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| 2. |
- Arshad, Waqas, 1973-
(författare)
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A Low-Leakage Linear Transverse-Flux Machine for a Free-Piston Generator
- 2003
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The presented work is about a new low-leakage lineartransverse-flux electrical machine.The machine is dimensionedfor a free-piston generator. The intended application is inseries-hybrid vehicles. The free-piston generator is anintegration of a free-piston combustion engine with a linearelectrical machine. It provides benefits in e±ciency,emissions and multi-fuel capabilities, to name a few. Thesecombustion benefits are only possible if the total mover massis kept quite low (e.g.≤ 4 kg for a 50 kW unit). Thelimited mover mass and competitiveness demands result in a verystringent set of specifications for the electrical machine.Most existing electrical machine topologies arepreliminarily investigated. Of these only a transverse-fluxmachine (TFM) is found to be promising. Even for the case ofthe TFM, a factor 2 improvement in the performance indices ofthe existing TFMs is required. Different surface-mounted TFMsare investigated for improvement. It is found that theanalytically acceptable solutions fail during thethree-dimensional finite-element analysis (3D-FEM). Thesedesigns suffer from a very significant pole-to-pole armatureleakage flux in the stator. Similar design improvements for theburied-magnets TFMs are also investigated. Fortunately, in thiscase a promising design is derived. This design exhibits lowleakage and is verified with FEM analysis. A down-scaledone-phase 1.7 kW proof-of-concept TFM of this design isdimensioned. It is analysed thoroughly, both analytically aswell as by 3D-FEM computations. It is found that for thisparticular prototype the design suffered from saturation. It isshown how this design can be modified to yield the desiredforce with the allowed mover mass. The efficiency is found tobe only slightly lower than the demands but a poor power factoris noted.The prototype is built. Unfortunately, all the desiredmeasurements could not be carried out. This is because thebuilt-machine sufferedwith two major manufacturing defects.The magnets are found to be demagnetised to about 50% of theirmanufactured value. It is also found that the translator poleson the average are 7% longer (in the axial direction) than thestator poles. Low values of no-load induced voltage and forceare thus measured. The correlation between the manufacturingdefects and the measurements is established. Due toinsufficient measurement data, this relationship is mainly ofan indicative nature only. A more correct picture requiresconstruction of a new prototype. Still, the results do pointtowards a near fulfillment of the objectives. Other addressedtopics include, an investigation into a separate free-pistongenerator project with reduced specifications, application ofthe presented work to renewable energy sources like wind- andwave-energy and a brief introduction to the free-pistonapplication aspects.Keywords:Free-Piston Generators, Linear Machines,Transverse-Flux Machines, Hybrid Vehicles, Electrical MachinesDesign.
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| 3. |
- Carlsson, Fredrik, 1974-
(författare)
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On impacts and ride-through of voltage sags exposing line-operated AC-machines and metal processes
- 2003
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- During the last decade, power quality has been recognised asa global problem. Among different types of power qualityproblems, voltage sags have been identified to be one of themost severe problems for different process industries. The mostcommon reason to voltage sags is lightning strikes in powerlines. Protection equipment, usually located at switchyards,disconnect faulted power lines as soon as possible, which isapproximately 100 ms. Thus, the duration of voltage sags areapproximately 100 ms. The sensitivity to voltage sags ofelectrical equipment in process industries can be observed asfor instance malfunction, automatic turnoff or damages.This thesis gives an overview of three metals processes withfocus on the sensitivity to voltage sags and interruptions. Theinherent energy in the process is used to find the sensitivity.This energy may also be used to obtain "ride-through" for theprocesses. The three metals processes are a blast furnaceprocess, a hot rolling mill process and a cold rolling millprocess. The main attention in this thesis is paid to the blastfurnace process, which is powered by a line-operatedsynchronous machine.The thesis shows that the protection equipment forelectrical machines can be adjusted to avoid unnecessaryshutdowns. It is also explained why there are high torque andcurrents during voltage sags as well as after voltage sags. Itis shown that the first peak torque and current during thevoltage sags is almost proportional to the voltage change, thatis the voltage magnitude before the voltage sag minus thevoltage magnitude during the voltage sag. The first peak torqueand current after the voltage sag depends sinusoidal-like onthe duration of the voltage sag and almost proportional to thevoltage change during the voltage sag. There is no fluxsaturation during voltage sags, however after voltage sagssaturation is very likely to occur. The thesis explains why andalso how the flux is changed during and after voltage sags.The duration of voltage sags is in many cases set by theprotection equipment located in switchyards. It is shown thatthe durations of voltage sags can be changed to durations thatwill cause less peak torque and current after voltage sags forline-operated AC-machines. It is also shown how this istheoretically achieved.Keywords:Rolling mill, Blast furnace, Power Quality,Synchronous machine, Asynchronous machine, Voltage sag, Voltageinterruption, Ride-through, Process disturbances, Simulation,Modelling
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- Fischer de Toledo, Paulo
(författare)
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Feasibility of HVDC for city infeed
- 2003
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- It is well recognized that direct current and direct voltageoffer special advantages for both land and sea cable systems,both with regard to power transmission capability, losses, aswell as possible transmission length due to no capacitivecurrents. As cable systems were used very early in largecities, one of the first applications considered for HVDC wasto use it for city infeed and some schemes were also built.However, it turned out that the cost for the stations was toohigh and that the savings on the cable part were not highenough to justify the high costs of the converter stations,even considering other possible benefits of the HVDC techniquessuch as fast control of active power and almost no contributionto fault currents.During the 1990s new HVDC Voltage Source Converters, VSC,and new HVDC cables with solid insulation have been developedand the relative cost for the converters has been steadilydecreasing. It was, therefore, found justifiable to reexaminethe feasibility of using HVDC, especially based on the new VSCtechnique, for feeding electrical power to large cities. It wasalso decided, that the study should clarify the specialrequirements that had to be considered in the planning of cityinfeed systems as no good survey could be found. Because ofthis the study has been performed in close co-operation with anumber of utilities responsible for the power supply of somemedium sized and large cities. One such requirement, that alsojustified the study, was that it is expected that in the futuremore overhead lines in the cities or close to the cities haveto be replaced by cables.Although the transmission and distribution of electricalpower will be preferably made with conventional AC technique,but HVDC transmission would offer special advantages for longtransmission cable, systems with especial requirements withregard to power flow control, systems with restrictions toshort circuit currents, and other relevant issues related tocity center infeed. The use of HVDC transmission to feed powerinto city centers will also be preferable when severerestrictions exist in the system that would require significantadditional measures to mitigate using conventional ACtechnique. In those cases, the cost of these additionalmeasures can be significant enough to justify the use of analternative technique. Or, the implementation of those measureswill make the system too complex to operate. In these cases,HVDC transmission would have advantages over the conventionalAC solution, simplifying the operation of the system orresulting in a more economical solution.Keywords:HVDC (High Voltage Direct CurrentTransmission), Line Commutated Converters, Voltage SourceConverters, City Center Infeed, Underground CableTransmission.
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- Krah, Julius
(författare)
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Modelling approaches for audio frequency power transformers
- 2003
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The development of HVDC converters, local generation andtractions systems increases the demand for power converters.While a lot of studies have been conducted on the powerelectronics, there are still open questions concerning thecontained power transformer that operates at a few kilohertz.As power and loss increase with frequency, a smallertransformer can be constructed supposed cooling is increased orlosses are reduced. Losses can be reduced by use of a low losscore material. A DC-DC converter is likely to be of resonanttype, which puts constraints on the leakage inductance of thetransformer and other parasitic elements. These factors can beinfluenced by changes in the topology of the transformer, i.e.the geometry.Novel modelling approaches have been studied and comparedwith existing models. This concerns the electrical, magneticand thermal behavior of the transformer. The overall model hasbeen run for a realistic geometry. A new field homogenizationprocedure for a magnetic lamination is suggested. Simulationsprove that this method allows reducing the required number oflumped elements. A novel 2D measurement set-up forcharacterization of magnetic sheets has been designed andconstructed to operate up to a few kilohertz. The experimentalverification of the set-up proves that the magnetic fieldamplitude and especially homogeneity increased considerably.The latter allows larger pick-up coils. The set-up is muchsmaller than the previous and allows small samples, which isimportant in the audio frequency range, where core materialoften is manufactured on small rolls. In addition, thenecessary electric excitation decreased substantially.Measurements are required to characterize and evaluatetentative transformer core materials such that they can be usedas a material database for a simulator program. The first stepwas the development of a measurement program includingautomatic demagnetization. The hysteresis model developed atthe department was tested successfully. An idea is formulatedto optimize the model numerically.Keywords:Power transformer, Audio frequency, Loss,Hysteresis, Eddy currents, Skin effect, Leakage inductance,Parasitic elements, Thermal effects, Modelling approaches,Magnetic measurement set-up, Magnetic materials.
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| 6. |
- Lembke, Torbjörn A.
(författare)
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Induction bearings
- 2003
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- A self stabilizing homopolar induction bearing withintegrated touch down bearings has been developed forhigh-speed applications like flywheels, small gas turbines andcompact vacuum cleaners.Stability is achieved without any control electronics thanksto stabilizing eddy currents induced by permanent magnets. Eddycurrent losses are reduced to a minimum using a homopolardesign with ring magnets instead of multipole or halbacharrays.The bearing currents and forces are simulated using steadystate 3D-FEM analysis, which is enabled thanks to theimplemented Minkowskij transform. From these results ananalytical model has been developed, and the results arecompared.Efforts are made to develop a qualitative understanding ofthe bearing physics. Results are converted into usefulrotordynamic data that is easily understood by machineengineers.Finally some experiences from the first experimental testruns at 90.000 RPM are discussed.Keywords:Magnetic bearing, eddy current, inductionbearing, eddy current bearing, high speed, homopolar, gasturbine, flywheel
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| 8. |
- Mårtensson, Eva, 1964-
(författare)
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Modelling electrical properties of composite materials
- 2003
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Composite field grading materials are used to avoid stressconcentrations in high voltage applications such as cableaccessories and generator or motor end windings. The compositematerials consist of an insulating matrix filled with suitableconducting or semi-conducting particles. Silicon carbide (SiC)powder is one such filler that is being employed. The compositematerials display complex electrical characteristics that aredependent on filler properties, particle concentration,frequency and electric field. Optimization of the stressgrading properties would be facilitated if the characteristicof a specified material mixture could be calculatedapproximately.In this thesis a microscopic model for the local behavior atthe SiC grain contacts as well as a macroscopic model for theglobal performance of the composite material are presented. Thedc and ac characteristics of different SiC powders and variouscomposite materials have been studied by experiments andsimulations. The electrical properties of ethylenepropylene-diene monomer (EPDM) rubber filled with the SiC grains havebeen characterized by several time and frequency domainmeasurement techniques.It is shown that the SiC grain contacts can be modeled bySchottky-like barriers. The SiC powders are heavily doped andthe dominating conduction mechanismin the major part of thefield range is tunneling by field emission, amplified bypre-avalanche multiplication. The frequency dependentproperties are governed both by the interfacial barrier regionsand by the surrounding dielectric.A three-dimensional electrical network model for describingthe frequency dependent electrical properties of the compositematerials has been developed. Accounting for different types ofcontacts between the filler grains is fundamental for theresulting characteristics. The distribution of the conductingparticles in the matrix also affects the electrical propertiesand a well dispersed, and not only random, arrangement is morerealistic. Non-linearity is incorporated in an amended version,which treats the timedependent case.The model has been implemented in a MATLAB® program andthe calculations have been compared to measurements on EPDMrubber filled with SiC grains. It is demonstrated that thenetwork simulations reproduce the general characteristics ofrelevant concentration, frequency and field dependentexperimental results.Keywords:field grading; composite materials; SiC;rubber; electrical properties; nonlinear; time-dependent;model; network; conduction mechanisms; Schottky-barrier;
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| 9. |
- Nordlund, Erik
(författare)
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Simulation and Design of a Radial-Radial Four-Quadrant Transducer for Hybrid Electric Vehicles
- 2003
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This report is my licentiate thesis at the Royal Instituteof Technology, (KTH) in Stockholm. The work was done at thedepartment of Electrical Engineering, division of ElectricalMachines and Power Electronic.An internal combustion engine (ICE) in a conventionalvehicle operates rarely at optimum efficiency. The optimumefficiency is about 30-40% and can only be obtained in a narrowspeed-torque interval. The mean efficiency is much lowerbecause of the small maximum efficiency region. To operate acombustion engine at optimum efficiency during all drivingconditions, a full control of the torque and speed isrequired.In a series hybrid vehicle this control is achieved but theelectric machines are dimensioned for the maximum power and allenergy has to be converted from mechanical to electric and backto mechanical with losses at every conversion stage. Theparallel hybrid vehicle has a smaller electric motor butinstead the ICE is mechanically connected to the wheels via agearbox which limits the control of the ICE to only thetorque.In the novel Four-Quadrant Transducer (4QT) concept which isa further development of the work on the Integrated EnergyTransducer previously made at KTH, the advantages of the seriesand parallel hybrid systems are combined. This givespossibilities to operate the ICE totally independent of theroad load.The 4QT is an electric machine made up of two combinedconventional radial flux machines. The 4QT has two rotors; oneinner rotor with windings and one outer rotor with magnets, anda stator with windings. This gives the possibility to controlboth the torqueand the speed of the ICE independent of theroad load which will keep the operation of the combustionengine at the highest possible efficiency.In this thesis the 4QT system is presented and simulatedduring different driving conditions. The simulations have beendone on a twelve ton truck equipped with a 4QT and a dieselcombustion engine. According to the simulations the 4QT systemdecreases the fuel consumption in urban traffic by about 30%compared to a conventional vehicle of the same type.A prototype 4QT has been designed and constructed to verifythe simulation results and to test the performance of the4QT.
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