Sensorless speed detection of inverter fed induction motors using rotor slot harmonics and fast Fourier transform

A novel approach to sensorless speed detection for adjustable-speed AC drives is described. No a priori knowledge is required about the motor construction, electrical parameters, or load condition. In addition, no external tuning is needed for the system. The technique is based on instantaneous spectral estimation using the fast Fourier transform, whereby the speed-dependent slot ripple harmonic frequency is determined. For the assessment of this technique, an all-digital speed detector has been built around a general-purpose 386 microcomputer. The performance of this detector over a wide range of inverter frequencies and load conditions is discussed