SR motor is operated by switching the phase currents in synchronism with the region of rising inductance as determined by the rotor position. A shaft position transducer such as an opto-interrupter with a slotted disk or a high precision encoder is usually employed for this purpose. However, the incorporation of these sensors in a SR motor drive increases the cost and complexity, and tends to reduce the reliability of the drive system. In order to overcome these issues, sensorless control technique is used to determine the rotor position indirectly.
Several indirect position sensing methods have been published for sensorless rotor position control of SR motor drives. Most of the methods make use of the instantaneous phase inductance variation to detect the rotor position. This is possible since the current-position-inductance characteristics vary significantly between the aligned and unaligned position of the stator and rotor poles. Overall, the methods can be classified into two broad categories of non-intrusive methods and active probing methods as illustrated in Figure 13.