Phasor Diagrams of Symmetrical Components for Vector Space Decomposition Transformation in Symmetrical Nine-Phase Machine

In this paper, the phasor diagrams of the symmetrical components of a symmetrical nine-phase motor with a single neutral point have been studied. The symmetrical nine-phase motor consists of three identical three-phase winding sets connected in star to a single isolated neutral point. The symmetrical component transformation is widely used in the field of electric drives because it simplifies the modelling of a symmetrical multiphase machine. Additionally, it is the basis for the vector space decomposition (VSD) method. The VSD method is founded on the transformation of the input signals into different decoupled subspaces by summing two symmetrical components. Using the phasor diagrams of the symmetrical components of a nine-phase machine, we can then analytically predict how different voltage excitations of the system are transformed into the decoupled subspaces. Of great interest is the prediction of how the various voltage excitations affect the first two-dimensional subspace, which is responsible for torque generation in the machine. In this paper we will analyse input voltages with higher time harmonics and the application of different voltage amplitudes to the individual three-phase winding sets. With the latter example, we can also simulate the open-winding fault when one or more three-phase winding sets are not fed. Based on the acquired information, the control method of the analysed motor drive can be corrected accordingly.