Continuous Space Vector Modulation for Symmetrical Six-Phase Drives

The two common three-leg, two-level voltage source inverters of a six-phase drive are able to produce 64 different switching states, representable as voltage space vectors in two orthogonal subspaces. The discrete output voltage of the inverter legs is usually controlled by means of a space vector modulation (SVM) technique, the aim of which is to produce the same volt-seconds as the continuous reference voltage vectors in both subspaces within a certain time frame (switching cycle). Using a simplified phase model of the machine consisting in each subspace of a voltage source and a leakage inductance, the phase currents are found to exhibit unwanted distortion harmonics. A proper sequence of voltage vectors has to be applied in order to reduce these harmonics. By allowing each inverter leg to change its output twice during the switching cycle (continuous modulation), 518400 different space vector sequences can be generated. This paper demonstrates that, among these sequences, only 11 need to be considered to modulate the reference vectors. Further simulation shows that, for a certain pair of reference vectors and leakage inductance ratio, only one of them reduces the distortion to a minimum. Due to the high computational effort, however, this optimization problem is not solvable online. To overcome this drawback, four different modulation strategies, the standard three-phase SVM, the fragmented sector mapping, the optimal interleaved and the approximated interleaved SVM, are derived from and compared with an offline optimization. Each strategy is evaluated regarding the resulting current distortion and the influence of the reference vectors as well as the inductance ratio. The harmonic current of these online modulation strategies is calculated in simulation and validated by experiments. Applying the proposed modulation techniques leads to a great reduction in the machine losses caused by switching.