The power rating of wind turbines is constantly increasing; however, keeping the voltage rating atthe low-voltage level results in high kilo-ampere currents. An alternative for increasing the powerlevels without raising the voltage level is provided by multiphase machines. Multiphase machinesare used for instance in ship propulsion systems, aerospace applications, electric vehicles, and inother high-power applications including wind energy conversion systems.A machine model in an appropriate reference frame is required in order to design an efficientcontrol for the electric drive. Modeling of multiphase machines poses a challenge because of themutual couplings between the phases. Mutual couplings degrade the drive performance unlessthey are properly considered. In certain multiphase machines there is also a problem of high currentharmonics, which are easily generated because of the small current path impedance of theharmonic components. However, multiphase machines provide special characteristics comparedwith the three-phase counterparts: Multiphase machines have a better fault tolerance, and are thusmore robust. In addition, the controlled power can be divided among more inverter legs by increasingthe number of phases. Moreover, the torque pulsation can be decreased and the harmonicfrequency of the torque ripple increased by an appropriate multiphase configuration. By increasingthe number of phases it is also possible to obtain more torque per RMS ampere for the samevolume, and thus, increase the power density.In this doctoral thesis, a decoupled d–q model of double-star permanent-magnet (PM) synchronousmachines is derived based on the inductance matrix diagonalization. The double-star machine isa special type of multiphase machines. Its armature consists of two three-phase winding sets, which are commonly displaced by 30 electrical degrees. In this study, the displacement anglebetween the sets is considered a parameter. The diagonalization of the inductance matrix resultsin a simplified model structure, in which the mutual couplings between the reference frames areeliminated. Moreover, the current harmonics are mapped into a reference frame, in which they canbe easily controlled. The work also presents methods to determine the machine inductances by afinite-element analysis and by voltage-source inverters on-site.The derived model is validated by experimental results obtained with an example double-star interiorPM (IPM) synchronous machine having the sets displaced by 30 electrical degrees. Thederived transformation, and consequently, the decoupled d–q machine model, are shown to modelthe behavior of an actual machine with an acceptable accuracy. Thus, the proposed model is suitableto be used for the model-based control design of electric drives consisting of double-star IPMsynchronous machines.
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