Permanent Magnet Generator Design for offshore wind farms: Multi Pole Permanent Magnet Generator-PMG Machine Design for Gearless Offshore Wind Turbine Drives with multiple terminal interfaces
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- Institutt for elkraftteknikk 
The primary objective of this thesis is to investigate the design of the low speed direct driven multiphase PM generator for wind applications in which high voltage system is established by the use of a multilevel converter topology. An approach taken in this thesis for design of multiphase generator is that the generator is a multiphase open winding generator (i.e. both terminals (ends) of each winding are made accessible resulting in 12 terminals for a six phase machine). The generator studied in this thesis is a multi-pole, 6 phase open winding PM generator with concentrated winding system. Two types of such 6 phase generators are investigated in this thesis paper. One is 6 phase open winding generator with 60 degree phase shift between each phase and the other is a dual 3 phase open winding generator with 30 degree phase shift between each set. This generator configuration allows the use of multilevel converter topology with cascaded dc links which is not possible in generators with multi-star configurations. For the design and analysis of 6 phase open winding generator, a 120 slot, 116 poles PM generator with single layer concentrated winding has been chosen. The star of slot method has been used to obtain the winding configuration required for the generatorThe numerical and analytical methods are used to calculate different parameters of the test generator 2 like air gap flux, average flux density in the air gap, flux linkage in a phase, induced EMF, flux density distribution, inductances of the machine and also the cogging torque. Moreover, the effect of slotting of the generator on the flux density distribution due to magnet has also been investigated using analytical approach.The dynamic model equations for both 6 phase open winding RFPM generator (i.e. 60 degree phase shift generator and 30 degree phase shift generator) have been developed using the dq theory. Based on those equations, the simulation models have been developed which are used to study the behavior of generator on connecting 2 level converter topology with cascaded dc link and boost chopper.A lab experiment has also been carried out on the test generator to study the behavior of 6 phase open winding generator when it is connected to a 2 level converter topology. Out of many results observed in the lab, one prime result was the voltage stress in the winding insulation and it was found to be close to but lower than the induced voltage per phase. Therefore, it can be concluded that the voltage stress in the winding insulation is not a problem for this generator configuration. The value of cogging torque obtained from the lab test and that obtained from the analytical approach are satisfactorily close. Moreover, it is very low compared to the nominal torque of the PM generator which is one the main advantages of single layer concentrated winding PM machines. However, the other results obtained from the lab and those obtained from the simulations i.e. current drawn and the electromagnetic torque, do not match. So, more work is required on the improvement of generator model so that it can emulate the lab results to a satisfactory level.However, from the simulation results, it was also observed that for the same rectifier-converter-load topology, from the generator s perspective, whether the 6 phase open winding generator is configured with 60 degree phase shift or is configured as dual 3 phase open winding generator with 30degree phase shift, the two models of generator behaves similarly. There is not any typical advantage of dual 3 phase open winding generator with 30 degree phase shift over 6 phase open winding generator with 60 degree phase shift or vice versa. The above conclusion has been drawn based on the generator s behavior only (i.e. output generator voltage, current drawn from the generator and mean electromagnetic torque) but if we see from the converters perspective, the conclusion might be different and since it is not in the scope of this thesis, it has not been considered here.