Sensorless field orientation control of an induction machine by high frequency signal injection

This paper describes a new scheme to find the rotor flux angle from stator voltages and currents by injecting a high-frequency signal. The signal is not a rotating one but a fluctuating one at a reference frame rotating synchronously to the fundamental stator frequency. When the estimated rotor flux angle coincides with the actual angle, the proposed method makes virtually no ripple torque, no vibration and less audible noise caused by the injected signal. The difference of impedances between the flux axis and the quadrature axis at high-frequency signal injection on the rotor flux angle is explained by the equivalent circuit equation of the induction machine. The difference is verified by experiments on test motors under various testing conditions. A sensorless field orientation algorithm is proposed and experimental results clarify the satisfactory operation of the algorithm with one hundred and fifty percent load torque at zero stator frequency