3 Rotor Position and Speed Estimation

The accurate value of the rotor position is crucial for independent control of the PMSM torque and flux. The rotor position can be obtained either by using dedicated rotor position sensors (sensored techniques), or by using position estimators (sensorless techniques).

The sensorless technique based on the back-EMF signals is a popular class of sensorless techniques. The observer takes the phase currents and voltages as input, then determines the rotor back-EMF of the PMSM. The back-EMF in turn is used to calculate the rotor position. One major drawback of these techniques is that it requires a minimum value of back-EMF to estimate the electrical position of the rotor. Therefore, these techniques require an open loop start-up procedure to start the motor before executing FOC.

The following section describes a simple open loop start-up procedure, followed by a back-EMF based angle and speed estimation technique with a Sliding Mode Observer.