Maximum efficiency control method in 7-phase BLDC motor by changing the number of the excited phase windings

This paper proposes a maximum efficiency control method in 7-phase brushless DC motors (7-phase BLDCMs) used for the ship propulsion system. In the same load condition, the system loss of the 7-phase BLDCM is dependent on the number of the excited windings and the instantaneous phase currents. Thus, it is necessary to determine the excitation number of the phase windings according to the load condition. The 2- and 4-phase excitation methods are proposed for the maximum efficiency operation. The phase excitation method of the minimum system loss according to the load condition can be obtained from the loss analysis. In addition, by using the loss analysis, the optimal speed which is the changing point of the phase excitation to select the optimal excitation methods (2-, 4- and 6-phase excitation) can be calculated and derived. The proposed method can be easily implemented by controlling the conductive angle of the phase current at the suitable phase windings. The maximum efficiency operation can be achieved by using the proposed excitation method and 6-phase excitation method. The usefulness of the proposed control algorithm is verified through simulation results.