Induction motor speed estimator and synchronous motor positionestimator based on a fixed carrier frequency signal

A speed and position estimator, based on the introduction of a constant-frequency carrier signal (400-1000 Hz) in AC motor stator currents has been developed. Speed and position are estimated through the detection of irregularities in the rotor and, hence, the results are independent of machine parameters. These irregularities generate amplitude oscillations in the currents generated by the high frequency carrier, which are proportional to the rotor position. At the same time, the frequency of these amplitude modulations is proportional to machine speed. In induction machines, the amplitude of these signals is too low to estimate position accurately. However, special electronic circuits, designed and implemented with analog multipliers and filters, have permitted measurement of very low speeds with high precision. Computer simulations and some experiments have been carried out, and they have shown that it is possible to measure speeds as low as 20 r.p.m. In the case of salient-pole synchronous machines, the method is useful to measure rotor position due to the high irregularities produced by the poles, which are easily computed because of the important amplitude variations detected in the high frequency carrier