Magnetomotive Force Balance-Based Open-Circuit Fault Diagnosis for T-Type Three-Level Inverter-Fed Dual Three-phase PMSM Drives

This paper proposes a fast diagnosis method for single-open-switch and single-open-phase faults in T-type three-level inverter-fed dual-three-phase PMSM drives, based on magnetomotive force (MMF) balance. By extracting MMF balance degree features between the abc and def three-phase subsystems, this method enables hierarchical fault diagnosis across three stages: identifying the fault-occurred three-phase subsystems, localizing the exact faulty bridge arm/phase within them, and pinpointing the faulty switch. Firstly, the diagnosis is activated upon detecting abnormal MMF imbalance between subsystems, and the faulty subsystem is locked. Then, phase-sequence similarity analysis between the faulty subsystem's actual MMF and theoretical fault reference MMF narrows the fault scope to a specific bridge arm and preliminarily identifies potential faulty switch pairs. Finally, due to the difficulty in distinguishing open-switch faults between upper/lower arms and the neutral-point arm under low modulation indices, a hybrid modulation mode is introduced. This mode maintains three-level modulation for healthy bridge arms while applying two-level modulation to the faulty bridge arm, thereby enhancing fault characteristics for precise fault identification. Open-phase faults are diagnosed through synchronous alarms during consecutive positive and negative current half-cycles. The method is easy to implement, requires only a single feature and two thresholds without additional hardware or model parameters. Experiment demonstrate fault detection within 0.4 ms and accurate localization within 1/15 fundamental period under both dynamic and steady-state operating conditions.