Impact of iron loss on speed estimation in sensorless vector controlled induction machines

Methods of sensorless vector control of induction machines rely on rotor speed estimation using either stator current spectral estimation techniques, or mathematical model of an induction machine in conjunction with one of the modern control methods. The first approach enables speed estimation which is independent of parameter variations in the machine. The accuracy of all the schemes that belong to the second group is however strongly affected by parameter variations. The impact of iron loss on operation of vector controlled induction machines has been examined in detail for the drives that do possess a speed sensor. The aim of this paper is to examine the impact of iron loss on accuracy of speed estimation in sensorless rotor flux oriented induction machines. The scheme elaborated in the paper is the well-known MRAC based method that relies on the mathematical model of an induction machine. Error in speed estimation is evaluated on the basis of experimentally identified iron loss for a particular machine in the speed range from zero up to twice the rated speed, in steady-state operation. It is shown that the omission of iron loss representation in the speed estimator leads to a typical speed estimation error of two to three RPM in the base speed region (for a four pole 50 Hz machine) and that the speed estimation error increases in the field-weakening region, reaching eleven RPM at twice the rated speed with rated torque command. The results of the paper show that the speed estimation error due to iron loss is comparable with speed estimation errors due to other parameter variation effects.