A wide range sensorless speed estimator based on rotor irregularities for induction motors

This work presents a method of speed estimation for induction motors, based on the detection of side-band frequencies which are produced by dynamic eccentricities of the rotor. The method uses two different ways to measure speeds: one for speeds lower than 600 rpm and the other for higher speeds. In the first case, a high frequency carrier signal is introduced, and in the second case, the normal frequency current of the inverter is used. In both the cases the stator current signal is filtered and then self-multiplied. The rotor irregularities will produce in this current signal, amplitude modulations proportional to rotor speed and independent of the machine parameters. This information is carefully isolated through the help of programmable filters and high precision comparators to get the rotor speed. 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