A direct model predictive torque control approach to meet torque and loss objectives simultaneously in permanent magnet synchronous motor applications

Abstract

When using model predictive torque control (MPTC) for controlling permanent magnet synchronous motors (PMSM), often more than one objective is considered. A common approach is to combine different objectives with weighting factors into one cost function. Due to conflicts between the objectives, a deviation from the commanded reference for each objective results. For a PMSM, the tracking of a torque reference with a preferable small torque ripple as well as the tracking of a stator current reference to reduce the losses are typical objectives in MPTC. However, commonly used voltage-source inverters cause an unavoidable torque ripple due to the switching principle. Thus, the presented MPTC approach forces the torque to remain within certain tolerance bands and achieves a loss-minimum operation simultaneously. Only the torque reference and allowed upper and lower bounds are commanded to the controller. The MPTC provides the optimal switchover time points directly to each phase of a 2-level inverter at a defined upper switching frequency. The compliance with both the specified torque bounds and the loss-minimum operation is proven in simulations for a linear and a non-linear PMSM-model. The performance is compared to field-oriented control (FOC) and direct torque control strategies respectively, using a voltage-source inverter. For an evaluation of the performance in presence of non-linearities, saturation and angle dependencies of the PMSM are modeled. In the linear case, the achieved torque ripple with the MPTC is comparable with the ripple which arises from the the usage of the FOC. In the non-linear case, the MPTC shows a superior torque behavior when using the non-linear model during prediction.