A Sequential Direct Torque Control Scheme for Seven-Phase Induction Machines Based on Virtual Voltage Vectors

The traditional direct torque control (DTC) strategy utilizes the largest 14 voltage vectors for a seven-phase induction machine (7PIM), which is directly extended from the three-phase counterparts. This method suffers from the issues of severe harmonic currents and torque ripple for multiphase ones. This article first puts forward the concept of virtual vectors for a 7PIM. This can efficiently eliminate the third- and fifth-order harmonic currents. However, the DTC strategy based on the virtual voltage vector has insignificant effect on reducing the torque ripple. Then, the torque ripple caused by these virtual vectors in different torque and speed conditions is investigated. Analysis shows that there are some additional degrees of freedom while choosing the applied voltage vector. A novel sequential strategy integrated with duty ratio optimization algorithm is proposed to select the optimal voltage vector that can significantly alleviate the torque ripple. Experimental results verified that the proposed method can effectively reduce the dynamic and steady-state torque ripple as well as the harmonic current.This article utilizes a 7PIM for an illustration, but it should be mentioned here that the proposed strategy is applicable for multiphase machines with any number of phases.