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Rotor slot harmonics are found in the stator current waveforms for most squirrel-cage induction motors. These harmonics are caused by the finite number of rotor slots in a motor, and their frequencies are inherently correlated with the motor's rotational speed. A frequency demodulation approach is proposed in this paper to continuously and accurately track the rotational speed for induction motors that are operated at either dynamic or steady-state conditions from fixed-frequency power supplies. First, a complex current vector is synthesized from polyphase electrical current measurements. Second, a local oscillator and a mixer are cascaded with a digital filter to heterodyne a specific rotor slot harmonic and suppress adjacent interferences. A finite impulse response differentiator is then employed as a frequency demodulator to approximate the time derivative of the phase of this specific rotor slot harmonic and to resolve its instantaneous frequency. Finally, the induction motor speed is calculated from this resolved instantaneous rotor slot harmonic frequency. Experimental results demonstrate that the proposed scheme is capable of interleaving data acquisition with real-time computation, iteratively estimating motor speed on a sample-by-sample basis.