Induction machine speed sensor based on stator current measurement

The detailed analytic study of the induction machine, taking the slotting effects into account, allows the influence of the rotor speed on the harmonic content of the stator currents to be shown. This result has also been confirmed by tests realised on wound and squirrel cage rotor machines. The magnitudes of the speed dependant current spectrum lines are very low because they are functions of the small e.m.f. induced in the stator, which are tied to the stator and rotor slot geometry, but also of the impedance of the electric circuit constituted of the stator phase reactance and the grid one. To achieve the realisation of the speed sensor the spectrum line magnitudes have first to be amplified. Then a narrow band switched capacitor analog active filter, in which central frequency is controlled by a phase locked loop, is used. This device produces a logic signal with a frequency which is an exact multiple of the rotor speed. A logic counter is employed to display the result.