A precise induction motor speed estimator, based on a fixed carrier frequency signal

A speed estimation method, based on the introduction of a constant frequency carrier signal, is presented. This frequency carrier signal produces a constant speed rotating field from which the speed information in a wide range is obtained. The speed is estimated through the detection of irregularities in the rotor. These irregularities generate frequency pulsations proportional to the rotor speed, which are independent of the parameters of the machine. Because these signals are extremely low, special electronic circuits, implemented with analog multipliers and filters, have been designed, and they are able to measure very low speed values. Computer simulations and some experiments have been carried out, and they have shown that it is possible to measure speeds as low as 20 RPM