A smooth and stable open-loop I-F control for a surface mount PMSM drive by ensuring controlled starting torque

The back-emf based sensorless vector control method for a surface mount permanent magnet synchronous motor (SMPMSM) provides sufficiently good performance in medium and high speed range with simple control structure. Since back-emf based sensorless control is ineffective in low speed range, I-f open loop method is used for starting and a change over to sensorless control is performed. However, loss of synchronism may occur if the frequency ramp rate of I-f control is chosen arbitrarily. In this paper, a controller is proposed to dynamically adjust the slope of frequency ramp in the ramp-up interval, which works in conjunction with the conventional I-f control. The controller controls the total torque demand below the torque capability of the motor by adjusting the frequency slope. To overcome the inherent mid-frequency instability of I-f control, a reference speed modulator is included in the control. Stable starting of a 25kW PMSM drive with I-f control using the proposed method is validated with experimental results.