Bandwidth-Invariant MTPA Speed Control of Induction Motor Drives With Application to Airborne Wind Energy Systems

Efficiency-oriented control of induction motor drives reduces rotor flux at partial load, but the resulting reduction in speed-loop gain degrades closed-loop bandwidth under fixed-gain control. The maximum torque per ampere (MTPA) constraint permits an exact resolution. Enforcing MTPA as a structural control law reduces the conventional two-input field-oriented-control plant to a single-input single-output system. Its linearized transfer function is a second-order plant with a real zero at twice the rotor-flux pole frequency. A gain-scheduled proportional-integral controller, whose gain is inversely proportional to the stator-current magnitude, cancels the operating-point dependence and maintains constant crossover frequency and phase margin without a flux observer or look-up table. Four strategies are compared in simulation and experimentally on an 18.5 kW induction machine under airborne-wind-energy pumping cycles: standard field-oriented control, fixed-gain MTPA, the proposed SISO-MTPA, and loss minimization. SISO-MTPA is the only strategy that improves both net harvested energy and speed tracking relative to the FOC baseline while remaining undominated: its energy is within 10% of the MTPA-optimal strategies, its speed-error RMSE is approximately one third, and its maximum deviation is over 60% lower. Against an industrial drive under high-power generation, it reduces speed-tracking RMSE by 47% with comparable generated energy.