Modeling, analysis and design optimization of a permanent magnet assisted synchronous reluctance machine

ABSTRACT This Master Thesis deals with the electromagnetic and thermal performance of a permanent magnet assisted synchronous reluctance machine for traction application, carrying out a comparison between two windings configuration and between two technics for torque ripple reduction. In the first chapter, an excursus about the kinds of electrical machines and their application areas is presented. In the second chapter, the mathematical model and the main features of an IPM motor are presented. The capability curves and the circle diagram are introduced to better describe the IPM performances; at the end the concept of the interaction between the model parameters and the saturation effects and cross saturation in a real machine are shown. In addition a brief introduction to FEM analysis is presented. In the third chapter, the electro-magnetic performance of the reference machine are presented. In the fourth chapter a preliminary comparison between toroidal windings configuration and distributed windings configuration in terms of electromagnetic performance is presented. At the chapter end an efficiency comparison for different motor dimensions is presented. In the fifth chapter, the thermal problem for electrical machine is introduced and the main heat removal equations are presented. The equivalent lumped parameters thermal circuits for each windings topology are modeled with the purpose to continue the comparison in terms of thermal performance defining the toroidal winding advantages and drawbacks for different motor dimensions. In the sixth chapter, two reduction technics of ripple torque are introduced. The purpose is find a way that permit to reduce the torque ripple without worsen the performance. Fractional slot windings and a new rotor design are introduced and FEM analysis provide and validate the torque ripple results

An Analytical-Numerical Approach to Model and Analyse Squirrel Cage Induction Motors

Nowadays, finite element analysis represents the most accurate tool to analyse electrical machines. However, the time domain resolution of electromagnetic problem, in some cases, requires long simulation time due to the induced nature of the currents. The computational burden increases when the machine features a skewed layout on the stator or rotor structures, since this requires 2D multi-slices approximated analysis or even a full 3D model. In this paper, a general analytical method to model electromagnetic devices is applied to a squirrel cage induction motor featuring a skewed rotor structure. The modelling approach is wisely implemented and adapted to pursue a fair balance between accuracy of the analysis and computational burden, taking advantage of all the symmetries existing in the rotor cage of the machine, aiming to minimize the model complexity. A comparative analysis in term of the inductances between analytical and finite element is proposed. The results provided by the model developed are compared with respect to the corresponding values provided by both finite element and experimental test performed on the reference machine. Such comparisons show that the proposed model is actually able to achieve a pretty good balance between accuracy and computational efficiency

Rotor Slot Design of Squirrel Cage Induction Motors with Improved Rated Efficiency and Starting Capability

Among the electro-mechanical devices transforming energy from electrical to mechanical, the squirrel cage induction motor can be surely considered a workhorse of the industry due to its robustness, low cost and good performance when directly fed by the a.c. grid. Being the most influencing motor topology in terms of energy consumption, optimizing the efficiency of squirrel cage induction motors could lead to a great impact towards the reduction of the human environmental footprint. The induction motor design aided by finite element analysis presents significant challenges because an accurate performance prediction requires a considerable computational burden. This paper makes use of an innovative fast and accurate performance evaluation method embedded into an automatic design procedure to optimize different rotor slot geometries. After introducing the performance estimation approach, its advantages and limits are discussed comparing its prediction with the experimental tests carried out on an off-the-shelf induction motor. Different rotor cage structures with increasing geometrical complexity are then optimized in terms of starting and rated performance adopting the same design optimization process, the same stator geometry and constituent materials. The analysis of the optimal solutions shows how it is possible to improve the rated efficiency without compromising other performance indexes. The presented results can be used as general design guidelines of squirrel cage induction motors for industrial applications

Rotor Design Optimization of Squirrel Cage Induction Motor - Part I: Problem Statement

Squirrel cage induction motor is the most widely adopted electrical machine in applications directly fed by the main grid. The analysis, design and optimization of this machine topology has been addressed by a considerable amount of literature over the last century. Although its wide adoption, the induction motor design, especially when carried out in an automatic fashion, still presents significant challenges because the accurate prediction of the performance requires time-consuming finite element analysis. This work proposes a systematic approach to perform the design optimization of a squirrel cage induction motor focusing on the rotor slot geometry, being this the major player in defining the torque-speed characteristic. Structured as a two-parts companion papers, this first part presents an innovative performance evaluation methodology which allows a very fast estimation of the torque and efficiency behaviour preserving the results' accuracy. The proposed performance estimation technique is assessed against experimental tests carried out on an off-the-shelf induction motor. The selection of the performance indexes to be optimized is justified in detail along with the description of a generalized rotor parametrization which allows a comprehensive exploration of the research space

High Speed Permanent Magnet Assisted Synchronous Reluctance Machines - Part I: A General Design Approach

The design of synchronous reluctance machines with and without permanent magnets assistance constitutes a challenging engineering task due to the numerous design variables and performance indexes to be considered. The design complexity increases even further when the application requires high speed operation, with consequent rotor structural constraints and and related effects on the electromagnetic performance. Structured as two-parts companion papers, this first part proposes a comprehensive design procedure able to consider all the non-linear aspects of the machine behaviour, greatly reducing the number of independent design variables, without worsening the computational burden. In particular, the non linear behaviour of the rotor iron ribs and the effect of the permanent magnets on the structural design are all taken into account with the proposed iterative design procedure targeting the achievement of a desired power factor. The proposed method will be then used to draw some preliminary design considerations highlighting the several trade-offs involved in the design of high speed permanent magnet assisted synchronous reluctance machine. Part I is setting the theoretical bricks that will be further expanded and experimentally validated in the companion paper Part II

High Speed Synchronous Reluctance Machines: Modeling, Design and Limits

An important barrier to the adoption and acceptance of synchronous reluctance (SyR) machines in different applications lies in its non-standardized design procedure. The conflicting requirements incurring at high speeds among electromagnetic torque and structural and thermal limitations can significantly influence the machine performance, leading to a real design challenge. Analytical models used for design purpose lack in accuracy and force the designer to heavily rely on finite element analysis (FEA), at least during the design refinement stage. This becomes even more computational expensive as the speed increases, as the evaluation of the rotor structural behaviour is required. This work presents a computational efficient hybrid analytical-FE design process able to consider all the main limiting design aspects of SyR machine incurring at high speed, namely structural and thermal. As a vessel to investigate the proposed design routine accuracy, several high speed SyR machines have been designed for a wide range of operational speeds (up to 70krpm). The thermal and mechanical factors limiting the high speed operation are deeply analyzed aiming at maximize the mechanical output power. The proposed design approach is then validated by comparison against experimental measurements on a 5kW-50krpm SyR prototype

Modular Power Sharing Control for Bearingless Multi-Three Phase Permanent Magnet Synchronous Machine

This paper proposes a modular approach to the power sharing control of permanent magnet synchronous bearingless machine. The selected machine topology features a winding layout with phases distributed into non-overlapping three phase groups, a solution whose twofold aim is to increase the fault tolerance and to allow for the radial force generation. The three phase sub-windings are supplied by standard three-phase inverter, leading to a modular system architecture. A throughout explanation of the methodology used to develop the control algorithm is presented considering the torque and force control in combination with the power sharing management of the machine. Special emphasis is also placed on validating the modelling hypotheses based on a finite element characterisation of the machine electro-mechanical behaviour. The proposed control strategy is also extended to cater the possibility of one or more inverters failure, thus validating the intrinsic advantage of the redundancy obtained by the modularity of the system. An extensive experimental test campaign is finally carried out on a prototyped multi-three phase permanent magnet synchronous drive. The obtained results validate the bearingless power sharing operation in healthy and faulty scenarios, both at steady state and under extreme transient condition

High-Speed Synchronous Reluctance Machines: Materials Selection and Performance Boundaries

This article presents a comprehensive comparative design exercise of synchronous reluctance (SyR) machines considering different soft magnetic materials and a wide range of speeds. First, a general design methodology able to consider all the consequences of selecting different materials is presented. In fact, magnetic nonlinearities, rotor structural limitations, and the rise of both stator and rotor iron losses are all considered. The adopted design approach allows achieving optimal stator and rotor geometries balancing all these competitive multiphysics aspects and keeping constant the cooling system capability. Both silicon–iron (SiFe) and cobalt–iron (CoFe) alloys with optimized magnetic and mechanical performance are examined to assess the maximum capabilities achievable with an SyR machine technology. The adoption of CoFe alloys leads to machines that outperform the SiFe counterparts up to a certain speed, above which, machines with SiFe provide better performance. Indeed, in the lower speed range, the effect of the higher saturation flux density of the CoFe materials is dominant, while for higher design speeds, their higher iron losses and lower yield strength, with respect to the SiFe ones, make the latter more convenient. All the design considerations are finally validated by comparing the predicted performance with the experimental test results on a 6.5-kW, 80-kr/min SyR machine prototype

Airgap Length Analysis of a 350kW PM-Assisted Syn-Rel Machine for Heavy Duty EV Traction

Synchronous reluctance (Syn-Rel) machines with embedded permanent magnets (PMs) are research hotspots in variable speed motor drives due to their robust rotor structure and wide constant power speed range (CPSR). In this paper, the potential of PM-assisted Syn-Rel machine to be next generation heavy duty traction motor solution has been investigated, with special attention put on one key geometric parameter, i.e., airgap length. Careful machine design and optimization has been conducted based on geometric parametrization including airgap length variation, for 15000rpm peak speed and 350kW peak power output. In low speed operations, the influence of airgap length on different torque components has been analyzed in detail based on the frozen permeability method. In field weakening region, the variation trend of several key parameters such as output power, torque ripple, and power losses have been investigated along with airgap length. It is found that with high electric and magnetic loading, reducing the electromagnetic airgap length is not always beneficial. There exists a suitable airgap length value to comprehensively balance torque/power density, cooling capability, efficiency and reliability. Numerical FEA and experimental tests of the prototype are combined to verify the conclusions