Position sensorless control of synchronous reluctance machines (SynRMs) is not easy due to nonlinear correlations caused by extensive magnetic saturation. Although the d-axis inductance that varies largely depending on the magnetic saturation is not required, the active flux (AF) method cannot utilize the q-axis current to estimate the rotor position. The fundamental saliency (FS) method can utilize both the d- and q-axis currents, but it suffers from a model mismatch that variable components of the inductances are not exactly sine or cosine functions of double the rotor position. This paper proposes the rotational frame fundamental saliency (RFFS) method which basically performs the FS method in the rotational frame. The RFFS method can overcome challenges regarding the model mismatch and utilize both currents for position estimation. Experimental verification revealed that the RFFS method improved torque accuracy at very low speed and increased the maximum torque for stable operation by exploiting both currents compared to the AF method.
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