A Method for Measuring Torque of Squirrel-Cage Induction Motors Without Any Mechanical Sensor

This paper presents a method for estimating the torque of three-phase squirrel-cage induction motors. The method is not invasive, since it does not rely on mechanical sensors and only the measurement of the stator current is required. The slip and the rotational speed of the motor are estimated from the spectrogram of the stator current signal either in stationary or in nonstationary operating conditions. The torque is estimated from the ratio of the estimated slip and the rated one. A test platform equipped with a rotational speed sensor and a dc motor for measuring the torque of a three-phase induction motor fed by a pulsewidth-modulated voltage source inverter operating under scalar and vector control was implemented to verify the correctness of the proposed method. The validity of the proposed approach is demonstrated by the close agreement between the estimated values of the speed and torque obtained with the proposed method and the ones that are obtained from direct measurement.