An improved two-motor three-phase drive using FCS-MPC based flux and torque control with voltage constraint consideration

This paper presents a flux and torque control scheme, based on finite-control-set model predictive control (FCS-MPC), for two three-phase induction motors supplied by a five-leg two-level inverter. The reduced-switch-count topology with leg sharing inherently imposes an additional constraint on the voltages in the system. In the best available PWM-based control scheme for this topology, the constraint means that, in simple terms, the sum of speeds of two machines cannot exceed the rated speed of one machine, in order to avoid over-modulation and large torque oscillations. In essence, no provision exists to account for the additional voltage limit of the topology. It will be shown here that the FCS-MPC can consider the voltage constraint dynamically in the control loop, and hence, apart from preserving the independent control of the two machines, it can significantly widen the speed operating range. Three different cost functions, corresponding to three operating modes, are considered. The unique way in which the MPC handles tracking errors allows the motors to operate dynamically in the base speed region with field weakening, without requiring any external change of the flux references. Simulation and preliminary experimental results verify the theory.