Synchronous Reluctance Machines: Analysis, optimization and applications
2: Marelli Motorsport, Italy
3: Magna Powertrain, Austria
Reluctance motors induce non-permanent magnetic poles on the ferromagnetic rotor; the rotor does not have any windings and torque is generated through magnetic reluctance. Synchronous reluctance motors (SyRMs) have an equal number of stator and rotor poles. Reluctance motors can deliver high power density at low cost, so they are finding increasing application in the transport sector. Disadvantages include high torque ripple and the complexity of designing and controlling them. Advances in theory, computer design, and control electronics can overcome these issues. This hands-on reference covers the concept and design of synchronous reluctance motors. It conveys all key topics required to understand this technology. Chapters cover magnetic materials, geometry, modeling, design and analysis, optimization, production technology, fault-tolerance, experimental validation, and self-sensing-oriented optimization. Synchronous Reluctance Machines: Analysis, optimization and applications is ideal for researchers working on electrical machines and motors, particularly electric vehicles. The writers - experts from academia and industry - provide the reader with an excellent background and understanding of the core concepts involved in synchronous reluctance motors such that they can engage in their own R&D.
Inspec keywords: optimisation; fault tolerance; magnetic flux; rotors; torque; permanent magnet motors; reluctance motors; finite element analysis
Other keywords: reluctance machines; permanent magnet motors; fault tolerance; finite element analysis; torque; magnetic flux; rotors; reluctance motors; optimisation
Subjects: Education and training; Finite element analysis; Synchronous machines; Optimisation techniques
- Book DOI: 10.1049/PBPO186E
- Chapter DOI: 10.1049/PBPO186E
- ISBN: 9781839532634
- e-ISBN: 9781839532641
- Page count: 369
- Format: PDF
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Front Matter
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1 Introduction
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SyR motors are becoming attractive in many fields and applications requiring high dynamics, high torque density and fault-tolerant capabilities such as traction and household appliances. A substantial advantage is also the virtual absence of rotor losses, leading to lower rotor temperatures if compared with induction motors. The design methodology is well established since works have been carried out in the field of torque ripple reduction or power factor improvement. The key benefits of this technology are a rotor structure made of cut-outs and iron parts only, rotor with neither excitation coils nor PMs leads to a very cost-effective structure. The most recent revolution of electrical machines was due to the introduction of Rare-Earth (RE) permanent magnets. They are characterized by a very high magnetic energy density compared to previous magnetic materials. Fault-tolerance is the ability to ensure proper operations even in or after an event of failure. A dual-three-phase system is applied to an SyR motor to increase the reliability of the overall drive. In addition, it is shown that such a motor is suitable when a short-circuit failure occurs.
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2 Magnetic materials
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All materials can be classified in terms of their magnetic behavior falling into one of five categories, depending on their magnetic susceptibility. The two most common types of magnetism are diamagnetism and paramagnetism, which account for most of the periodic table of elements at room temperature (see Figure 2.1). These elements are usually referred to as nonmagnetic, whereas those that are referred to as magnetic are actually classified as ferromagnetic. The only other type of magnetism observed in pure elements at room temperature is antiferromagnetism. Finally, magnetic materials can also be classified as ferrimagnetic, although this is not observed in any pure element but can only be found in compounds, such as the mixed oxides, known as ferrites, from which ferrimagnetism derives its name (Table 2.1).
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3 Synchronous reluctance motor geometry drawing
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This paper is about the design rules adopted to draw different Synchronous Reluctance (SyR) rotor geometries. At first, the fluid and segment shaped flux-barriers rotor drawings are described. Then, a design methodology to choose the proper SyR rotor parameter. the fluid flux-barriers are to align their edges to determined flux lines that would appear in the case of a d-axis magnetization of a solid rotor geometry. The flux-barrier edges overlap the flux lines obtained with a solid rotor. This design strategy allows one to minimize the d-axis flux lines distortion and effectively block the q-axis ones with beneficial effect on the rotor anisotropy. The segmented flux-barriers rotor presents a simple geometry and, for this reason, it is often preferred to the fluid flux-barriers one even though it yields a lower rotor anisotropy. A further advantage of this geometry is that it allows one to achieve a high flux-barriers filling with the common sintered Permanent Magnet (PM) without affecting the rotor geometry.
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4 Reluctance network model of high-speed synchronous reluctance machines
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The aim of this chapter is to describe the magnetic behaviour of the high-speed (hs)-Synchronous Reluctance (SyR) rotor through an Improved Reluctance Network (IRN) that takes into account the radial ribs contribution on the rotor saturation and the cross coupling between d-axis and q-axis using nonlinear reluctances.
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5 Nonlinear analytical model for synchronous reluctance machines
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This chapter presents a nonlinear analytical model of the Synchronous Reluctance (SyR) machine at the air gap, used to derive both average and torque harmonics as a function of the rotor geometry. Saturation coefficient is assigned to each of Flux line paths and applied at the air gap. The analytical model proves to be fast and fairly accurate in any relevant calculation. Therefore, it is quick and easy to obtain the behavior of the average torque and torque ripple as functions of the rotor flux-barrier geometry. The result is presented using maps that are essential for finding proper combinations of barrier angles that maximize the average torque and minimize torque ripple. The torque maps are compared with those obtained from both linear analytical and Finite Element (FE) models. The maps computed analytically show good agreement with those derived by means of FE analysis, and they are obtained in a much shorter amount of time.
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6 Design criteria of flux-barriers in synchronous reluctance machines
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In this chapter, a design criterion for multi flux-barrier Synchronous Reluctance (SyR) machines is proposed. In particular, the focus is on the proper flux-barrier design. An analytical model is adopted to compute the impact of the rotor flux-barriers on the torque, focusing on its ripple.
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7 Structural analysis with GetDP
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The chapter looks at finite element structural analysis of a synchronous reluctance rotor using GetDP, a free open-source scientific software for the numerical solution of integrodifferential equations.
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8 Efficiency map computation
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One of the most effective graphical ways to describe the performance of an electric machine is the efficiency map. The efficiency map is a constant-efficiency contour plot in the torque, or power, versus speed plane. Each point of such a map corresponds to a specific current supply condition that exhibits the maximum efficiency for a given torque and speed load point. The number of rows and columns depends on the size of the torque and speed arrays selected to build the evenly spaced grid. The maximum torque depends on the operating condition and current limit. In general, it can be set equal to the rated or overload torque. The maximum speed depends on the mechanical constraint of the machine. The voltage is automatically taken into account in the efficiency map computation.
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9 Multi-objective optimization
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This paper is about the Reluctance motors induce non-permanent magnetic poles on the ferromagnetic rotor; the rotor does not have any windings and torque is generated through magnetic reluctance. Synchronous reluctance motors (SyRMs) have an equal number of stator and rotor poles. Reluctance motors can deliver high power density at low cost, so they are finding increasing application in the transport sector. Disadvantages include high torque ripple and the complexity of designing and controlling them. Advances in theory, computer design, and control electronics can overcome these issues. This hands-on reference covers the concept and design of synchronous reluctance motors. It conveys all key topics required to understand this technology. Chapters cover magnetic materials, geometry, modeling, design and analysis, optimization, production technology, fault-tolerance, experimental validation, and self-sensing-oriented optimization. Synchronous Reluctance Machines: Analysis, optimization and applications is ideal for researchers working on electrical machines and motors. The writers - experts from academia and industry - provide the reader with an excellent background and understanding of the core concepts involved in synchronous reluctance motors such that they can engage in their own R&D.
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10 Design and optimization of a PMaSyR motor for low-voltage E-scooter applications
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This chapter proposes a multi-objective PMASYR motor design procedure that (a) evaluates the feasibility of the drive system in overload and flux-weakening condition and (b) computes the optimization cost functions in the same operating condition to allow easy and direct considerations of the Pareto fronts, without any further computations and checks after the optimization.
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11 Synchronous reluctance motor optimization for pumping application
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This paper is about A multi-objective optimization of a Synchronous Reluctance (SyR) motor. The aim is to substitute some permanent magnet servomotors with SyR motors for variable-speed pumping applications. In particular, a torque of at least 90 N m was required at 2 000 rpm. The overload torque is double the rated one, which represents quite a demanding requirement for this kind of machine. Lastly, a FW speed range of at least two to one was required. The optimization has been performed using the provided stator. The most common way to optimize the SyR machine is to maximize the average torque and minimize the torque ripple. Since the stator is fixed, the efficiency is directly proportional to the average torque, so it is not necessary to include it among the optimization objectives. The optimization parameters were the three flux-barrier angles, and the magnetic insulation ratio kair. the torque waveforms show a higher harmonic content than predicted by Finite Element Analysis (FEA). The torque ripple value is consistent for different current values, as it can be verified from the overload current.
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12 High-torque low-speed permanent magnet assisted synchronous reluctance motor design
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Direct-drive electric machines represent a valid solution for low-speed applications. This ensures lower weight, noise, axial length and lower mechanical losses. Such machines find applications in energy production, electrical propulsion, industrial automation, low-head pumps and so on. They typically employ strong Rare-Earth (re) magnets with high pole numbers. However, in this application ferrite magnets have to be preferred, because of the high cost of RE magnets. This means that the machine should exploit the reluctance torque component as much as possible. Therefore, a Permanent Magnet assisted Synchronous Reluctance (PMASYR) machine is chosen. The structure of the chapter is as follows: specifications and constraints, parametric analyses, multi-objective optimization coupled to finite-element analysis is performed. Finally, a thorough electromagnetic analysis is performed on one possible design solution.
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13 Bonded magnets in PMaSyR machines
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The aim of this chapter is to present some experimental investigation about the impact of different production technologies on the BMs properties and to evaluate their suitability for PMaSyR machine applications. In the first part a brief description of the motor geometry and its design is presented. Then, the steps for BMs preparation and magnetization are described. In particular, two BM typologies are tested: one prepared with compression molding and the other with injection molding. Next, three motor prototypes are manufactured: one SyR machine and two PMaSyR machines with compression and injection molding BMs.
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14 High-speed synchronous reluctance machines
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This chapter describes a guideline to properly design Synchronous Reluctance (SyR) machines suitable for High-Speed (hs) applications. The main target is to give some guidelines for an accurate design of the rotor geometry to obtain a robust mechanical structure, a quite high-torque density and a low-torque ripple. A particular care is paid to the rotor rib thicknesses, designed to guarantee the structural integrity of the rotor and to minimize the q-axis magnetic flux. In fact, the thickness of the seiron ribs has to be wide enough to mechanically sustain the rotor and it increases as the design speed increases. This is a significant drawback, since a quite large part of the magnetic flux flows through them, reducing the available saliency and, consequently, the machine torque.
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15 Overview of fault-tolerant SyR machines
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The Fault-Tolerant (FT) reliability of an electric drive system is a mandatory requirement in those applications where the failure could cause economic costs or, more importantly, injure the human safety as in the automotive, aerospace and military applications. The fault-tolerance of a drive system is defined as the ability to ensure proper speed or torque reference tracking in the event of fault. In other words, more the fault-tolerance is high, more the drive system performance in fault condition approaches the rated one. Faults can be first classified on the basis of their nature: mechanical and electrical. The most common mechanical faults are due to eccentricity problems and misalignment of bearings. In AC drives, a failure of the inverter switches can cause Short-Circuit (SC) of the DC bus or a disconnection of one or more phases. In electrical machines, the winding is typically subject to faults due to insulation failures unbalanced supply conditions, a bad connection at the motor terminal, or high resistance contact. The Induction Motor (IM) is definitely the most used electric machine thanks to its robustness, reliability and mature technology. Most of the proposed solutions to increase the FT of IM drive are based on the principle of systems redundancy.
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16 Impact of winding arrangement in dual three-phase synchronous reluctance machine
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This chapter introduces the study of the FT-Synchronous Reluctance (SyR) machine investigating the impact of different dual three-phase winding arrangements on the machine performance by means of Finite Element (FE) analysis.
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17 Optimization of a synchronous reluctance machine for fault-tolerant applications
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Synchronous Reluctance (SyR) machine with the dual three-phase winding arrangement W-12-12 exhibits, in general, good performance in both Full Winding Mode (FWM) and OC-FWM except for the torque ripple that is quite high in the second operating condition. The aim of this study is to improve the Fault-Tolerant (FT) reliability of such a drive system focusing on the SyR rotor design. In particular, a standard geometry is selected for the stator lamination, while two alternative Finite Element (FE)-based optimization approaches are proposed for the rotor design. On the basis of the optimization results, the most promising individual is selected and analyzed in detail by means of FE analysis in FWM and Open Circuit Half Winding Mode (OC-HWM).
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18 Experimental validation of a synchronous reluctance machine for fault-tolerant applications
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The Synchronous Reluctance (SyR)machine design for Fault-Tolerant (FT) applications optimizing both stator and rotor geometry. Moreover, in order to permit a single-layer winding, a 48-slot 8-pole configuration is adopted. Two alternative stator geometries are proposed. The first one is a classical stator with slots all equal and evenly spaced along the stator periphery. The second one still presents all the slots of the same shape, but their displacement is not uniformly distributed along the stator periphery. The rotor presents three fluid flux-barriers per pole. In both the optimization, the strategy full-half winding mode optimization. On the basis of the optimization results, an optimal individual is selected and manufactured. Such a prototype is tested in laboratory to verify the reliability of the proposed design procedure and the machine suitability for FT applications. The short-circuit currents are measured by means of three different current probes and then visualized on the oscilloscope screen. It can be observed that their behavior and maximum amplitude are in good agreement with the FE results discussed before. The small difference between the waveform is due to the skewing effect of the experimental test. This chapter dealt with the optimization and analysis of an SyR motor with dual three-phase winding for FT applications. The experimental results confirmed the FE predictions since very low induced voltages have been observed.
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19 Self-sensing-oriented optimization of synchronous reluctance machine design
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This chapter deals with self-sensing-oriented optimization of Synchronous Reluctance Machines (SYRMs). This kind of machine is among the most challenging ones to control without the position sensor at low speed. In fact, typical position estimations adopt High-Frequency (HF) voltage injection that heavily relies on the intrinsic machine saliency. However, both at low and high currents, such saliency is not guaranteed due to the presence of the iron ribs and to the saturation of the iron material, respectively. Furthermore, the estimation algorithm could also become unstable due to the absence of convergence points. The aim of the chapter is to tackle this issue, embedding proper sensor-less control capability into the design through multi-objective optimisations.
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20 Conclusions
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Due to the rapid interest in designing electric machines with no rare-earth materials, the synchronous reluctance machine is a perfect candidate. It can be without magnets at all or even assisted by low-energy magnets as ferrite magnets. The synchronous reluctance machine reaches performance slightly lower than the permanent magnet machines, but it is characterised by other interesting advantages. Among the others, a cheap and robust structure, capacity to sustain high overload, a high fault-tolerant capability. The possible drawbacks of such a machine, such as torque ripple, low-power factor or low power in flux-weakening operations, have been deeply analysed and several remedies have been proposed.
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Appendix A: Iron losses insights
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Appendix B: MMF distribution along the stator periphery
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Appendix C: HS-SyR analytical model constants
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Appendix D: GetDP elasticity formulation
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Appendix E: Maxwell stress tensor derivation
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Appendix F: High-frequency signal injection mathematical model
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Appendix G: Incremental permeability simulations for differential inductances computation
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Back Matter
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