Application of linear-phase filters in induction motor speed detection

In rotor-slot-harmonic-based induction motor speed detection schemes, a motor´s rotational speed is resolved from the phase of a rotor slot harmonic signal. In such schemes, frequency-selective digital filters are often applied to attenuate interferences in sampled motor currents before rotor slot harmonic signals are extracted. This paper demonstrates that the use of linear-phase filters, which is a family of digital filters with linear phase responses, helps preserve the rotor slot harmonic signal´s phase during the filtering operation. The paper also shows that for induction motors with periodically time-varying loads, the rotor slot harmonic signal´s frequency is modulated by periodic speed oscillations, and hence contains multiple sidebands in the frequency spectrum. Experimental results further show that linear-phase filters allows for a more accurate detection of the induction motor speed than filters with nonlinear phase responses.