DSP Based Torque Estimation in Three-phase Cage Induction Motors

The knowledge of the instantaneous electromagnetic torque produced by an induction motor is highly desirable in innumerous applications and is required in several closed loop control strategies. Direct measurement has shown not to be convenient/feasible due to the need of external sensors and special installation arrangements and costs involved. The way is to estimate the torque, which in turn depends on the flux estimation. The present work is based on the study and implementation of a previously proposed technique based on the voltage-model-based stator flux estimation. It reports on the flux and torque estimation using a compensated low pass filter to achieve the stator flux via integration of the stator counter emf voltage signal. The pure integration of the counter emf is not recommended because it is sensitive to offsets and initial phase conditions that cause drift and undesirable offsets in the output signal. The compensation scheme used overcomes these problems. The scheme was implemented using a DSP board. Very good experimental results were achieved. The paper explains in detail the strategy used for flux/torque estimation, and shows a comparison of experimental and simulated results.