0 Literature reviewWind energy is an inexpensiverenewable energy source which has an ability to make significant contributionto the electricity utility network. However, there are two problems associatedwith the construction of wind power generators which has to be addressed, thefirst one being the instability of the wind speed and the second one isrotating speed of the wind turbine which is low due to the large diameter ofthe rotor blades. The technologies have been developed to estimate the variablespeed constant frequency to counter the instability of the wind speed. Thelater issue was addressed by using conventional solution of gear box toincrease the speed which helps to reduce the size of generators.
Althoughsolution of gear box has several disadvantages as it generates noise andvibrations, losses in gear drive is high since it is a mechanical device, nedof constant lubrication and periodic maintenance (Fengxiang, 2005). The cost ofgear box is also high. Direct driven variable speedpermanent magnet machines are being a subject of attention due to variousadvantages offered by them such competitive cost and possibility to eliminatethe gear box from wind turbine structure.
Energy capture is increased in suchapplication by using variable speed. Due to removal of gearbox weight andlosses in wind mill reduces drastically which enhances system efficiency.Frequency of periodic maintenance is also reduced saving finances of theorganization.
Although, large numbers of poles are required to construct agenerator due to low rotational speed; it is necessary for the generator thatit should be efficient naturally with competitive cost. To supply power to thegrid, frequency converter is required due to the variable speed scheme.According to (Fengxiang et al, 2005) small pole pitch can be achieved byincorporating large pole numbers with permanent magnets. A simple and effectivegenerator construction is shown in figure 1 of appendix 1 in the form of disctype axial flux configuration. The stator in the figure shown is a toroidalwound accommodating rectangular coils which forms an air gap winding.
Permanentmagnets are attached to the rotor disc located on both side of stator.According to (Spooner et al, 1996), the assembly of the permanentmagnet machine is the crucial problem during its construction. No strong forcesare present at the time of assembly as the assembly of the magnet is carriedout individually and iron parts are already located in the position in themodern assembly practice of permanent magnet machines.To reduce the assembly problemsof PM generators the modular construction is proposed by to (Spooner et al,1996). The paper presented by to (Spooner et al, 1996) says that, for the largegrid connected wind turbines, direct coupled, permanent magnet, synchronouswith radial field and multipole machines can be used. The power rating could bebetween 100 kW to 1 MW and pole numbers could be between 100 to 300.
Employingmodular constructions help to reduce need of detail design, number of tools anddrawings. The modular assembly practice can be utilised in vast ranges ofmachines. The standard ferrite magnet blocks are used in the rotor module,whereas the stator module is formed by single rectangular coil embedded insimple E-cores. The assembly of the magnetised parts can be arranged easilywhich improves the efficiency of the machine with low reluctance. The multipolepermanent magnet is shown in figure 2 of appendix 1 whereas modular arrangementof magnet is shown in figure 3 of appendix 1 which help to visualise thedifference. 1.0 Progress of the workThe progress of the work shownhere is divided in to two parts,1.
Designof permanent magnet machine2. Finiteelement analysis of the design5.1 Design of permanent magnetmachineDuring the study of permanentmagnet machine, various important design parameters were studied which isincorporated in the design of the permanent magnet machine, Ratedpower of the machine- Velocityof the wind speed and speed ration of driving shaft governs the rated output ofthe generator. For this project work, minimum wind speed of 4 km/hour isconsidered, the rpm produced by the shaft and output of the generator withsingle phase connection will be calculated theoretically and evaluated duringFEA analysis if the design.
Numberof phases and poles- Numberof stator poles decide the number of phases in the machine. The thumb rule isthe number of stator poles are twice than number of phases. It has been foundthat during research, torque ripple increases with small numbers of phaseswhere as cogging torque reduces with large pole numbers. Considering theseconstraints, three phase machine is selected. By using electrical engineeringhandbook by (Chen, 2004), 24 number of stator poles are selected to reduce thetorque ripples. The rotor pole is selected considering the relation between therotor and stator.
Framesize- Dimension for allelectrical machines are freeze by International Electro-Technical Council knownas IEC, which comply the ISO regulations. The stator and rotor ratio selectedat this moment is 1:16 but it may change during the course of designingdepending upon the need. The structure size is yet to be finalized althoughpreliminary selection of the ratio fixes the frame structure.Airgap- The probability of coggingtorque increases due to use of permanent magnets therefore to reduce the sameand to increase the flux density air gap will be limited to a range of 0.5 to1.0 mm.Machinespecification- Although design ofpermanent magnet machine is ongoing, and all the parameters are not fixed yet.Even though tentative specification of permanent magnet rotor is given belowbased on selection through engineering handbook and ratio, just a note, belowspecification might change depending on the design requirements,Table1: Initial specification of permanent magnetmachine No Parameters Value 1 Rating 3000 W 2 Stator poles 24 3 Magnet poles 8 4 Phases 3 5 Poles per phase 8 6 Length of air gap 0.
5 to 1.0 mm 7 Stator diameter 0.16 m 8 Rotor diameter 0.
1 m 9 Length of magnet 0.015 m 10 Length of back iron 0.016 m Based on initial specification ofpermanent magnet machine, initial CAD model is generated which shown in figure4 of appendix 1. 5.3 Finite element analysis The design of PM machines will bevalidated in FEA (Finite Element Analysis) software developed in theuniversity. The FEA software is a powerful tool which help to analyse thedesign and allows necessary changes before actual production and assembly. Variousscenarios can be simulated to check the performance of the PM machine.
In this project work, FEA will beused to check the designed structure of the machine, excitation of the rotor,material properties and torque produced due to different rotor position as wellas wind current. The step by step approach will be used to find solution ofcontinuum problem during FEA analysis; for example, elements are created bydividing the continuum region by using different shapes of elements. It may bepossible that different shapes of element produce similar solution for thecontinuum. It has been found during familiarization of software that it isquickly possible to express material properties, constraints and excitationalthough it is comparatively difficult to express. Other important parameterswill be calculated by using solution of system equation, such as,electromagnetic problems, Components of magnetic flux density are nodalunknowns.
By using these components torque, induction and several otherelectromagnetic parameters will be calculated and compared with the design one.Following assumptions are made todetermine distribution of magnetic field inside the machine. These assumptionsare primary and may changed during actual simulation of the design,1. Since themagnetic field outside the status stamping is almost negligible hence themagnetic vector potential line of outer periphery of the status stamping istreated as zero2. Hysteresiseffects are neglected as magnetic material is isotropic for stator and rotorstampings3.
Componentsof Z- directions are Current density (J) and magnetic vector potential (A)4. Distributionof magnetic field along the generator’s axial direction inside the generator isconstant5. Endeffects are considered to be zero 2.0 Summary and conclusionThe progress report includesbrief overview of permanent magnet machine with its application in windturbine, benefits of the same if used in the wind turbines followed by aim,objectives and glimpse of literature review which primarily emphasis on construction,assembly and capacity of permanent magnet. Project management section showsgnat chart in detail along with its completion status. Progress report on theother hand shows the completed work so far in design part as well as futureconsiderations and assumptions for FEA.Considering the work put togetherin progress report, it can be concluded that progress of the report is as perthe schedule and it will be completed according to plan provided in projectmanagement section. Initial design specification for the PM machine iscompleted, along with initial CAD drawing for the same.
Detail design procedureis in process and it will be completed by end of week 8. Study of FEA softwareby using similar case studies have been carried out. Once the final design ofthe PM machine is completed then CAD modelling and FEA simulation will commencewhich is around start of week 9. Any alteration required in the design will becarried out considering the results of FEA. Comparison of initial and finaldesign along with FEA justification will be provided.