Very low speed sensorless control of induction motor drives using high-frequency signal injection

This paper addresses the control issues of speed sensorless induction motor drives at very low speeds, I r/min to 30 r/min. Low voltage high-frequency signals superimposed posed on the two-axis voltage commands in the synchronously rotating reference frame are injected into the motor stator. Due to inherent parasitic saliencies of the cage rotor, magnetic saturation in particular, the resultant stator current harmonics contain frequency components that depend on the synchronous frequency. Band-pass and low-pass filtering techniques combined with coordinate transformations are used to extract the synchronous angular position embedded in the stator current harmonics. The rotor speed can be computed by subtracting the estimated slip frequency from the extracted synchronous frequency. The experimental results further show that effective speed sensorless control can be achieved at very low speed operating range.