Dynamic behaviour of electric machine stators: Modelling guidelines for efficient finite-element simulations and design specifications for noise reduction

Boosted by the increasing interest of industries such as automotive, 100% electric engine technologies power more and more affordable vehicles for the general public. In spite of a rather favourable common opinion about the low noise emitted by electric motors, controlling the vibratory and acoustic performances of such machines remains a very costly challenge to take up. Associating the expertise of the company Vibratec and the institute Femto-ST Applied Mechanics Department, this industry-oriented Ph.D. thesis aims at improving the current knowledge about the mechanical behaviour of electric machines. New finite-element modelling methods are proposed from homogenisation approaches, experimental analyses, model updating procedures and variability studies in temperature and frequency, in order to predict the behaviour of an electric motor more efficiently.Dopées par un intérêt croissant des industries telles que l’automobile, les technologies de motorisation 100% électriques équippent de plus en plus de véhicules à la portée du grand public. En dépit d’une opinion commune favorable sur les faibles émissions sonores des moteurs électriques, la maîtrise des performances vibratoires et acoustiques d’une telle machine reste un challenge très coûteux à relever. Associant l’expertise de l’entreprise Vibratec et du département Mécanique Appliquée de l’institut Femto-ST, cette thèse CIFRE vise à améliorer les connaissances actuelles sur le comportement mécanique de machines électriques. De nouvelles méthodes de modélisation par éléments finis sont proposées à partir d’approches d’homogénéisation, analyses expérimentales, recalage de modèles et études de variabilité en température et en fréquence, pour permettre une prédiction plus performante du comportement vibratoire d’un moteur électrique

Unit-wave response-based modeling of electromechanical noise and vibration of electrical machines

The primary aim of the thesis is to develop a method for the rapid electromechanical sound power calculation of electrical machines to be used in industry. The core idea is that the numerical simulation of sound radiation is carried out only once. Then, a number of characterizing curves, known as unit-wave responses, are extracted from the results and stored. Finally, the unit-wave responses, together with the magnetic excitation force waves, are used for fast sound power estimation. An experimental method for the determination of unit-wave responses is also developed, which serves especially the process of model verification. The secondary aim is to study the items crucial for the modeling of the sound radiation of electrical machines, such as the effect of tangential force waves on the vibration response, the correlation of force waves in the case of a DTC converter supply, the effect of impregnation on the material properties of the stator core, and the feasibility of the approximate methods for sound power calculation. The most important results of the work done in the thesis include the following: 1) the unit-wave-based sound power calculation performs well, provided that force waves with different wave numbers are weakly correlated, which was verified for the case of a machine supplied by a DTC converter; 2) tangential force waves may have a remarkable effect on the response, which is observed as either an increased or decreased response level, depending on the phase difference of the radial and tangential force waves; 3) the VPI impregnation affects the structural material properties of the stator core considerably, which manifests itself as increased stiffness and decreased damping of the stator core, and 4) the high-frequency boundary element method seems appropriate for the fast and approximate sound power calculation of electrical machinery

Design of a high speed high power switched reluctance motor

PhD ThesisAn increase in the price of rare earth materials in 2009 prompted research into alternative motor technologies without permanent magnets. The SRMs have become more of an attractive solution as they are relatively simpler to construct than other machines technologies hence cost effective. Furthermore, the rugged structure of the rotor makes it suitable for high speed operation, if appropriately designed. This thesis investigates the design, analysis and prototype manufacture of an SRM, that from electromagnetic point of view, meets the power output of the PM machine used in the Toyota Prius, although operating at a higher speed of 50,000 rpm. As a result, the required torque is considerably less than an equivalent motor with the same output power running at lower speed, hence this approach allows for much smaller frame sizes. To achieve the required torque, careful choice of stator/rotor tooth combination, coil number of turns and number of phases is needed. Running at high speed, increases the AC copper loss (consisting of skin effect and proximity effects) and iron loss. These shortcomings are extensively discussed and investigated. The mechanical design of this motor requires careful consideration in order to minimise the high mechanical stresses acting upon the rotor, which are due to the high radial forces caused by the centripetal force at high speed. In order to address the mechanical constraints caused by the hoop stress, a structure common to flywheels is applied to the rotor. In this approach, the shaft bore is removed and the laminations are sandwiched together using cheek plates, which are secured using tie rods. The cheek plates have their extending shafts, which consequently will transfer the torque to the rest of the system. The proposed model is analysed for both the electromagnetic and mechanical aspects, successfully demonstrating a promising rotor topology for the design speed. A high speed motor design needs to take into account shaft design, rotor design and bearing design. The high speed operation of the salient rotor gives dramatic rise to the windage loss. These factors are carefully considered in this work and the results are presented

Structural Dynamic Modeling and Simulation of Acoustic Sound Emissions of Electric Traction Motors

The acoustic behavior of electric drive systems is one of the main comfort criteria of electromobility. Due to its high-pitched sound emissions, the electric motor plays an important role. The corresponding noise is predominantly determined by the vibrational behavior of the electric machine given by the structural transfer function. The early phase consideration of the vibrational behavior of electric machine structures becomes even more relevant if one takes into account the strong requirements towards lightweight design and spatial restrictions inside vehicle applications. One of the most important tools inside the early stage development is the structural dynamic simulation. In order to be able to sustainably predict the vibrational behavior of an electric machine, the corresponding simulation model needs to sufficiently represents all acoustically relevant structural effects and at the same time remain practical and numerically solvable in a reasonable amount of time. This conflict is dealt with in this dissertation. The acoustic behavior of electric machines is strongly coupled to the vibrational behavior of the electric machine stator. The microscopic representation of the strongly heterogeneous stator structure is elaborate and requires a large computational effort. Therefore, so-called homogenized substitutional materials are typically employed in structural dynamical simulations of electric motors. The homogenized materials intend to represent the effective stiffness and damping properties of the underlying heterogeneous structure by an anisotropic substitutional material. Typically, the corresponding effective stiffness and damping properties of the homogeneous material are reversely obtained from experimental investigations on the particular structure. However, this approach presumes the physical existence of prototypes that can be tested. In this thesis, different so-called homogenization techniques will be investigated that allow the identification of homogenized material properties based micromechanical models of the underlying heterogeneous structure. Therefore, various numerical and analytic approaches will be investigated. The resulting modeling approaches will be validated based on different experimental analyses on an exemplary stator structure and subsequently be employed in a comprehensive acoustic simulation of an entire electric drive train. However, the simulation and optimization of the mostly broadband acoustic behavior of electric motors remains time-consuming. In order to efficiently predict the acoustic behavior of electric machines the use of model order reduction methods can be advantageous. Model order reduction methods typically involve mathematical algorithms that yield the effective reduction of the model’s degrees-of-freedom. In this thesis, different model order reduction techniques will be applied and evaluated regarding their usability in the area of vibrational simulations of electric machines. A particularly efficient model order reduction could be achieved by using so-called Krylovsubspaces. By employing the Krylov-subspace method the solution time for particular operation points of the electric machine could be reduced to less than 10% of the original solution time. The integrated modeling procedure, presented in this thesis, yields the sustainable and efficient representation of the vibrational behavior of electric machines. It allows the early phase evaluation and optimization of the acoustic behavior of different electric machine designs. This thesis differs from similar research so far that a generic approach was used to make the representation of the global dynamic behavior of the electric machine possible. The process includes micromechanical models which add a unique robustness and sustainability to the approach

High Performance Cooling of Traction Brushless Machines

The work presented in this thesis covers several aspects of traction electric drive system design. Particular attention is given to the traction electrical machine with focus on the cooling solution, thermal modelling and testing. A 60 kW peak power traction machine is designed to achieve high power density and high efficiency thanks to direct oil cooling. The machine selected has a tooth coil winding, also defined as non-overlapping fractional slot concentrated winding. This winding concept is state of the art for many applications with high volumes and powers below 10 kW. Also, these have been proven successful in high power applications such as wind power generators. In this thesis, it is shown that this technology is promising also for traction machines and, with some suggested design solutions, can present certain unique advantages when it comes to manufacturing and cooling.The traction machine in this work is designed for a small two-seater electric vehicle but could as well be used in a parallel hybrid. The proposed solution has the advantage of having a simple winding design and of integrating the cooling within the stator slot and core. A prototype of the machine has been built and tested, showing that the machine can operate with current densities of up to 35 A/mm^2 for 30 seconds and 25 A/mm^2 continuously. This results in a net power density of the built prototype of 24 kW/l and a gross power density of 8 kW/l with a peak efficiency above 94%. It is shown that a version of the same design optimized for mass manufacturing has the potential of having a gross power density of 15.5 kW/l which would be comparable with the best in class traction machines found on the automotive industry. The cooling solution proposed is resulting in significantly lower winding temperature and an efficiency gain between 1.5% and 3.5% points, depending on the drivecycle, compared to an external jacket cooling, which is a common solution for traction motors

Segmental rotor switched reluctance drives

One of the well-known drawbacks of switched reluctance machines is the relatively high output torque ripple. Techniques aiming to reduce machine torque ripple either compromise the machine performance or the simplicity of the inverter and the controller. The work presented in this thesis shows that low torque ripple over a wide speed range can be achieved without severe penalties in terms of the machine performance and the size, cost and complexity of the power electronics and the controller. This is achieved by designing a 6-phase machine and driving it from a three-phase full bridge circuit. Switched reluctance motors with segmented rotors are a relatively recent advancement in the electromagnetic design of doubly-salient reluctance motors, having only been introduced in 2002. By replacing the conventional toothed rotor with individual segments, it has been proven that higher torque density than conventional switched reluctance machines could be achieved. Early work by Mecrow and El-Kharashi has demonstrated the operation of prototype machines with short-pitched and fully-pitched windings. The machine design work presented here builds on this early work by examining aspects of the machine design and its operation. Two six-phase machines – one with a segmented rotor and the other with a toothed rotor - have been designed. Performance comparisons have been made between the two six-phase machines and a three phase segmented rotor machine that was previously designed at Newcastle University. Additionally, a three phase single tooth winding and a two phase segmented rotor switched reluctance machine have been studied in simulation and experimentally. Detailed comparison of inverter ratings and machine efficiencies are made under equal conditions for a 2-phase machine driven from h-bridge and asymmetric half-bridge inverters. This is achieved with results from a test rig and the use of accurate dynamic simulation. Simulation models for 3-phase and 6-phase machines have also been generated. Detailed comparison of inverter ratings and machine efficiencies are made under equal conditions for the 3-phase and 6-phase drives in the dynamic simulation. Comparisons between simulated and measured results are shown to be very good for all of the drives.EThOS - Electronic Theses Online ServiceGBUnited Kingdo