Optimal frequency modulation of carrier waves and its application to induction motor drive systems

Induction drive systems are widely used due to their low cost, long-term operational capability, and high reliability. These AC motors typically rely on inverters to convert DC power to AC. The DC/AC power conversion systems must meet the dual requirements of providing a high-quality power supply and offering flexible control for induction motor (IM) systems. Currently, multilevel neutral point clamped (NPC) inverters are particularly advantageous. Multilevel NPC inverters are efficient as they use fewer passive components and DC sources while producing outputs with low harmonic distortion. However, IM systems exhibit strong nonlinearity. The lack of optimization in generating pulse width modulation (PWM) control pulses for NPC inverters results in poor output quality and fails to meet high-performance control requirements for IM systems. This issue arises because existing PWM modulation techniques for multilevel NPC inverters are predominantly based on fixed-frequency carriers. To address the technical limitations of fixed-frequency carriers and enhance the flexibility of PWM pulses, this paper proposes a frequency modulation technique for the carrier signal, referred to as FM modulation. By employing the proposed FM modulation technique, the output voltage quality of multilevel NPC inverters is improved, with reduced total harmonic distortion, lower switching frequency, and extended component lifespan. This ensures robust control for highly nonlinear systems such as IM systems. The effectiveness of the proposed FM modulation method is validated through comparisons with classical modulation techniques such as in-phase disposition, phase opposition disposition, and alternate phase opposition disposition, all of which use fixed-frequency carriers for PWM pulse generation. Simulations and real tests conducted on actual IM systems confirm the efficacy of the proposed FM technique.