ОBSERVERS OF ROTATION FREQUENCY OF AN ASYNCHRONOUS MOTOR ON THE PRINCIPLE OF DETECTING SLOT CURRENT HARMONICS

Abstract

Definition of the asynchronous motor rotational speed by indirect measurements is possible by application of special observer algorithms. The input values of observers are, as a rule, the instant values of currents and stresses of the stator. It becomes possible here to exclude the speed encoder from the structure of the electric drive, thus reducing its cost and increasing its reliability. In order to implement the methods of indirect speed definition considered in this paper, only main structural parameters of a machine should be known - the number of stator and rotor slots and the number of pairs of poles. Observers of rotational speed are investigated, that imply the effect of appearance of slot current harmonics in their operation - higher harmonics caused by the difference of values of the magnetic induction in the air gap in front of slots and rotor teeth. This periodic modulation of magnetic flux develops higher harmonics of electromotive force and stator current with their frequency depending on the rotational frequency. The drawback of this method is that amplitudes of slot harmonics are decreased at decreasing the rotational frequency. The second method is based on detecting the slot harmonics of the stator current at simultaneous addition of a high-frequency signal of the supply voltage. At this expense, symmetrical secondary slot harmonics are formed to the left and right of the high frequency spectrum component. The amplitude frequency spectrum of the stator current is investigated at the presence of a high-frequency component of the supply voltage. Software is developed for observing the rotational frequency by fast Fourier transform of the stator current, the software applying two described methods. The paper presents such characteristics of operation of rotation frequency observers as the accuracy at static and dynamic modes, bandwidth and noise resistance.