An I-f Startup Method for Back-EMF based Sensorless FOC of PMSMs with Improved Stability during the Transition

The I-f control is often used as a startup method for back-EMF based sensorless field-oriented-control (FOC) of permanent magnet synchronous machines (PMSMs). In this hybrid control scheme, usually a large current magnitude is used for the I-f control to start the machine. After the machine reaches a certain speed when the position and speed estimations from the back-EMF based method are accurate enough, the control can be switched to FOC. To obtain a smooth transition, a popular method reported in the literature is reducing the current vector magnitude to move the current vector to the q-axis before the transition. However, the current reduction methods usually assume that load torque does not change during the process of reducing the current vector magnitude. In cases where the load torque changes, since the current vector magnitude cannot be increased automatically to reject the load disturbance, synchronism can be lost and the transition to FOC may fail. To achieve a smooth transition with improved stability during the transition process, this paper proposes an I-f startup method with compensation loops, which can move the current vector to the q-axis and fix it there even when load varies. Since the current vector is already on the q-axis in the I-f control mode, the switching to FOC is made very smooth. Experimental results verify the effectiveness of the proposed method.