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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.

This study provides an insight in the analytical estimation of the electric field along the surface of a stress grading system (SGS) in the end-winding region of an electrical machine. An analytical approach is derived for the field grading materials (FGMs) having power-law electric-field-dependent conductivity. Despite some limitations, the introduced model can be used for the preliminary analysis of SGS with specific geometric and dielectric parameters, both during sinus and pulsed voltage condition. The analytical considerations are verified with finite element simulations for 15 markedly different nonlinear FGMs, which were previously reported in the applications related to the medium voltage rotating electrical machines. Post-print / Final draft

5-kW high-efficiency induction motor is analyzed using two-dimensional (2-D) open-source platform and the results are compared with commercial software and measurements. The importance of equivalent circuit parameters in 2-D analyses is highlighted. The noncommercial and commercial software give similar results. The motor is further analyzed in the 2.5-D domain and the results are compared with measured results. The losses obtained with 2-D, 2.5-D, and using the IEC segregation procedure of losses procedure are compared. The performance of an induction machine depends significantly on different parameters such as air gap and stator tooth tip height. Their impact on losses is studied with open-source finite-element analysis. The benefit of this open-source platform in the electrical motor analysis is the ability to use parallel-computing effectively and reduce the time required to solve the electromagnetic problem. Other benefits are the transparency of all features of the software and license-free usage. It is concluded that in 2-D analyses, the open-source software can be used on industrial-scale problems as it showed acceptable results for losses, and lower computational time when using appropriate mortar and conforming boundary conditions.

New global energy efficiency classification standard IEC 61800-9-2 for frequency converters and motor systems has been recently launched. It classifies the energy efficiency of the converters and motor-drive systems and introduces three different methods to determine voltage-source converter (VSC) losses for the classification. In here, experimental tests are carried out using the input–output method and the calorimetric method to determine the losses and efficiency of a 160-kW commercial VSC. The measurement results with these two independent methods are compared and the uncertainty of the methods is analyzed. The converter electric input and output power distributions are analyzed in the frequency domain and are used further to estimate the electric power measurement uncertainty. The measurement uncertainty related to losses obtained with the input–output method is compared with the loss difference between the two methods. This is the first research work to accurately evaluate the experimental methods for the determination of the power losses and examine the validity of the energy efficiency classes of the frequency converters according to new IEC61800-9-2. An urgent need to update the loss boundaries of the IE-classes has been found and the measurement uncertainty effect on the IE-classification is discussed.

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.

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.

Typically, active magnetic bearings have been applied to high-speed rotors in medium to high power range to replace ball, roller, and oil-film bearings. They require less maintenance and provide number of unique benefits owing to contactless suspension and active control. Integrated compressor or turbines result in predictable rotor dynamics. This allows use of model-based controllers. The model-based centralized controllers outperform decoupled transfer function controllers, but they do require accurate plant models. For integrated wheels on a single rotor the control models comprise a rigid rotor and lowest frequency bending modes. The bending mode parameters related to node locations can be identified yielding controllers tuned to the applications. This work introduces drive train modelling and magnetic levitation control of 2 MW rotor and external load with flexible coupling. The model-based control is tested in the experimental setup and drive train frequency responses are compared to the modelled multi-rotor drive train dynamics.

High-speed electric machines are special types of electric machines which aim for compact, direct-driven high-speed applications and highly efficient operation, especially, when gearbox can be avoided. Design of these types of machines is highly iterative combining multiphysics optimization and leads to custom types of machines which fulfill the application-specific requirements. Synchronous Reluctance Machines (Syn-RMs) might have an axially-laminated solid rotor structure, which combines magnetic and non-magnetic layers rigidly bonded to each other by vacuum brazing, hot isostatic pressing, soldering, explosion welding or even by additive manufacturing. In the study five non-magnetic materials and nine magnetic materials are cross compared and the results show clear differences in performance, efficiency and physical properties of the rotor when made of different material combinations, and thereby can suggest the best pairs when application-specific performance criteria are known. The study is carried out on a 12 kW machine with a maximum speed of 24000 rpm.

Solid rotors are preferred choice of topology for high-speed applications due to their robustness against high centrifugal forces at high speeds, ease of manufacturability, and higher temperature range. However, for peripheral speeds lower than 200 m/s, laminated rotor structure is preferred because of lower eddy current losses resulting in higher efficiency. However, laminated rotors are complex to manufacture, sensitive to temperature and have vibration and mechanical integrity related issues. As a compromise between these two designs in terms of mechanical strength and efficiency, this study investigates a radial flux 2 MW, 15 krpm induction motor rotor core made of thick laminations. The baseline dimensions of the thick-lamination rotor design are calculated using analytical equations considering aspects such as mechanical stresses, rotordynamics, and bearing parameters. Lastly, the lamination to lamination contact behavior under unbalance load is analyzed for a simplified model and their effect on natural frequencies is studied.

Brushless synchronous generators (BSG) are typically used to produce an island network whose voltage is close to the nominal voltage of the generator. Generators are often used also in test-field applications where also zero output voltage is needed. The exciter construction and magnetic remanence may lead to a situation where the non-loaded generator terminal voltage cannot be controlled close to zero but a significant voltage is always generated because the exciter remanence. A new brushless synchronous generator excitation and de-excitation converter topology for test applications is proposed. The purpose is to achieve full voltage control from zero to nominal level without modifications to the generator. Insulated-gate bipolar transistor (IGBT) and Field-Programmable Gate Array (FPGA) technology are used to achieve the required fast and accurate control. In the work, simulation models were first derived to characterize the control performance. The proposed converter topology was then verified with the simulation model and tested empirically with a 400 kVA brushless synchronous generator. The results indicate that the exciter remanence and self-excitation can be controlled through the exciter stationary field winding when the proposed converter topology controls the field winding current. Consequently, in highly dynamical situations, the system is unaffected by mechanical stresses and wear in the generator.

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

A high-speed squirrel-cage induction machine requires a totally different design compared to the traditional squirrel-cage industrial motor because of the mechanical limitations caused by the high speed. This results in a more complicated rotor construction and expensive material selection, and sets higher standards for the manufacturing precision. The objective of this paper is to demonstrate the design aspects, material selection, and manufacturing of a squirrel-cage rotor for high-speed applications. In this paper, the rotor dimensioning approach based on equations and data analysis is presented. Rotor material selection and construction topology influence on the electrical machine design are discussed. The results are illustrated with the design of a 6-kW, 120 000-r/min induction machine for a turbo circulator. The influence of rotor parameters on the electromagnetic performance of the designed machine is demonstrated. Mechanical stresses for different topologies are studied with finite-element method analysis. Several manufacturing methods for producing a high-precision rotor are described and compared. The presented rotor design approach, which enables high electromagnetic performance and robust construction, is verified by the testing of a prototype.

In this paper, a direct liquid cooling method is proposed for a radial-flux permanent-magnet motor. To demonstrate the feasibility of the cooling method, a test motor with a rated output of 205 kW was designed, constructed, and tested in an actual vehicle application, an electric city bus. The energy consumption tests were conducted by applying a heavy-duty chassis dynamometer capable of simulating the inertia, weight, and road loads that the buses are subjected to in the normal on-road operation. The electricity consumption on the real bus route of the Espoo line 11 in Finland was 0.61 kWh/km. The test results of the cooling solution show that the motor is capable of meeting the most challenging requirements of the load cycle even with a full payload. The highest winding temperature rise in the test driving cycles was only 26 °C, which proves the effectiveness of direct-liquid-cooled coils in a vehicle motor.

A permanent magnet traction motor integrated with a planetary gearbox is studied. A machine of this kind can be employed as a propulsion motor in off-road machines like agricultural tractors that have to produce either very high traction forces at low speeds or reach higher traveling speeds at lower torques. In principle, a constant power curve as a function of speed is desired, which means that the output torque of the drive system should be inversely proportional to the operating speed of the off-road machine. Such driving conditions are challenging as the electric motor has to be heavily overloaded at the lowest speeds. Therefore, it is essential to accurately evaluate not only the electromagnetic performance but also the thermal performance of the machine. This paper studies the integration principles of an electrical machine and a planetary gear. The integration poses some new challenges to the design. For example, also the lubrication and cooling can, and, in practice, must be integrated into the system. The thermal performance of the motor and cooling with the lubrication oil of the gear were analyzed. The long-term tests with the oil cooling system were carried out to verify the successful integration.

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.

Off-road traction applications require extremely high torque, especially, at low speeds. Such a high torque can be reached by gearing up the torque of an electrical machine. A tooth-coil permanent magnet synchronous machine with an integrated planetary gear is proposed as a solution for heavy off- and on-road traction applications. The key benefits of tooth-coil permanent-magnet synchronous machine (PMSM) in the proposed drive system and its integration with a planetary gearbox are explained in detail. A multi-objective optimization is performed to determine the slot/pole combination of the PMSM and the design process of the proposed drive system is entirely clarified. The proposed tooth-coil PMSM is investigated by electromagnetic and structural finite element analysis to obtain the final design and highlight the machine performance. A prototype is built to validate the proposed system. The results show that the traction drive system works well in laboratory and offers means to serve as a traction system in heavy off- and on-road applications.

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

Pulse-width-modulated (PWM) voltage source inverters (VSI) are a common way to drive induction motors in variable speed applications. As switching hardware improves, motors are driven with increasing switching frequencies, extending the frequency range of PWM harmonics. Discrete Fourier Transform (DFT) can be used to analyze the differences in the active power spectrum as switching frequency is changed. This paper covers the basic methodology on DFT-based analysis of input power measurements. Analysis on the input active power spectrum of a PWM VSI driven 5-kW induction motor is conducted at different switching frequencies.

Effective and educative induction motor (IM) Direct Torque Control (DTC) simulation with Simulink software is presented. Motor model is realized in stator reference frame and the inverter modulation is controlled with DTC principle. Inverter non-idealities are neglected and inductances and resistances of the motor are considered as constants. The DTC model is created with multiple subsystems which allows to break down the whole control system to smaller and easily understandable parts. This way the basics of the DTC can be comprehended easier, and the model used in the education of electrical drives. In addition, the Simulink model utilizes only the basic Simulink blocks, so no additional libraries are required.

This paper investigates the segregation of losses in a 5-kW IE3-rated induction machine. The stator and rotor copper, iron core, stray and mechanical losses are experimentally determined using the IEC segregation of losses-method. The loss components determined from the laboratory measurements are compared with simulated ones. Two alternative methods are used to calculate the losses using a finite element software. The first method is post-processing and the second method is to emulate the loss segregation measurement procedure. By emulating the measurements, FEM output values can be used to calculate variable and fixed losses. The results indicate that the measured losses show a good correspondence with both utilized methods.