Asymmetric Voltage Vector-Based Initial Rotor Position Detection in Four-Switch Inverter Fed BLDC Motors

In position-sensorless brushless direct current (DC) motors (BLDCMs) fed by a four-switch three-phase (FSTP) inverter, only two phases are fully controlled, while the remaining phase is tied to the midpoint of the split DC-link capacitors. The voltage pulses required by inductance-based initial position detection can cause unequal discharge of the series capacitors, shifting the neutral-point voltage away from half of DC-link voltage$(\boldsymbol{U}_{\mathbf{dc}}/\mathbf{2})$. This neutral-point drift breaks the spatial symmetry of the inverter voltage vectors, so the 360° electrical period can no longer be evenly partitioned into six sectors during initial rotor position detection. To address this issue, this paper proposes a detection-pulse injection sequence that explicitly accounts for the asymmetric voltage vectors of the FSTP inverter. With the proposed sequence, the initial rotor position can be identified within a 30° electrical sector. The method requires no additional voltage or current sensors, and experimental results confirm its feasibility.