Ing. Filip Baum

This paper introduces a novel overmodulation strategy tailored for dual-inverter topologies with galvanically isolated DC-links and evenly distributed DC-link voltage. The core contribution of the paper is the utilization of the multilevel capabilities of the dual-inverter topology. At the maximum modulation index, the presented overmodulation method achieves what is referred to as a 12-step operation. The 12-step operation improves the voltage waveform and offers superior harmonic performance compared to the conventional 6-step operation. The utilized approach leverages a hybrid Space Vector Pulse Width Modulation (SVPWM) method, a well-established technique in two-level inverters, to achieve this extended operation range. Furthermore, the paper presents a nonlinear gain compensation characteristic vital for real-time motor control applications and provides a comparative analysis of the modulation's harmonic performance. The proposed approach is validated through simulations and experiments. The experiments were conducted using a custom inverter based on Gallium-Nitride (GaN) transistors where simple volt/hertz control of an induction motor was programmed. The results of these experiments affirm the strategy's effectiveness, showcasing superior current waveform quality and a smooth transition from linear mode to 12-step operation while keeping a simple implementation of the algorithm.

The increasing popularity of electric drives employing an isolated dual-inverter (DI) topology is motivated by their superior DC-link voltage and power utilization, fault-tolerant operation, and potential for multilevel operation. These attributes are significant in battery-powered transportation, such as electric vehicles and aviation. Given the considerable freedom in modulation and control of the DI topology, this paper researches the impact of reference voltage vector distribution between the two individual inverters. The study also evaluates the influence of two well-established asynchronous modulation strategies—Space Vector PWM (SVPWM) and Depenbrock’s Discontinuous Modulation (DPWM1). Since simulation tools nowadays play a crucial role in power electronics design and concept verification, the results are based on extensive and detailed models in Matlab/Simulink. Employing the basic field-oriented control of a 12 kW induction motor with precisely parameterized SiC switching devices for accurate loss calculation, this research reveals the possibility of significant energy savings at multiple operating points. Notably, optimal efficiency is achieved when one inverter operates up to half of the nominal speed while the other solely establishes a neutral point for the winding. Moreover, the results highlight DPWM1 as a superior strategy for the DI topology, showcasing reduced converter losses. Overall, it is shown that the system’s losses can be significantly reduced just by the design of the voltage vector distribution in the drive’s operating range and the modulation strategy selection.

Mathematical models of induction motor (IM) used in direct field‑oriented control (DFOC) strategies are characterized by parametrization resulting from the IM equivalent circuit and model‑type selection. The parameter inaccuracy causes DFOC detuning, which deteriorates the drive performance. Therefore, many methods for parameter adaptation were developed in the literature. One class of algorithms, popular due to their simplicity, includes estimators based on the model reference adaptive system (MRAS). Their main disadvantage is the dependence on other machines’ parameters. However, although typically not considered in the respective literature, there are other aspects that impair the performance of the MRAS estimators. These include, but are not limited to, the nonlinear phenomenon of iron losses, the effect of necessary discretization of the algorithms and selection of the sampling time, and the influence of the supply inverter nonlinear behavior. Therefore, this paper aims to study the effect of the above-mentioned negative aspects on the performance of selected MRAS estimators: active and reactive power MRAS for the stator and rotor resistance estimation. Furthermore, improved reduced‑order models and MRAS estimators that consider the iron loss phenomenon are also presented to examine the iron loss influence. Another merit of this paper is that it shows clearly and in one place how DFOC, with the included effect of iron losses and inverter nonlinearities, can be modeled using simulation tools. The modeling of the IM and DFOC takes place in MATLAB/Simulink environment.