Ayman Mohamed Fawzi EL-Refaie

Analysis of surface permanent magnet machines with fractional-slot concentrated windings

This paper presents a closed-form analytical technique for analyzing surface PM machines equipped with fractional-slot concentrated windings. Since this class of winding configuration deviates significantly from conventional sinusoidal distributions, classical steady-state phasor or dq analytical techniques cannot be used to provide accurate results. The presented analytical model provides a fast and reliable method to analyze and compare candidate machine designs. Stator slotting effects are taken into consideration and a wide range of concentrated winding configurations can be analyzed. This technique is capable of analyzing the machine both below (constant-torque) and above (flux-weakening) base speed. Average torque, cogging torque, and ripple torque are all evaluated. Analytical results are verified using finite element analysis.

Comparison of Interior and Surface PM Machines Equipped With Fractional-Slot Concentrated Windings for Hybrid Traction Applications

Electric drive systems, which include electric machines and power electronics, are a key enabling technology for advanced vehicle propulsion systems that reduce the petroleum dependence of the ground transportation sector. To have significant effect, electric drive technologies must be economical in terms of cost, weight, and size while meeting performance and reliability expectations. This paper will provide details of the design, analysis and testing of two permanent magnet (PM) machines that were developed to meet the FreedomCar 2020 specifications. The first machine is an Interior PM (IPM) machine and the second machine is a surface PM (SPM) machine. Both machines are equipped with fractional-slot concentrated windings (FSCW). The goal of the paper is to provide a quantitative assessment of how achievable this set of specifications is as well as a comparison with the state of the art. The paper will also quantitatively highlight the tradeoffs between IPM and SPM FSCW machines especially in the context of traction applications.

Winding Inductances of Fractional Slot Surface-Mounted Permanent Magnet Brushless Machines

Permanent magnet (PM) brushless machines equipped with fractional-slot concentrated-windings (FSCW) have been receiving considerable attention over the past few years, due to the fact that they have short end-windings, a high slot fill factor, a high efficiency and power density, and good flux- weakening and fault-tolerance capabilities. A key design parameter for such machines is the phase winding inductance since this has a significant impact on the performance, as well as on the magnitude of any reluctance torque. The paper describes a detailed investigation of the various components of the winding inductance in machines equipped with both overlapping and non- overlapping windings and different slot/pole number combinations. It also examines the influence of key design parameters, which affect the inductance components, with particular reference to the inductances of machines in which all the teeth are wound and those in which only alternate teeth are wound. It is shown that the main component of the winding inductance is the relatively large slot leakage component. Both analytical and finite element models are employed and predicted results are validated on several prototype machines.

Effect of Magnet Types on Performance of High-Speed Spoke Interior-Permanent-Magnet Machines Designed for Traction Applications

Interior permanent magnet (PM) machines are considered the state of the art for traction motors, particularly in light-duty hybrid and electrical vehicles. These motors usually use neodymium-iron-boron (NdFeB) PMs. These magnets include both light rare-earth materials such as neodymium (Nd) as well as heavy rare-earth materials such as dysprosium (Dy). The main purpose of Dy is to enhance the magnet coercivity to avoid demagnetization under both high temperatures as well as flux weakening. One of the key risks in terms of using these rare-earth magnets is the significant fluctuation/increase in their prices over the past few years. Applications that use large quantities of these magnets, such as traction motors and wind generators, are the most affected by these fluctuations. There has been an ongoing global effort to try to reduce or eliminate the use of rare-earth materials (particularly Dy which is the most expensive) without sacrificing too much performance. This paper will focus on advanced spoke designs targeting traction applications. The goal of this paper is to come up with new spoke designs using various grades of Dy-free magnets as well as ferrites targeting the same set of specifications. This paper will provide a detailed comparison between the various designs highlighting the key tradeoffs in terms of power density, efficiency, flux-weakening capability, and magnet susceptibility to demagnetization. Also, a prototype using ferrites has been built and tested, and the experimental results will be presented.

Eddy-Current Loss Minimization in Conducting Sleeves of Surface PM Machine Rotors With Fractional-Slot Concentrated Armature Windings by Optimal Axial Segmentation and Copper Cladding

When a nonmagnetic high-strength metallic retaining sleeve offers advantages over a nonmetallic (e.g., carbon fiber) one, it is possible to consider the application of a high-conductivity shield ldquocoatingrdquo on this sleeve to reduce the surface eddy-current losses due to nonsynchronous fields. One can start by using a Maxwells equation-based analytical model to ldquoscreenrdquo for the optimal shield thickness and then employ a ldquo2.5 Drdquo finite-element method that accounts for periodic fields and finite rotor length, including axial segmentation and/or copper cladding. These are quantified to help design a low-loss rotor sleeve for a surface permanent-magnet machine with fractional-slot concentrated armature winding. With this type of winding, the sleeve losses can be significant due to its rich (read parasitic) asynchronous harmonic armature reaction MMF content.

Effect of Number of Layers on Performance of Fractional-Slot Concentrated-Windings Interior Permanent Magnet Machines

Interior PM machines equipped with fractional-slot concentrated-windings are good candidates for high-speed traction applications. This is mainly due to the higher power density and efficiency that can be achieved. The main challenge with this type of machines is the high rotor losses at high speeds/frequencies. This paper will thoroughly investigate the effect of number of winding layers on the performance of this type of machines. It will be shown that by going to higher number of layers, there can be significant improvement in efficiency especially at high speeds mainly due to the reduction of the winding factor/magnitude of the most dominant stator mmf subharmonic component. It will also be shown that there is significant improvement in torque density. Even though there is reduction in the winding factor of the stator synchronous torque-producing mmf component, this is more than offset by increase in machine saliency and reluctance torque. The paper will provide general guidelines regarding the optimum slot/pole/phase combinations based on torque density and efficiency. Sample designs of various slot/pole combinations are used to quantify the benefit of going to higher number of layers in terms of torque density, efficiency, and torque ripple.

Effect of Number of Phases on Losses in Conducting Sleeves of Surface PM Machine Rotors Equipped With Fractional-Slot Concentrated Windings

High-speed machines with a solid rotor or a high- strength retaining sleeve could offer design and performance advantages. For a specific application, if the use of a high-strength nonmagnetic metallic retaining sleeve is more advantageous than a nonmetallic (e.g., carbon fiber) one, one needs to evaluate the eddy-current losses due to armature reaction space and time harmonics and/or tooth ripple, as they can be significant. This problem is aggravated furthermore in fractional-slot concentrated-winding machines due to their inherent sub- and super nonsynchronous MMF harmonic components. In this paper, the impact of the number of phases is quantified to help design a lower eddy loss rotor sleeve for a high-speed surface-mounted permanent-magnet rotor machine with fractional-slot concentrated armature winding, FSPCW-SPM. The goal of this paper is to provide a general method for laying out preferred FSPCW configurations. Also, a general method for screening and choosing the optimal slot/pole combinations is presented.

Winding inductances of fractional slot surface‐mounted permanent magnet brushless machines

Purpose – Permanent magnet (PM) brushless machines equipped with fractional‐slot concentrated‐windings (FSCW) have been receiving considerable attention over the past few years, due to the fact that they have short end‐windings, a high‐slot fill factor, a high efficiency and power density, and good flux‐weakening and fault‐tolerance capabilities. A key design parameter for such machines is the phase winding inductance since this has a significant impact on the performance, as well as on the magnitude of any reluctance torque. The purpose of this paper is to describe a detailed investigation of the various components of the winding inductance in machines equipped with both overlapping and non‐overlapping windings and different slot/pole number combinations. It also examines the influence of key design parameters, which affect the inductance components, with particular reference to the inductances of machines in which all the teeth are wound and those in which only alternate teeth are wound.Design/methodolo...

Analysis of the Torque Production Mechanism for Flux-Switching Permanent-Magnet Machines

This paper investigates the principles underlying torque production in a flux-switching permanent magnet (FSPM) machine. Because the phase windings and permanent magnets in FSPM machines are both located on the stator, the torque production mechanism is not the same as for a conventional PM synchronous machine. Spatial harmonic analysis is applied to examine the frequency components present in the electric loading and magnetic loading of the machine. Since torque is proportional to the product of the electric and magnetic loading, understanding the source of the principal harmonics in these waveforms yields powerful insights into the components that result in torque production. The analysis is first presented for a specific FSPM machine (12-slot, 10-pole) and then extended to a general FSPM machine. The primary torque-producing harmonics in the airgap flux density waveform are found to be the heterodyned harmonics of the MMF produced by the stator magnets and the airgap permeance seen by the stator looking into the rotor. Analytical results are compared to results from finite element (FE) analysis and exhibit good agreement.

Lumped parameter magnetic circuit model for fractional-slot concentrated-winding interior permanent magnet machines

This paper presents a simplified lumped-parameter magnetic circuit model (MCM) of a fractional-slot concentrated-winding (FSCW) interior permanent magnet (IPM) machine that provides rapid estimates of machine performance for use in machine design optimization software. This model incorporates several key nonlinear phenomena including (i) magnetic saturation; (ii) cross-saturation effects between the d- and q-axes affecting both flux linkages and inductances; (iii) stator slotting effects; and (iv) localized effects due to rotor bridges. A coupled permeance element is proposed that captures the cross-coupling saturation effects along with airgap modeling that captures both radial and tangential airgap flux densities. The MCM is configured to predict key machine performance variables including airgap flux densities, back-EMF, phase and dq-axis inductances, characteristic current, and torque. Finite element (FE) analysis results are presented that match the MCM results quite closely, building confidence in the MCM model.