Fractional-slot Concentrated-winding Machines Research Articles

Analysis of MW-Level Offshore Wind Turbine Generators with Dual Star–Delta Fractional-Slot Windings

This paper investigates the use of fractional-slot concentrated windings (FSCWs) in large-scale (MW level) offshore wind generators. It focuses specifically on a power rating of 3 MW and uses an existing direct-drive synchronous PM machine (DD-SPM) with 480s/160p and dual three-phase integer-slot winding (ISW) as a baseline. A multiple of the common 12s/10p FSCW machine is used that matches the electrical frequency of the ISW machine, yielding a 192s/160p dual three-phase machine. The hybrid star–delta connection has grown increasingly popular owing to its unique harmonic cancellation properties, which can help reduce rotor and PM eddy current losses in FSCW machines. In this paper, two dual three-phase star–delta-wound machines are scaled to 3 MW and included in the investigation. Specifically, a 384s/160p dual three-phase and dual star–delta winding machine, which is a multiplication of the 24s/10p machine, and a 192s/176p dual three-phase and dual star–delta winding machine, which is a multiplication of the 24s/22p machine, are used. These machines are investigated using finite element analysis (FEA) and compared on the basis of their air-gap flux density harmonics, open-circuit electro-motive force (EMF), torque performance, and losses and power. It is found that the proposed 384s/160p dual star–delta winding machine has the best electromagnetic performance of all, with a stator power that is 1.2% greater than that of the baseline ISW machine. However, this machine has a coil pitch of 2 and so loses the manufacturing and fault-tolerant advantage of having concentrated windings. If concentrated windings are desired, then the proposed 192s/176p dual star–delta winding machine is the best choice, with the stator power only 2.6% less than that of the baseline ISW machine, but unfortunately still has significant rotor and PM eddy current losses.

A Natural Position Observer With Vertical Detection Coil for FSCW Machines

This paper presents a sensorless control strategy using the novel detection coil for permanent magnet synchronous motor (PMSM) with fractional slot concentrated winding (FSCW). In order to eliminate the armature component in the coil voltage, the detection coil distribution is determined based on the analysis of synchronous inductance and leakage inductance for FSCW machines. Hence, the rotor position and angular velocity are estimated from the terminal voltage of detection coil, without any knowledge of motor parameters. Both finite element analysis (FEA) results and experiments are carried out to prove the effectiveness of the proposed method at steady and transient states.

Design Method and Performance Evaluation of Modular Multiphase PMSM with Hybrid Single/Double Layer Fractional-Slot Concentrated Winding

This paper proposes a general design method of modular multiphase permanent-magnet synchronous machine with hybrid single/double layer (HL) fractional-slot concentrated winding (FSCW), which indicates that double-layer (DL) FSCW machines satisfying two preconditions can be transformed into HL FSCW machines. HL FSCW machines with different slot and pole number combination are designed to prove the feasibility and universality of the proposed method. Performance of single-layer (SL) FSCW machine, DL FSCW machine, and HL FSCW machine are compared, from aspects of magnetomotive force, back electromotive force, average torque, torque ripple, core loss, eddy current loss in PMs (PM loss), efficiency, cogging torque, and flux density distribution. Besides, phase-to-phase magnetic isolation of HL FSCW machine, indicating by flux lines distributions and phase-to-phase mutual inductances, is also researched. HL FSCW machine has advantages of higher torque, lower PM loss, higher efficiency, better phase-to-phase thermal, physical and magnetic isolation compared with SL FSCW machine and DL FSCW machine.

상그룹 집중권선을 이용한 모듈형 영구자석 전동기에 관한 연구

Large scaled machine such as wind-power generators high reliability and fault-tolerant is becoming much important ever. In order to achieve the reliability and fault-tolerant, using 36-slot/ 34-pole, a novel modular dual 3-phase permanent magnet machine with a phase-group coil is proposed to reduce magnetic mutual interference between phases. Two modular dual 3-phase machines having a fractional slot concentrated winding (FSCW) are selected to show the comparison results such as torque, back-electromotvie force (EMF), mutual inductance permanent magnet (PM) joule loss, and iron loss. One is a conventional FSCW machine and the other is the proposed FSCW machine. Based on coil magneto-motive force vectors deleting two coils in the same a phase-group the magnetic mutual coupling effect is disappeared. From the several simulation results with the help of finite element analysis (FEA), the validity of the proposed concept is confirmed.

Third Harmonic Current Injection in Different Operating Stages of Five-Phase PMSM With Hybrid Single/Double Layer Fractional-Slot Concentrated Winding

In this paper, 30-slot/24-pole five-phase PMSM with hybrid single/double layer (HL) fractional-slot concentrated winding (FSCW) is designed for wheel-driving application in electric vehicles (EVs). Third harmonic current injection in different operating stages of the machine is investigated, including normal operation and fault-tolerant operation. HL FSCW machine is validated to be suitable for injecting third harmonic current to enhance torque by both theoretical analysis and finite-element analysis (FEA), and compared with 20-slot/24-pole single-layer (SL) and double-layer (DL) FSCW machine under constant rms and peak harmonic injection. Besides, a novel flux weakening control method with third harmonic current injection is proposed, which improves torque and efficiency of HL FSCW machine. The influence of nonlinearity caused by stator core saturation and higher order harmonics contained in back electromotive force (EMF) is included in flux weakening operation with harmonic injection. Finally, fault-tolerant control of HL FSCW machine with one-phase open-circuit fault is investigated, and two compensatory strategies with and without third harmonic current injection are proposed. Torque ripple caused by one-phase open-circuit fault is suppressed effectively by the proposed strategies.

Analysis of Inter-Turn-Short Fault in an FSCW IPM Type Brushless Motor Considering Effect of Control Drive

A comprehensive analysis of inter-turn-short fault (ITSF) in a fractional slot concentrated winding (FSCW) interior permanent magnet synchronous motor (IPMSM) is presented in this article. During online fault detection, the fault signatures can be influenced by the controller action in a closed-loop control system. Therefore, this study focused on the comparison of the ITSF characteristics in an IPMSM using open-loop and closed-loop control drives. The variation rate in various parameters, such as flux density, input current, circulating current, torque ripple, and vibration under healthy and ITSF conditions are compared. Conventional six-step square-wave control (SSC) and field-oriented control (FOC) drives are respectively used for open-loop and closed-loop analysis of ITSF. Furthermore, this study also focuses on mitigating the capability of the control drives for achieving extended postfault life of the motor. Based on our analysis a fault mitigating strategy for the benchmark IPMSM is also suggested. A multidomain cosimulation technique is used to integrate the finite element method-based model of IPMSM with the control drives under different severities of ITSF for the simulation. Finally, experiments on a three-phase 400-W IPMSM are performed for verification. The result shows that in the FSCW machine the ITSF can be mitigated and FOC has a better capability of ITSF mitigation than SSC.

Multiphase Modular Fault-Tolerant Permanent-Magnet Machine With Hybrid Single/Double-Layer Fractional-Slot Concentrated Winding

This paper proposes a multiphase modular fault-tolerant permanent-magnet (PM) machine with hybrid single/double-layer fractional-slot concentrated winding (FSCW). Comparisons between the proposed machine and conventional FSCW machines are made to indicate the special characteristics in aspects of armature magnetomotive force harmonic distribution, cogging torque, magnetic isolation ability, and torque capability. Then, influences of specific structural parameters, including the angle between single-layer slot (SLS) and double-layer slot (DLS) and the opening widths of SLS and DLS, on machine performances are investigated, and power density of the machine is improved by selecting the parameters appropriately. In addition, two stator modular design schemes are proposed. The influences of gaps between modules on machine performances, such as torque, losses, inductances, and short-circuit current, are investigated.

Modulation Effect of Slotted Structure on Vibration Response in Electrical Machines

Analytical method for vibration and noise prediction in electrical machines is computationally efficient and widely used. In conventional analytical approaches, the slotted stator is simplified as a cylinder, and it is generally believed that the vibration is more sensitive to the air-gap forces with low spatial orders, hence, those high-order air-gap forces are usually neglected. In this paper, the force distribution on the slotted stator is analyzed using a force transformation model and a new phenomenon, i.e., the modulation effect, is found, which indicates that a high-order air-gap force could also induce a low-mode stator vibration with a large amplitude. As a result, the conventional analytical methods are not so accurate for machines with a strong modulation effect, such as fractional-slot concentrated-winding machines. The existence condition for the modulation effect is also discussed in this paper. Finite-element analysis and experiments have been done to verify the analysis results.

Advanced optimization methods for fractional slot concentrated windings

Usage of fractional slot concentrated windings (FSCWs) in electrical machines offers several advantages due to their simplicity and compactness. However, the magnetic field of FSCWs has more space harmonics, including sub-harmonics that lead to undesirable effects. High rotor losses, noise, vibrations and thermal problems are the main drawbacks caused by FSCWs. These disadvantages limit the suitability of this winding type for some applications. In order to improve winding performances, several methods and techniques have recently been developed and analysed. This paper provides an update of latest research activities in terms of optimization of FSCW machines. These include efforts to reduce only sub-harmonics, to simultaneously reduce sub- and high harmonics or to use a higher harmonic for rotor excitation in current-excited synchronous machines. Simplest and most effective methods are considered, and their functionalities are explained briefly. Moreover, the effectiveness of said optimization methods is shown by simulation and experimental results, obtained from various optimized FSCW machines.

Investigation Study of the Influence of Pole Numbers on Torque Density and Flux-Weakening Ability of Fractional Slot Concentrated Winding Wheel-Hub Machines

Fractional slot concentrated winding machines (FSCWMs) with the low operation speed and large diameter usually have a large number of poles and slots; thus, numerous pole/slot combinations are feasible. The common practice to choose pole/slot combinations by multiplying the basic combinations may neglect some competitive candidates. Taking 36-slot FSCWMs as examples, this paper investigates the influence of pole numbers on torque density and flux-weakening ability, the two most vital performances of wheel-hub machine. It is shown that the machines with pole number slightly less than the slot number have the highest torque densities. Each component of synchronous inductance is separately analyzed, and its variation against pole numbers shows obvious regularity. Machines with pole numbers larger than the slot numbers have an excellent flux-weakening ability due to the large inductance and small permanent magnet flux linkage. The measurements together with the finite element analysis results confirm that the stator leakage inductance contributes the most to the superior flux-weakening ability. The identical analysis is also performed on 54-slot and 81-slot FSCWMs, with similar regularities observed.

Reduction of Tooth Harmonic in Fractional-Slot Concentrated-Winding Permanent-Magnet Machines Using New Stator Design

Fractional-slot concentrated windings (FSCWs) are characterized with many advantages. However, their magnetic fields have rich space harmonics. These harmonics, especially tooth harmonics, have negative effect on machine performances. This paper proposes a new method to reduce the tooth harmonics in FSCW permanent-magnet (PM) machine, in which the magnetic distribution in air-gap will be affected by winding span radian. Also, a new stator structure is designed on the method. By using the new stator design, each order tooth harmonic can be reduced significantly and the working harmonic can also be increased slightly. Simultaneously, the unbalanced radial magnetic force can be significantly weakened. The basic principle of tooth harmonic reduction method can be analyzed by using the stator structure of the FSCW machine with single layer windings. Then, a three phase 18-slot/16-pole FSCW machine with double layer windings is newly designed, which can effectively weaken the tooth harmonic components of FSCW machines. It is shown that, under the fixed electrical and geometrical constrains, the designed FSCW machine has better performances, such as loss, vibration and noise, than the conventional ones.

Thermal Conductivity Evaluation of Fractional-Slot Concentrated-Winding Machines

The use of fractional-slot concentrated windings (FSCW) in electrical machines allows more compact, efficient, and reliable design with respect to machines equipped with distributed windings. However, an electromagnetic design linked to a thermal analysis of the electrical machine is mandatory to achieve the desired performance, and to fulfill the requirements of efficiency and reliability. One of the most critical issues in thermal design of electrical machines is to assign fair values for the input parameters of the thermal simulation models, particularly those related to the stator winding insulation system. This paper deals with the assessment of the equivalent thermal conductivity of the insulation system of FSCW machines. For this purpose, three FSCW electrical machines for different applications were evaluated via an experimental method based on a dc thermal transient test. Whereas the investigated machines present different characteristics among themselves, different approaches were required to properly estimate the thermal conductivity.

Study of Direct-Drive Permanent Magnet Synchronous Generators with Solid Rotor Back-Iron and Different Windings

Direct-drive permanent magnet (PM) synchronous generators (DDPMSGs) are gaining more and more attention and application for wind power. This paper presents a comprehensive performance comparison of an external rotor surface-mounted PM synchronous generator equipped with three commonly used winding types, i.e., integral-slot distributed winding (ISDW), fractional-slot concentrated winding (FSCW), and fractional-slot distributed winding. The tradeoff between torque and the power factor, loss distribution, current, torque, and PM demagnetization characteristics under a short-circuit fault and the steady temperature rise are investigated and compared using finite-element analyses. It is found that, generally, FSCW PM generators exhibit high rotor losses compared with ISDWs, which will lead to an overly high temperature rise. Measures such as segment PMs and the adoption of lamination steel in the rotor can reduce the rotor loss and greatly improve the machine efficiency. However, the temperature rise of FSCW machines is still higher than that of ISDW machines. The conclusion is intended to help choose the proper winding types.

Winding Inductances of an Interior Permanent Magnet (IPM) Machine With Fractional Slot Concentrated Winding

This paper investigates inductance characteristics of a novel 14 pole/18 slot, fractional-slot concentrated-winding (FSCW) IPM machine. The variations of the self and mutual inductances were calculated analytically and also obtained from the finite element model. These results were compared with experimentally measured values of the self and mutual inductances of the prototype machine. Study of the inductance characteristics reveals that both self and mutual inductances vary with rotor positions nonsinusoidally. Presence of harmonics in the magnetomotive force (MMF) is responsible for this nonsinusoidal variation of inductances. The conventional distributed winding machine does not have these harmonics. This paper analyses the contribution of these harmonics in the <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Ld</i> and <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Lq</i> parameters in a <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">dq</i> -model of the prototype FSCW machine constructed. The self and mutual inductances were represented by a Fourier series in the inductance matrix of the machine for <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">dq</i> -transformation. It was seen that the difference between <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">d</i> - and <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">q</i> -axis inductance is mainly contributed by the harmonic terms of the mutual inductance.

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 this 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. This paper will also quantitatively highlight the tradeoffs between IPM and SPM FSCW machines especially in the context of traction applications.

Rotor End Losses in Multiphase Fractional-Slot Concentrated-Winding Permanent Magnet Synchronous Machines

Fractional-slot concentrated windings (FSCW) have been gaining a lot of interest in permanent magnet (PM) synchronous machines. This is due to the advantages they provide including shorter nonoverlapping end turns, higher efficiency, higher power density, higher slot fill factor, lower manufacturing cost, better flux-weakening capability resulting in wider constant power versus speed range, and fault tolerance. This paper focuses on eddy-current losses in the rotor clamping rings. Additionally, the loss in the nonmagnetic shaft with the option of i) metallic, ii) nonmetallic, and iii) metallic with shielding laminations clamping rings is analyzed. The study is based on finite element analysis (FEA). Desirable slot/pole combinations for different number of phases with both single- and double-layer windings are investigated. Experimental results for a three-phase 12 slot/10 pole design are presented to confirm that the losses in the rotor clamping rings can be very significant in case of FSCW machines and should not be overlooked during the design phase.

A Design Method of Fractional-Slot Concentrated Winding SPMSM Using Winding factor and Inductance factor

This paper proposed a calculation method of the torque and power density especially for the fractional-slot concentrated winding machines. The torque coefficient and the inductance coefficient to transform from m-slot/n-pole machine to 3-phase/2-pole machine are used. Some machines were designed and these torque densities and power densities are compared. Experimental results showed the effectiveness of the proposed calculation method and have good accordance with both calculation results and FEA results.