quasi-skew-asymmetric rotor structure. In the paper it is shown that this method achieves similar performance advantages as of the traditional stepped skewing method and can apply several quasi-skew steps without need to apply actual skewing. This helps to avoid extra complicity of the rotor structure and extra costs related to the skewing. Further, the proposed method is very robust in terms of appearing of unwanted torque ripple and cogging torque components if some motor parameters are changed (e.g. pole geometry, stator geometry or air gap flux density harmonics), compensating all the possible torque ripples and cogging torque in a similar manner as in a traditionally skewed machine.

The conductivity of carbon-nanotube-based yarns is developing gradually. Individual carbon nanotubes (CNTs) offer very high conductivity levels but yarns spun of them still are in their infancy and develop slowly. Currently, CNTs are typically grown with chemical vapour deposition from methane, methanol or other carbon-rich hydrocarbons. Theoretical ideas of using graphene as a basic material to create high-conductivity nanotubes have also been presented lately. However, already present-day CNT-based yarns demonstrate conductivity levels that are considered good in principle for certain applications. This paper studies the applicability of CNT-based winding materials in electrical machine design.

Increasing the specific torque and power of a propulsion permanent magnet synchronous machine (PMSM) is important for the improvement of the performance of electrical vehicles. Among all technologies, increasing the current density and at the same time increasing the cooling capacity accordingly might be the most promising one. In this paper, the specific power of an outer-rotor PMSM is investigated by using different winding impregnation techniques. Both stator water jacket and winding direct liquid cooling are utilized. The continuous power of the designed PMSM is estimated by 3D thermal finite element analysis (FEA). For the sake of simplicity of the 3D FEA, equivalent thermal conductivities in the whole conducting region with different materials are first estimated by more detailed 2D FEA. After that, the estimated equivalent thermal conductivities are applied in 3D thermal FEA. The continuous power of the PMSM is further estimated based on temperature constraints. It is found out that the winding impregnation technique has a significant impact on the equivalent liquid-cooled winding thermal conductivity and, as a result, on the continuous power of the PMSM.

The paper concentrates on the design challenges of a 1.5 kW, 50 Hz direct on-line synchronous reluctance motor with a rotor topology defined by natural-flux-line-curve approach. A direct rotor substitution of a squirrel cage induction motor by a rotor of synchronous reluctance motor equipped with a damper winding does not guarantee a sufficient starting capability. To design a direct on-line synchronous reluctance motor with stronger starting capabilities the natural-flux-line-curve approach was modified. The proposed approach was applied in optimisation of the direct on-line synchronous reluctance motor using a genetic algorithm. The optimisation results show a clear tendency in the rotor design when trying to achieve the reliable starting capability and to maximise the steady-state performance of the synchronous reluctance motor. Both the stator design and the rotor diameter of the optimised direct on-line synchronous reluctance motor are different from those of a corresponding squirrel cage induction motor.

A self-holding three-position electric tubular actuator (SHTPETA) is described in the paper. The proposed linear actuator can be applied in a clutch system, where it is necessary to change certain mover positions over time. Other potential applications of such actuators can be shift of gears, coupling (decoupling) of various moving (rotating) mechanisms, or electromagnetic valves. The self-holding force of the proposed SHTPETA is achieved by the reluctance force between the armature and mover cores, where a magnetic flux is excited by a permanent magnet located in the armature and an excitation current is not needed, whereas the moving force is determined by the DC voltage (current) applied to two coils located in the stationary part of the SHTPETA. The paper proposes a relatively fast and simple approach for the SHTPETA design process based on the required forces and moved distance. Post-print / Final draft

Being much more rugged than a rotor made of traditional steel sheets, a solid rotor is frequently used in high-power high-speed induction machines (IMs). Furthermore, by a solid-rotor machine, a higher rotating speed can be achieved. However, the solid rotor also has certain disadvantages, the relatively high solid-rotor eddy-current losses being the most serious one. Basically, solid-rotor eddy-current losses are mainly induced by time–spatial air-gap flux density high-order harmonics. The suppression of the high-order harmonics is the key means to mitigate the rotor eddy-current losses. In this paper, a novel computational model is proposed for evaluating the rotor harmonic eddy-current losses of a 2 MW, 12 000 r/min IM. The model was built on a special machine referred to as a virtual permanent magnet harmonic machine (VPMHM). The model was constructed by applying the finite-element method (FEM), and it is based on the time–spatial harmonics produced by a rotating magnet with sinusoidal magnetization. The VPMHM model links the air-gap flux density harmonics directly to the specific rotor eddy-current losses. Furthermore, according to the proposed model, three options, namely, the use of air-gap permeance modifying notches on the stator teeth, semimagnetic wedges, and their combination, were investigated in detail for mitigating the rotor eddy-current losses in the motor nominal operating point. The results and conclusions in this paper are based on the FEM analysis.

High-power high-speed induction machines (IMs) are gaining ground in various industrial applications, especially in replacing traditional motors and gearboxes driving compressors or high-pressure pumps, etc. However, the high-speed technology presents extra challenges for the mechanical structure of the machine including bearing systems. Therefore, it is often reasonable to use active magnetic bearings (AMBs) instead of traditional bearings as they provide much smaller friction and allow a not perfectly centered orbit of the rotor without affecting the lifetime of the bearings. Using AMBs in a machine, the nonuniformity of the rotor leads to an inevitable rotor eccentricity; the level of which depends on the control tolerance and dynamic characteristics of the AMB and the maximum force generated by the AMB. This rotor eccentricity results in an unbalanced magnetic pull (UMP) of the rotor, which has to be compensated by the AMB. This paper analyzes the UMPs of a 2 MW, 12 000 r/min IM in the static eccentricity, dynamic eccentricity, and mixed eccentricity conditions. The AMB system was designed and optimized with the consideration of gravity and UMP effects. Furthermore, the required current compensation for the forces in the horizontal and vertical directions was studied during the operation and verified by the finite-element method.

Traditional industrial electrical machines are designed avoiding strong iron saturation in the magnetic circuit. Flux-switching permanent magnet synchronous machines (FSPMSMs), however, differ significantly by their construction and magnetic circuit from the traditional PMSMs. In PMSMs, local saturation in the stator at no load can be avoided by a proper stator tooth and yoke design, but in FSPMSMs, the flux accumulates in the teeth toward the air gap, and the armature steel often becomes heavily saturated in the region close to the air gap. Therefore, it is difficult to avoid local saturation in an FSPMSM, and proper design tools are required to take this saturation into account when the machine is designed. This paper demonstrates the importance of accurate knowledge of steel properties in regions with heavy magnetic saturation. It is shown that especially the design process of an FSPMSM should be supported by an accurate $B - H$ curve of the steel up to 2.2 T and beyond (or at the point where the steel is completely saturated), because it has a significant influence on the overall performance of the machine.

Efficient cooling is needed, for example, in traction motors which face regularly high torque peaks and generate high stator Joule losses. This paper studies the feasibility of the direct liquid cooling in the thermal management of a low-power low-voltage permanent-magnet machine. A tooth-coil axial-flux permanent-magnet double-stator-single-rotor test machine was first equipped with indirect liquid cooling using water cooling jackets and then with direct winding cooling. The winding material used is a hybrid conductor comprising a stainless steel coolant conduit tightly wrapped with stranded Litz wire. The performance of the motor is examined at various power levels using oil or water as the cooling fluid. The results confirm that the proposed direct cooling method is practical also in small machines, and furthermore, it offers significant improvements in the machine thermal management, especially, in cases where stator Joule losses dominate.

Permanent-magnet synchronous machines (PMSM) are gaining a foothold in a growing number of applications. However, the high cost of these machines compared with the prices of induction motors, whose production process is mature and material costs are low, limits the use of PMSMs with rare-earth permanent magnets in many potential cases. A viable design solution for a PMSM cost reduction is to use low-cost ferrite magnets instead of rare-earth ones. Nevertheless, it is challenging to apply ferrite magnets to a high-power rotor surface magnet PMSM because of their weaker magnetic properties compared with the rare-earth magnets and the risk of irreversible demagnetization. This paper aims to investigate the boundaries and limiting factors for achieving the maximum tangential stress and linear current density at a certain air gap diameter by using rotor surface ferrite magnets. The computed results are validated by a prototype, which was designed within the described boundaries.

A novel stainless core and submersible permanent magnet synchronous generator (PMSG) for tidal energy is investigated in this paper. The materials to be used must be resistant to marine environments. Therefore, there are two alternatives: using stainless magnetic materials and non-corrosion resistant materials can be used by protecting them by coating. The two alternatives are studied. Non-isotropic stator teeth width are investigated. This type of asymmetry consists of varying the distance of the slots of the same phase, keeping the slot width constant. This is achieved since each group of slots of the same phase are in parallel. Therefore, by modifying the arrangement of the stator slots, the magnetic circuit is modified, which allows adjusting the electrical parameters of the generator without changing its size. A hysteresis torque analysis is presented due to the magnetic materials used. A 1.7 kW, 80 rpm, prototype machine with 24 slots and 20 poles is proposed. Simulations by FEA are presented.

Different options of placing permanent magnets into a rotor are studied in a case of a permanent magnet synchronous motor. Rotors with permanent magnets in Halbach structures are compared with more conventional rotors in a 100 kW traction motor application. A rotor-surface-magnet motor is compared with four distinct interior magnet and Halbach rotor designs. One design is a full Halbach rotor, in which the whole rotor is structured from segmented magnets polarized in several directions. As a result, no iron core in the rotor is needed. Surface magnet Halbach and embedded partial Halbach structures are also introduced. The advantages and disadvantages of applying Halbach structures and especially interior magnets in a permanent magnet motor are investigated in detail. It is shown that the partial Halbach type with slightly embedded magnets can improve the maximum available torque without a considerable increase in material consumption.

High-speed electrical machines have their advantages, such as higher power density than in traditional electrical machines, and disadvantages, such as very demanding and tailored design and manufacturing process. The manufacturing of the rotor for a high-speed electrical machine is extra complex. In a high-speed electrical machine, the rotor in normal operation experiences various types of loadings, such as centrifugal forces from the rotation, temperature gradients from the operational environment, losses generated in the electrical machine and loads from the process. A failure related to the mechanical integrity of a rotor leads to a halt and very probably to the complete malfunction of the machine. The aim of this study is to compare three different rotor constructions in a high-speed induction electrical machine operating at the megawatt power range. The first one is a full copper squirrel cage rotor that consists of 25 pieces (22 copper bars, two copper end rings and a steel shaft), the second is a slitted solid rotor with copper end rings that consists of three pieces and two materials, and the third is a slitted solid rotor made solely of structural steel consisting of one piece. Further, these rotors are compared in terms of their electromagnetic performance, applying the same stator structure. The manufacturing challenges are directly related to the number of individual parts needed for the construction of the full rotor. The base material for the rotor core of the studied high-speed induction machine is often structural steel that is magnetically soft, has moderate electrical resistivity, and can be combined with copper or aluminum cage winding which is non-magnetic and has high electrical conductivity. From the mechanical perspective, these materials are very different; e.g. high-strength copper (CuCrZr) has a 50 % higher thermal expansion coefficient, 13 % higher density, 600 % higher thermal conductivity, and approximately 50 % lower yield strength than common structural steel (S355). This complicates the rotor manufacturing, as the required tolerances for the assembly of the rotor components are very strict and depend on the final manufacturing method of the rotor. Post-print / Final draft

This paper evaluates the feasibility of a direct liquid cooling approach in the thermal management of an axial flux permanent-magnet machine. To demonstrate the cooling method, a test motor was fitted with helical tooth coil windings formed from a hybrid conductor comprising a stainless steel coolant conduit tightly wrapped with a stranded Litz wire. The motor is a 100-kW permanent-magnet, axial-flux, double-stator, single-rotor machine. The proof of concept integrated the motor with a closed liquid coolant loop, appropriate instrumentation, and a data acquisition system. The general performance of the motor was examined at various power levels using polyalphaolefin oil as the cooling fluid. The results show the proposed cooling method to be feasible, and furthermore, to provide significant improvements to the machine thermal management.

This paper describes and classifies possibilities of using asymmetric geometry solutions in slotted electrical machines with nonlinear soft magnetic core. These asymmetrical optimizations can reduce cogging torque and torque ripple at the nominal load. Additionally, some of them can improve overload capability, and field weakening capability by proper optimization of over-saturated regions of soft magnetic core. In this paper it is shown that the geometry of the soft magnetic core regions of TC-PMSMs, exhibiting strong saturation due to the interaction of PM- and armature reaction fluxes, has a significant impact on the torque quality. Therefore, by their modifications (including asymmetrical) it is possible to improve the machine characteristics. Two types of PMSM rotor arrangements (embedded-magnet rotor PMSM and rotor-surface magnet PMSM) were simulated with 2D FEM. The results showed that slight modification of the TC-PMSMs soft magnetic regions near by the air gap which exhibit the strongest saturation can lead to significant torque quality variation as well as to synchronous inductance changes.

Direct liquid cooling (DLC) of electrical machine windings is a clearly effectual cooling method for high-torque and high-power-density applications needed, especially, in heavy vehicles. Usually, in electric vehicles, the stator copper winding losses are the most dominating losses during the accelerating or decelerating (recuperation) modes, and therefore, traditional windings may heat up significantly during high-torque periods. Removing the heat directly from the winding in which most of the heat is generated, allows the most straightforward and effective way of cooling. This paper investigates the feasibility of the direct liquid cooling in a radial-flux permanent-magnet traction motor. This case provides experimental information of the behavior of the direct liquid cooling of the windings. The prototype motor equipped with directly liquid-cooled windings is installed in an electric bus so that typical bus load cycles can be tested. Post-print / Final draft

Aspects of Direct On-Line Synchronous Reluctance Motor (DOLSynRM) design are considered in this paper. Steady-state performance of a DOLSynRM is dependent on the saliency and inductance difference along the d- and q-axis, which in turn depend significantly on the number of flux barriers. However, damper winding (DW), which is required for DOL start-up, can cause extra losses in the rotor part during steady-state operation. At the same time, DW design depends directly on the number of rotor barriers when it is implemented by filling the flux barriers with aluminium. This work considers the interconnection between rotor core geometry and DW design of a DOLSynRM. DW modification by means of rib design is another key aspect examined in the paper.

The possibility of applying low-cost ferrite magnets in a direct-on-line-start permanent-magnet-assisted synchronous reluctance machine (DOL PMaSynRM) for industrial applications, such as pump or ventilation systems was studied. The main target of this machine topology was to achieve a higher efficiency class compared to a counterpart induction machine (IM). Because of the weak properties of the ferrite magnets the motor had to be optimized for a low magnetic stress starting from the permanent magnets' point of view. The optimization procedure includes finding of a suitable rotor construction with a proper number of barriers, thickness of the barriers and thickness of the ferrite magnets. More detailed attention was dedicated to the analysis and elimination of irreversible demagnetization risk of ferrite magnets during start up period.

One of the essential parts of a Synchronous Reluctance Motor (SynRM) for direct-on-line (DOL) applications is a damper winding (DW) which makes the start of the motor possible. However, the DW can also be a considerable source of Joule losses caused by DW currents reacting on the stator current linkage and permeance harmonics. In order to get a comprehension of how DW bars' distribution influences the amount of induced rotor Joule losses at steady state different designs of DW of a SynRM are considered in the paper. Also the authors consider different methods of minimising stator current linkage distortion and permeance harmonics by stator part modification: magnetic wedges, reduction of slot opening and short pitching. A comparison with an induction motor with the same stator was done as well.

Electrical motors with ferrite Permanent Magnets (PM) are a possible solution when avoiding the usage of rare earth materials in rotating machinery. Irreversible demagnetization of the ferrite PMs is the one of the key challenges that needs to be analyzed in such designs. Nowadays electrical machine design is increasingly relying on Finite Element Analysis (FEA) software that require having accurate and easy-to-implement models for a magnet. Linear Recoil Curve (LRC) models are among the best candidates. This paper studies the LRC models for FEA simulation of ferrite PMs in rotating machinery. Comparison of the model-based results with high accuracy Vibrating Sample Magnetometer (VSM) measured data for a commercially available ferrite magnet grade show that none of the existing temperature dependent LRC models is universal. However, the further development of the studied modelling concept can offer an accurate tool for FEA simulation of ferrite PMs in electrical machines.

This paper introduces an important source of torque ripple in permanent-magnet synchronous machines with tooth-coil windings (TC-PMSMs). It is theoretically proven that saturation and cross-saturation phenomena caused by the nonsynchronous harmonics of the stator current linkage cause a synchronous inductance variation with a particular periodicity. This, in turn, determines the magnitude of the torque ripple and can also deteriorate the performance of signal-injection-based rotor position estimation algorithms. An improved dq-inductance model is proposed. It can be used in torque ripple reduction control schemes and can enhance the self-sensing capabilities of TC-PMSMs.

Permanent magnet synchronous machines (PMSM) are considered a viable option in various types of applications. However, particularly in consumer and low-power industrial applications, the price may be a factor that limits the use of PMSMs. In addition to a different technology, the main reason for the high price of PMSMs is the use of expensive neodymium or samarium-cobalt magnets. Their use is necessary only if a high motor torque T to linear current density A ratio (T/A) is required. Ferrite permanent magnets are low cost, abundant, and have negligible eddy current losses in low-frequency applications such as motor drives. They have a much lower energy product (BHmax) than the most modern magnets. Because of the high prices of rare earth magnets, many parties are seeking for opportunities to use ferrites instead. In the case of rotor surface ferrite magnets, the air gap flux density remains low. The air gap torque producing tangential Maxwell stress is proportional to the product of the air gap flux density Bδ[ Vs/m2] and the linear current density A [A/m]. If the flux density is low and A cannot be increased, the rotor has to be made larger than in machines having a high air gap flux density. In the case of multiple pole machines, the outer rotor approach, with its low rotor yoke height, offers an interesting alternative. The air gap diameter of these machines can be made larger than in conventional inner rotor type motors without increasing the machine outer dimensions. In this paper, an outer rotor PMSM with ferrite magnets is analyzed and tested. The machine characteristics in a fan drive are compared with an induction machine of the same power.

In order to be competitive, companies aim at improving their technologies in terms of production process simplification and reduction in the overall price of products. However, the use of lower-cost materials and a simpler construction can often degrade the machine performance. One of the possible production process simplifications of electrical machines is the use of modular permanent magnet synchronous machines (MPMSM). Some analytical approaches to analyze machines with modular stator can be found in the literature but these fall in the accuracy or do not consider the U-Shape core structure. This paper presents an analytical model to estimate the no-load air-gap flux density distribution of a U-shape MPMSM with tooth-coil winding. The results were verified with 2-D finite element analysis showing good accuracy.

In early papers about Permanent Magnet Synchronous Machines (PMSMs) it was often stated that there are no rotor losses in PMSMs. Later, eddy current analyses for sintered permanent magnets (PMs) have been dominating the rotor loss analyses. However, in some designs also hysteresis losses can be possible in PMs. This paper uses 2D Finite Element Method (FEM) program and 2D slotless analytical model to show the main factors affecting the hysteresis loss emergence and the applicability limitations of the existing analytical approaches in evaluating the hysteresis loss. A Tooth Coil Winding PMSM prototype with external rotor topology is used in the analysis as a reference.

This research investigates an axial flux singlerotor single-stator asynchronous motor (AFAM) with aluminum and copper cage windings. In order to avoid using die casting of the rotor cage winding an open rotor slot structure was implemented. In future, this technique allows using copper cage winding avoiding critically high temperature treatment as in the die casting processing of copper material. However, an open slot structure leads to a large equivalent air gap length. Therefore, semi-magnetic wedges should be used to reduce the effect of open slots and consequently to improve the machine performance. The paper aims to investigate the feasibility of using open slot rotor structure (for avoiding die casting) and impact of semi-magnetic wedges to eliminate negative effects of open slots. The results were mainly obtained by 2D finite element method (FEM) simulations. Measurement results of mechanical performance of the prototype (with aluminum cage winding) given in the paper prove the simulated results.

Stator winding structure and its manufacturing in permanent magnet synchronous machines (PMSM) is the vital part of the whole machine design. It is important to follow particular procedures in order to keep the winding losses at the acceptable level, which include DC resistance loss and additional losses due to alternative magnetic flux within the area of the stator winding. The winding transposition or using of Litz wire, which is effective way of proximity loss suppression, sometimes cannot be implemented due to challenges in assembling or cost aspects (especially in low cost PMSM solutions). In this case it is important to know consequences of using straight (non-twisting) windings in PMSMs with relatively high electrical frequency. This article studies impact of the number of stator winding parallel strands on the additional AC losses distribution in the slot where this winding is located. Also, thermal condition (which impacts the life time of the insulation) of the winding is analyzed with different number of parallel strands. Current and temperature distributions in the slot conductors were attained by finite element method (FEM). The total losses caused by additional AC losses in the winding were compared with the measurements results.

The saturation in the stator teeth of an 18-slots 16poles TC-PMSM with sinusoidal current supply caused by the interaction of armature reaction fluxes and rotor-PM-produced fluxes is studied. It is found that under load some teeth are saturated deeper than others. An increase in the width of these teeth reduces the oversaturation, which locally prevents the decrease in the inductance and back-EMF, and, consequently, reduces the torque ripple in the machine. The proposed technique can also be applied to other types of TC-PMSMs. Keywords—Armature reaction, current linkage, permanent magnet machines, TC-PMSM, tooth-coil winding, torque ripple, saturation.

A novel rotor structure for a tooth-coil-winding (concentrated winding) open-slot axial-flux permanent-magnet (PM) machine that reduces the eddy currents and increases the main flux linkage is proposed in this paper. A composite-body-based rotor assembly and a steel-laminated layer inserted on top of the magnets are used to reduce eddy-current losses in the PMs. A prototype of an axial flux generator for a heavy-duty vehicle following this structure is optimized, analyzed and measured. The tooth coil windings produce high-amplitude harmonics in the air gap and the large slot openings reduce the rotor flux. Consequently, a rotor structure that decreases the amount of harmonics travelling through the magnets is required. Steel laminations covering the magnets have positive effects on both phenomena. Two-dimensional and three-dimensional finite-element analysis is used to verify and optimize the geometry of the laminations. Computations are verified by experimental measurements of a 100 kW test machine prototype.