Achieving transducerless (sensorless) control of a.c. machines that is robust during sustained operation at or near zero speed (i.e. zero excitation frequency) is exceedingly difficult. This paper present an elegantly simple and viable, generalized approach to achieving robust, accurate transducerless state estimation in polyphase a.c. machines, including the estimation of flux, position and velocity in induction and synchronous machines at zero and low speeds. It is based upon the tracking of magnetic saliencies via inverter-generated high-frequency signal injection with demodulation incorporating heterodyning and a closed-loop observer. The saliencies may be saturation-induced, thus yielding a means of direct flux position estimation, or rotor-constructed and thus yielding rotor position and velocity estimation. Experimental results are provided demonstrating rotor position estimation with a permanent magnet synchronous motor.
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