Fast Stator Flux Trajectory Control for PMSM Operating Under Low Switching Frequency

In rail transit traction field, the traction motor needs to operate at a very low switching frequency to decrease the switching loss. Hybrid pulsewidth modulation scheme is usually applied for decreasing switching frequency and providing good harmonic performance. However, when the transition between different modulation schemes occurs or the voltage command steps, the stator flux would deviate from the expected trajectory, which would lead to current/torque shock even overcurrent. This article proposes a method which can provide more flexible choice of voltage vector and can fast eliminate the stator flux deviation. Compared with the previous method, the proposed one can provide better dynamic performance (less oscillation in current/torque and less settling time), especially when the pulse number is relatively low, like square-wave mode, and is easy to implement. The effectiveness of the proposed method is verified by the simulation and experimental results.