Today’s electrical machine technology allows increasing the wind turbine output powerby an order of magnitude from the technology that existed only ten years ago. However,it is sometimes argued that high-power direct-drive wind turbine generators will proveto be of limited practical importance because of their relatively large size and weight.The limited space for the generator in a wind turbine application together with thegrowing use of wind energy pose a challenge for the design engineers who are trying toincrease torque without making the generator larger.When it comes to high torque density, the limiting factor in every electrical machine isheat, and if the electrical machine parts exceed their maximum allowable continuousoperating temperature, even for a short time, they can suffer permanent damage.Therefore, highly efficient thermal design or cooling methods is needed. One of thepromising solutions to enhance heat transfer performances of high-power, low-speedelectrical machines is the direct cooling of the windings. This doctoral dissertationproposes a rotor-surface-magnet synchronous generator with a fractional slot nonoverlappingstator winding made of hollow conductors, through which liquid coolantcan be passed directly during the application of current in order to increase theconvective heat transfer capabilities and reduce the generator mass.This doctoral dissertation focuses on the electromagnetic design of a liquid-cooleddirect-drive permanent-magnet synchronous generator (LC DD-PMSG) for a directdrivewind turbine application. The analytical calculation of the magnetic fielddistribution is carried out with the ambition of fast and accurate predicting of the maindimensions of the machine and especially the thickness of the permanent magnets; thegenerator electromagnetic parameters as well as the design optimization. The focus ison the generator design with a fractional slot non-overlapping winding placed into openstator slots. This is an a priori selection to guarantee easy manufacturing of the LCwinding. A thermal analysis of the LC DD-PMSG based on a lumped parameter thermal model takes place with the ambition of evaluating the generator thermal performance.The thermal model was adapted to take into account the uneven copper loss distributionresulting from the skin effect as well as the effect of temperature on the copper windingresistance and the thermophysical properties of the coolant. The developed lumpedparameterthermal model and the analytical calculation of the magnetic field distributioncan both be integrated with the presented algorithm to optimize an LC DD-PMSGdesign.Based on an instrumented small prototype with liquid-cooled tooth-coils, the followingtargets have been achieved: experimental determination of the performance of the directliquid cooling of the stator winding and validating the temperatures predicted by ananalytical thermal model; proving the feasibility of manufacturing the liquid-cooledtooth-coil winding; moreover, demonstration of the objectives of the project to potentialcustomers.
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