Sensorless control of induction motor drives in the zero frequency speed range

An estimation method for sensorless control employing a test signal based reference model is presented. The method estimates the slip frequency by a comparison of the measured impedance curve while the machine is in operation with the reference model curves. The reference model is measured during DC magnetisation of a cold machine at steady state and constant rotor flux linkage. During normal machine operation the machine temperature and the stator frequency differ from these conditions and thus have a negative influence on quality of the estimation. The impact of these disturbances is investigated and two compensation strategies are presented. Measurements on two different standard induction machines which are not especially designed for sensorless control show that the estimation method allow safe operation at standstill and even at zero frequency.