At high speed range in voltage source inverter (VSI) fed permanent magnet synchronous motor gap flux is weakened to limit terminal voltage for saving the capacity of VSI. In weakening flux operation of salient pole PMSM, d-axis current and qaxis current can't be decided deductively due to reluctance torque. To cope with this problem wepropose feedback control of motor torque, which is calculated from current, voltage and motor speed. By this control method torque and terminal voltage can be determined without measuring them directly. In this paper, the proposed control system is discribed, the transient characteristic is studied and the test results. Lastly we show make it clear that our method is basically effective for railway vehicle traction.
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