On the use of sensorless induction motor drives as speed sensors

The present paper is a review of two techniques recently proposed by the authors to perform indirect speed estimation on sensorless field oriented induction motor drives. The two methods are presented to demonstrate that induction motor drives can suitably perform as flux and speed sensors showing accuracy and reliability comparable to those of standard sensors. Both techniques use a suitable high frequency current test signal that causes the generation of a zero sequence stator voltage component. However, while the first approach calculates the rotor speed from the angular frequency of the airgap flux and the estimated values of the stator currents, the second technique exploits a more general property of the carrier frequency signal injection as the motor speed is expressed as a function of the ripple generated on the rotor flux angular frequency and stator current components. Speed estimation according to the proposed approaches is fully insensitive to motor parameter variations and features a large bandwidth making induction motor drives an attractive alternative to standard speed sensors.