Higher pole number synchronous reluctance machines with fractional slot concentrated windings

This paper presents an examination of the advantages, disadvantages and remaining challenges of the application of fractional slot concentrated windings to higher pole number (pole-pairs > 2) synchronous reluctance machines. It extends previous work by assessing the effects of the available slotpole combinations on machine performance and construction. Finite element studies and figures of merit derived from d-q axis theory are used to compare and assess performance. Manufacturing and constructional issues are also discussed

Fault-Tolerant Operation of Delta-Connected Scalar- and Vector-Controlled AC Motor Drives

Operation and analysis of delta-connected ac motor-drive systems under fault-tolerant open-phase mode of operation is introduced in this paper for both scalar- and vector-controlled motor-drive systems. This technique enables the operation of the three-phase motor upon a failure in one of its phases without the need of a special fault-detection algorithm. It is mainly used to significantly mitigate torque pulsations, which are caused by an open-delta configuration in the stator windings. The performance of the fault-tolerant system was verified using a detailed time stepping finite element simulation as well experimental tests for a 5-hp 460-V induction motor-drive system and the results are presented in this paper This paper also compares the operation of this fault-tolerant mode of operation for the cases of scalar-controlled and closed-loop vector-controlled motor-drive systems

Reliable Multiphase Induction Motor Drives

A motor is said to be reliable if it can run at its rated operating condition for a specified period of time. With the widespread use of electric motors in newer applications, reliability is a major concern in terms of safety as well as revenue. About 30-40% of reported failures in induction motors are due to stator faults. It is well known that a stator fault starts as an inter-turn fault within a phase and then propagates into phase-to-phase and phase-to-ground faults that can then lead to complete shutdown of the motor. Two approaches have been taken in this dissertation to make an induction motor drive system more tolerant to stator faults; integration of an inter-turn fault detection method into a five-phase induction motor drive and design of fault-tolerant induction motors. The phase redundancy of five-phase motors makes it possible to achieve continued operation of the motor with an open phase. However, for true fault tolerance the drive must be able to detect an incipient fault and then transition to post fault operation. A low-cost diagnostic method based on DC voltage injection has been developed for detection of inter-turn faults in five-phase induction motor drive systems. It has been shown that difference in DC current response to an injected voltage before and after an inter-turn fault serves as a reliable fault indicator. The diagnostic is non-intrusive, requires no additional hardware and effectively integrates both fault detection and fault-tolerant control into the motor controller. The method has been successfully implemented and tested on low-cost microcontroller. The propagation of a stator inter-turn fault into a phase-to-phase fault is worsened in distributed winding induction motors where the different phase windings overlap each other at the end connections. Tooth wound or fractional slot concentrated winding (FSCW) stators have non-overlapping end connections and hence more physical and thermal isolation between the phases as compared to distributed winding stators. While FSCW configurations have been widely used for permanent magnet motors, their adoption for induction motors is a challenge. An FSCW configuration has been designed for outer rotor induction motors by using a dual slot layer stator structure and multilayer windings. Comparison with a conventional induction motor shows an 11% reduction in the copper usage in addition to having non-overlapping phase windings

Optimal design of the stator winding in six-phase ac machines involving frequency dependent parameters and magnetic saturation

Употреба електричних машина у електричним возилима и у електромоторним погонима великих снага често се базира на концепту вишефазних машина и претварача. Предмет научне расправе у оквиру дисертације је разматрање, утврђивање и уважавање електромагнетских појава које се јављају у току рада ових машина, а које могу утицати на одабир оптималне топологије статорског намотаја. Постоји потреба за анализом појава које се јављају при раду вишефазне машине, а које утичу на промене њених параметара и перформанси, што је искоришћено као темељ за формирање методологије пројектовања статорског намотаја шестофазне машине. Док су адекватни математички модели вишефазних машина за линеарне услове рада доступни, то није случај са моделима код којих је присутно засићење магнетског кола. У овој дисертацији обрађена је проблематика пројектовања намотаја статора на примеру шестофазне машине. Анализирана је појава засићења магнетског кола при различитим условима напајања, при чему се анализира потенцијална употреба постојећих модела засићених трофазних машина за потребе моделовања засићених вишефазних машина. Код модела шестофазне машине са векторски распрегнутим равнима анализирано је постојање међусобног утицаја између различитих равни при појави засићења магнетског кола, а што је од значаја за развој прецизнијих модела и алгоритама управљања. Дисертација се ослања на аналитичка разматрања, анализу применом методе коначних елемената и експерименталне резултате. Показано је да на засићење на путу главног флукса утичу компоненте струје које не доприносе стварању електромеханичког момента, тј. установљено је постојање међураванског унакрсног засићења, што доводи до потребе за развојем нових модела шестофазних и осталих вишефазних машина, који ће уважити овај ефекат. Поред засићења, анализиране су различите топологије статорских намотаја ради остваривања бољих перформанси шестофазне машине са становишта смањења површинских губитака у ротору. Установљено је да скраћење навојног корака намотаја често не доводи до смањења ових губитака. Као резултат анализе формирана је методологија оптималног пројектовања статорског намотаја шестофазне машине која се базира на основним научним доприносима ове дисертације.The use of electrical machines in electric vehicles and high-power drives frequently requires multiphase machines and multiphase inverters. The topic of the scientific discussion within the dissertation is the consideration, determination and appreciation of the electromagnetic phenomena that occur during the operation of multiphase machines, which can be related to the selection of the optimal topology of the stator winding. There is a need to analyze these phenomena, which influence the changes in parameters and performances. While appropriate mathematical models under the linear magnetic conditions are readily available for multiphase machines, the same cannot be said for the models of the saturated multiphase machines. This dissertation examines the saturation in an asymmetrical six-phase induction machine under different supply conditions and the possibility of extending the existing linear models to the saturated case. Specifically, in the vector space decomposed model, the mutual coupling between sequence planes is analyzed under saturated conditions, which is important for modeling and control purposes. The dissertation relies on analytical considerations, finite element analysis and experimental results. It is shown that the saturation of the main flux path is influenced by the current components in the orthogonal (non-fundamental) sequence plane. The existence of the cross-coupling effects between different subspaces is recognized, thus implies the need to develop new multiphase machine models which take this effect into account. In addition, various stator winding topologies have been analyzed in order to achieve better performance of the six-phase AC machine. An analysis of the different stator winding topologies and their influence on harmonic losses in the rotor cage, was presented. It has been found that the shortening of the winding pitch often has no effect on reducing these losses. The methodology of designing the stator winding of a six-phase machine is presented, which takes into account the contributions presented in the dissertation. A closer study of the dissertation research is the analysis of the phenomena which are significant in the design and optimization procedure when constructing a sixphase AC machine. The dissertation describes the phenomena that occur during characteristic operating regimes and are of importance during the design process. Therein, the magnetic circuit saturation effect was analyzed in greatest detail, while considering the existing methods for modeling in both three-phase and six-phase machines. It was concluded that the frequency dependence of parameters is present as in three-phase machines

In-wheel motors for electric vehicles

PhD ThesisThe in-wheel motor technology as the source of traction for electric vehicles has been researched recently because it is compact and ease-to-integrate. The motor is housed in the wheel. Since the room for the motor is tightly defined by the size of the wheel and there is no gearing system, the motor must have a high torque density to drive the vehicle directly and a high efficiency to keep cool. The existing motor uses a surface-mounted magnet topology. To make it more cost-competitive, the magnet material needs to be reduced while maintaining the torque performance at the rated operating condition. It is the motive of this Ph.D. research. The thesis starts with a brief introduction on the background of the electric vehicle. Then the major challenges of the in-wheel motor technology are summarised. With the derived specifications, an induction machine and a switched reluctance machine are then simulated and analysed. Still, the permanent magnet synchronous machine is proved to have the highest torque density. Change from surface-mounted to interior topology, six new magnet topologies are investigated. The V-shaped interior magnet topology shows superior torque-to-magnet-mass ratio and is easy-to-manufacture. It gives 96% torque while using 56% of the magnet mass compared to the existing motor due to the assist from the additional reluctance torque and the lower magnetic circuit reluctance. The key to use less magnet mass while avoiding the demagnetisation is the front iron shielding effect. The analytical explanation on the better resistance to demagnetisation in the V-shaped motor is provided. The magnet loss mechanism is discussed for proper segmentation. Detailed design adjustments are made to compromise between the torque-to-magnet-mass ratio and the manufactural practicality. Issues regarding to lower mechanical rigidity occurred in initial assembly of the prototype and solutions are proposed. Followed by successful assembly, experimental tests were conducted and results show good agreement with the simulation. A specific form of torque ripple is found in the V-shaped motor and occurs generally in all fractional-slot concentrated-winding machines with saliency. It is explained by an analytical model. This model is also extended to explain the generally lower reluctance torque in vi fractional-slot concentrated-winding machines. Potential design improvements are suggested and simulated for future versions.Protean Electri

Synchronous reluctance motors with fractional slot-concentrated windings

PhD ThesisToday, high efficiency and high torque density electrical machines are a growing research interest and machines that contain no permanent magnet material are increasingly sought. Despite the lack of interest over the last twenty years, the permanent magnet-free synchronous reluctance machine is undergoing a revival and has become a research focus due to its magnet-free construction, high efficiency and robustness. They are now considered a potential future technology for future industrial variable speed drive applications and even electric vehicles. This thesis presents for the first time a synchronous reluctance motor with fractional slot-concentrated windings, utilizing non-overlapping single tooth wound coils, for high efficiency and high torque density permanent magnet-free electric drives. It presents all stages of the design and validation process from the initial concept stage through the design of such a machine, to the test and validation of a constructed prototype motor. The prototype machine utilizes a segmented stator core back iron arrangement for ease of winding and facilitating high slot fill factors. The conventional synchronous reluctance motor topology utilizes distributed winding systems with a large number of stator slots, presenting some limitations and challenges when considering high efficiency, high torque density electrical machines with low cost. This thesis aims to present an advancement in synchronous reluctance technology by identifying limitations and improving the design of synchronous reluctance motors through development of a novel machine topology. With the presented novel fractional slot concentrated winding machine design, additional challenges such as high torque ripple and low power factor arise, they are explored and analysed - the design modified to minimise any unwanted parasitic effects. The electrical and electromagnetic characteristics of the developed machine are also explored and compared with that of a conventional machine. A novel FEA post-processing technique is developed to analyse individual air-gap field harmonic torque contributions and the machines dq theory also modified in order to account for additional effects. The developed machine is found to be lower cost, lower mass and higher efficiency than an equivalent induction or conventional synchronous reluctance motor, but does suffer higher torque ripples and lower power factor. The prototype is validated using static and dynamic testing with the results showing a good match with finite element predictions. The work contained within this thesis can be considered as a first step to developing commercial technology based on the concept for variable speed drive applications.Financial assistance was provided by was provided by the UK Engineering and Physical Sciences Research Council (EPSRC) in the form of a Doctoral Training Award and additional financial assistance was kindly provided by Cummins Generator Technologies, Stamford, UK, through industrial sponsorship of this wor

Advanced Non-Overlapping Winding Induction Machines for Electrical Vehicle Applications

This thesis presents an investigation into advanced squirrel-cage induction machines (IMs), with a particular reference to the reduction of the total axial length without sacrificing the torque and efficiency characteristics and analysis of recently found non-sinusoidal bar current phenomenon, which occurs under some certain design and operating conditions, and affects the overall performance characteristics of the IMs. As a first step, the most convenient method is determined by utilizing a fractional-slot concentrated winding (FSCW) technique, which has advantages such as non-overlapping windings, high slot filling factor, and simple structure. After implementing this technique, it is found that due to the highly distorted magnetomotive forces (MMFs) created by the FSCWs, significant high rotor bar copper loss occurs. In order to reduce the MMF harmonics without increasing the size of the machine, a new technique titled “adapted non-overlapping winding” is developed. This technique consists of the combination of the auxiliary tooth and phase shifting techniques, resulting in a stator with concentrated windings of two-slot coil pitches but without overlapping the end-windings. Thanks to this method a large number of the MMF harmonics are cancelled. Thus, a low copper loss IM with significantly reduced total axial length is obtained. Influence of design parameters; such as stator slot, rotor slot, and pole numbers, number of turns, stack length, stator and rotor geometric parameters, etc. on the performance characteristics of the advanced IM is investigated and a comprehensive comparison of advanced and conventional IMs is presented. This thesis also covers an in-depth investigation on the non-sinusoidal bar current phenomenon. It is observed that the rotor bar current waveform, usually presumed to be sinusoidal, becomes non-sinusoidal in some operation and design conditions, such as high speed operation close to synchronous speed, or fairly high electrical loading operation, or in the IMs whose air-gap length is considerably small, etc. Influences of design and operating parameters and magnetic saturation on the rotor bar current waveform and the performance characteristics of squirrel-cage IMs are investigated. The levels of iron saturation, depending on the design and operating parameters, in different machine parts are examined and their influences are also investigated, whilst the dominant part causing the non-sinusoidal rotor bar current waveform is identified. It is revealed that the magnetic saturation, particularly in the rotor tooth, has a significant effect on the bar current waveform