Torque Estimation of Variable-Speed Induction Motors Without Torque and Rotational Speed Meters

Abstract

This paper presents a torque estimation method for variable speed induction motors for factory tests that does not use a torque or rotational speed meter. A new algorithm, which takes into account the torque reduction component due to stray load loss that varies with the main flux level and rotational speed, is proposed to accurately estimate the torque, which is based on the cross product of the stator flux linkage and current vectors. Pre-determining two parameters, the parallel equivalent iron loss resistance and percentage stray load loss, is the key to accurate torque estimation using this algorithm. The authors also show a method to identify the two parameters only from a rated voltage no-load test and nameplate values. This allows determining the percentage stray load loss for individual test machines without performing actual load testing with torque and rotational speed measurements. The proposed method is carried out on V/f-controlled test machines fed by three types of voltage waveforms: sine, square, and pulse width modulation drives. The performance of the proposed torque estimation is verified by comparing the estimated and measured torques.