Sensorless Airgap Flux Position Estimation by Injection of Orthogonal Stationary Signals

A new method to compute the airgap flux position of induction motors at very low and zero stator frequency is presented that merges a sensorless third harmonic approach with the injection of a suitable pair of stationary sinusoidal stator voltage components. These two components feature different frequencies (245 and 135 Hz) and provide the machine with a permanent excitation that causes a modulation of the saturation level of the magnetic core according to the angular position of the airgap flux. As a result, the amplitude of the third harmonic of the stator voltage shows a ripple that contains information about the position of the airgap flux. Such a ripple includes two components having respectively the frequencies of the two injected signals and is demodulated using suitable filters and a closed loop state observer approach derived from the Resolver to Digital (R/D) conversion. The proposed technique Is suitable to provide an estimation of the position of the airgap flux at zero speed using a relatively simple hardware, such that equipping standard constant V/f industrial drives or automotive electromechanical actuators.