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One of the attractive benefits of slotless machines is low losses at high speeds, which could be emphasized by a careful stator core loss assessment, potentially available already at the pre-design stage. Unfortunately, mainstream iron loss estimation methods are typically implemented in the finite element analysis (FEA) environment with a constant-coefficients dummy model, leading to weak extrapolations with huge errors. In this paper, an analytical method for iron loss prediction in the stator core of slotless PM machines is derived. It is based on the extension of the 2-D field solution over the entire machine geometry. Then, the analytical solution is combined with variable- or constant-coefficient loss models, which can be efficiently computed by vectorized post-processing. Our case study compares different approaches to predict the loss phenomena. Moreover, the paper proposes a lookup-table-based (LUT) solution as an alternative approach. For validation purposes, the models are matched against a sufficiently dense dataset of 180 measured loss points for a lamination sample in the Epstein frame, covering a wide range of frequencies and flux density levels, which are usually not available from commercial datasheets. Finally, all the methods are compared for the slotless machine case, highlighting that the LUT method depends on a sufficiently dense experimental dataset to achieve satisfactory accuracy. Analytical Iron Loss Evaluation in the Stator Yoke of Slotless Surface-Mounted PM Machines

This paper presents a novel design concept for Synchronous Reluctance (SynRel) machines aimed at reducing the torque ripple. Two general sizing approaches based on the homothetic scaling principle are defined and compared. An in depth analysis on the torque ripple, for a wide range of scaled geometries, evaluated by finite element, has been carried out at different operating conditions. A further analysis is performed on 4 scaled geometries that have been optimized starting from 4 different rotor geometries. It is shown that the main rotor geometrical variables converge to similar values for all scaled machines. The accuracy of the proposed model is then validated by comparing the FE simulated torque ripple waveforms with the experimental data carried out, for a range of operating conditions, on a machine prototype. The outcome of this work is a fast and accurate scaling technique for the preliminary design of SynRel machines with reduced torque ripple.

The paper develops a method to choose the number of rotor bars in order to eliminate rotor slot harmonics in stator current spectrum and pulsation torques that are their consequence. Mains-fed, three-phase cage induction motors with the most common number of pole pairs and number of stator slots, that result in integer slot winding, are analyzed. The analysis is based on the recently derived general rule for optimal selection of rotor bars, valid for symmetrical multiphase machine with prime number of phases and integer slot stator winding. As a tool for validation of analytically predicted results, parameterized winding function (PWF) model is used. Electromagnetic torque ripple factor is used as a measure of goodness of the number of rotor bar selection. The practical motivation of the study is an attempt to supersede the many existing rules for rotor bar number selection that, depending on the source, may be different, and provide a unified general approach to the problem. One of the main findings derived in the paper is ascertainment that increasing the number of pole pairs increases the degree of freedom in choosing the proper number of rotor bars. The same applies when the number of motor phases increases.

Interior permanent magnet (IPM) machines with spoke-type design are very attractive for several applications, including vehicle traction, which requires a good flux weakening performance to achieve high speed. For this purpose, a high demagnetizing current is usually injected to reduce the machine flux at high speed, with detrimental effect in terms of efficiency. This problem can be mitigated by equipping the rotor with a mechanical flux weakening device, which activates during high-speed operation by centrifugal force. This paper addresses the analytical modeling of this special kind of IPM motor through a simplified magnetic equivalent circuit (MEC) approach, which incorporates saturation and slotting effects as well as the flux weakening device operation. It is shown how the MEC model can effectively describe the IPM machine behavior despite its simplicity. Results are successfully validated by comparison both with finite element analysis, which requires much more computational resources, and with measurements from laboratory prototype testing.

Fractional-slot concentrated windings (FSCWs) in three-phase permanent magnet machines are popular for several advantages but may cause severe performance issues due to their richness in air-gap magneto-motive force (MMF) space harmonics. This paper intends to explore the potentials and limits of FSCW optimization through multi-layer arrangements when applied to mitigate three problems, namely: permanent-magnet eddy-current losses, unbalanced magnetic pull (UMP) and torque ripple. It shows that, regardless of the algorithm adopted, the winding optimization is effective only on well-defined slot-pole combinations, which are strongly different depending what the optimization target is. Permanent magnet loss minimization is shown to be the field where the most effective results can be achieved, while UMP and torque ripple can be mitigated by winding optimization to a smaller extent and for a limited set of slot-pole combinations.

Large medium-voltage electric machine stators are usually equipped with form wound coils made of flat conductors (strands) and embedded in open (rectangular) slots. Air-gap magnetic flux lines can enter the slot and, sweeping the strands placed nearest the slot opening, induce eddy currents in them. Such eddy currents cause additional losses which can be much higher than usual skin-effect and proximity losses. In order to avoid dangerous overheating and hot spots, the additional losses in question need to be carefully predicted in the design stage. Time-stepping finite-element analysis (TSFEA) can be used for the purpose, which however implies a large computational burden and requires the machine geometry to be modeled in detail. This article proposes alternative methods based on time-harmonic finite-element analysis (THFEA) simulations performed on highly simplified machine models and with no need to take rotor motion into account. The proposed methods are shown to produce very accurate results, compared to TSFEA, but with very significant time and computational savings.

The identification of the optimal number of rotor bars in grid-fed, four-poles, three-phase cage induction motor is addressed in this two-part work for the frequent case of 36 stator slots. The optimization criterion being adopted is the minimization of the electromagnetic torque pulsations in fullload steady-state conditions. In Part I of the work a parameterized winding function (PWF) model was successfully validated through commercially available finite element software. In this Part II of the work, electromagnetic torque is investigated using the validated PWF model as it allows for investigating a wide range of design alternatives in a very short time, taking into account the skewing of rotor bars. Electromagnetic pulsations for different continuously changed number of rotor bars in a rather wide range are numerically evaluated using the torque ripple factor as a performance index. The evaluation is applied to both possible practical cases of unskewed rotor bars and rotor bars that are skewed by one stator slot pitch.

The use of slotless machines has acquired growing interest when features such as low vibrations and noise, along with low losses at high speed, are required. However, a careful assessment of the iron losses in the stator core will be relevant already in the design or pre-design phase. Standard methodologies for the iron losses estimation are typically FE analysis with constant loss coefficients, which can lead to weak extrapolations with huge errors. In this paper, an analytical method for iron losses prediction is derived. It is based on the extension of the field solution in the stator iron core, where an accurate 2-D field solution of the slotless machine topology is found. The analytical solution is combined with a variable loss coefficients model, which can be efficiently computed by vectorized post-processing. The case study proves that the approach can predict all of the loss phenomena over a wide operating range with improved precision. © 2020 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works.

Conventional rotating electrical machines are characterized by stator and rotor structures featuring a cylindrical geometry around the shaft rotational axis. Although advantageous for mechanical reasons, the cylindrical geometry results in overall machine shapes and dimensions that may be unsuitable for installation. This particularly occurs in shipboard applications, where electric motors and generators are subject to stringent room constraints and need to be fit in unusually shaped compartments. This paper presents the development and test of a dual-shaft rotating permanent-magnet electric machine prototype having a linear structure that facilitates its onboard use for such applications as electric propulsion and rudder actuation. In fact, the proposed machine topology has overall dimensions which can be adjusted to fit the space available for installation. The operating concept and the detailed electromechanical design of the machine are first described. Then the manufacturing and factory test of the prototype under inverter supply are illustrated. Finally, the validation of the prototype as a boat propulsion variable-speed inverter-fed motor is presented. It is proved that, despite of its highly non-conventional electromechanical design, the machine can be effectively fed from a general-purpose inverter for permanent magnet motors.

An accurate prediction of medium-voltage induction motor (IM) power factor in rated conditions is important in the design stage to verify the machine compliance with specifications according to international testing standards. Laboratory and industrial experiences suggest that significant errors in full-load power factor calculation can result for two-pole IMs in particular, due to the eddy currents induced in the solid-steel shaft at rated slip. Such eddy currents are responsible for rejecting the main flux into rotor laminated yokes causing an increase in their saturation and, hence, in the required magnetizing current with respect to no-load conditions. This article proposes a method to study the phenomenon through a combination of analytical and simplified Finite Element (FE) calculations as a computationally-efficient alternative to conventional FE simulations. The results of the proposed approach are experimentally assessed by comparison with measurements on a set of built and tested medium-voltage two-pole IMs of different sizes, showing very good accuracy and computational performance.

Slotless machines equipped with surface-mounted permanent magnet (SPM) rotor are used in such applications where the torque ripple and the additional losses due to the slotting effect are critical. For this kind of machine it is possible to have an accurate analytical solution for the magnetic fields in every machine cross-section domain. This paper derives an analytical expression for the magnetic field distribution inside the stator core of an SPM slotless machine for general load conditions. Different rotor magnetization patterns are also considered for board evaluation of the flux density. All results are compared with finite element analysis showing satisfactory accordance. © 2020 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works.

The identification of the optimal number of rotor bars in grid-fed, four-poles, three-phase cage induction motor is addressed in this two-part work for the frequent case of 36 stator slots. The optimization criterion being adopted is the minimization of the electromagnetic torque pulsations in full-load steady-state conditions. In Part I of the work results from the parameterized winding function (PWF) model are presented and successfully validated against time-stepping finite element simulations. In Part II the electromagnetic torque pulsations obtained from the PWF model are evaluated for different numbers of rotor bars. During this process motor rated power, stator winding design and main machine geometry are maintained invariant. The evaluation is applied to both possible practical cases of unskewed rotor bars and rotor bars that are skewed by one stator slot pitch.

The rotor structure of synchronous reluctance motor (SynRm) is usually provided with redial ribs to improve structural ruggedness for high-speed operation. While these radial ribs contribute to mechanical robustness, they also reduce the average torque and limit the flux-weakening capability due to an increase in inductance along the q-axis. To address this issue, the rotor design is refined by adopting non-magnetic radial ribs in this study. The introduction of titanium as non-magnetic material for radial ribs prevents magnetic leakage flux from flowing along the q-axis, and thereby improves the average torque, saliency ratio, efficiency and power factor of the SynRm. Furthermore, along with the electromagnetic analysis, the mechanical integrity of the proposed rotor design is also investigated. Two prototypes rated 1 kW at 1500 rpm are fabricated using both the conventional and the proposed rotor design, while the stator and other parameters are maintained constant in the two cases. The conventional design uses magnetic radial ribs whereas the proposed design uses non-magnetic radial ribs. Finally, a thorough comparative study, including inductance tests on the prototypes, is performed between the proposed and the conventional SynRm, confirming the strengths of the technology solution being set forth.

The complex relative permeance function is a suitable tool to predict the air gap magnetic field in slotted stator electrical machines. In the literature, the complex permeance function is usually identified by means of numerical techniques based on complex approaches such as conformal mapping or subdomain method. In this paper, an alternative approach is proposed. This method is based on solving the magnetostatic Laplace equation over a portion of the slotted air gap by imposing appropriate boundary conditions in the slot opening region. Such boundary condition tries to represent the theoretical trend of the magnetic field divergence near the corner-shaped ferromagnetic regions. The Carter theory for the slot fringing effect is also used. A fully analytical formulation for the complex relative permeance function is obtained, and its accuracy is assessed by comparison with Finite Element Analysis (FEA).

Energy conversion is one of the key steps for human utilizing the energy. In particular, as the key component for electromechanical energy conversion, electric machines play a significant role and greatly promote the development of modern civilization. Actually, the conventional types of electric machines, namely DC machines, induction machines, and synchronous machines, as well as surface permanent-magnet (SPM) machines and inserted-PM (IPM) machines, have been widely applied for various aspects in human lives. In recent years, emerging electric machines, including the double-salient type, hybrid-excitation type, PM memory type, magnetic-gearing type, PM vernier type, advanced magnetless type, multiport type, and some newly developing types, are proposed for new application occasions such as electric vehicles (EVs), electric ships, electric aircraft, electric robotics, wind power generation, unmanned propulsion systems, and so on. In addition, the corresponding advanced control strategies have presented for electric drives, which include the direct torque control (DTC), field oriented control (FOC), finite control set model predictive control (FCS-MPC), fault-tolerant control, and sensorless control. Thus, emerging electric machines and drives take active roles for applications with more electric or full electric. This special section brings new findings of electric machines and drives on their topologies, structures, analysis approaches, advanced control strategies, and new applications.

To meet the needs of the ongoing expansion in the gas industry, common requirements such as high reliability, efficiency, and dynamic performance must be satisfied while still keeping costs competitive. This article reports on an industrial research and development (R&D) project to find the best compromise between performance and production cost targets for a special direct-drive turbomachinery application. The development and testing of a 640-kW 10,000-r/min motor prototype characterized by a slotless stator and a Halbach-magnetized, magnetically levitated permanentmagnet (PM) rotor is discussed, including the technology selection, the design optimization processes, the machine manufacturing, and testing results.