Comparison of PM motor structures and sensorless control techniques for zero-speed rotor position detection

The rotor position of a synchronous Permanent Magnet (PM) motor can be detected by means of the superimposition of a high-frequency component to the steady-state stator voltage. Thanks to the anisotropy of the rotor, the corresponding high-frequency current is modulated and used to determine the rotor position. Two techniques can be considered: the first one adopts a pulsating voltage vector in the estimated synchronous reference frame, while the second one adopts a rotating voltage vector. These techniques are effective at zero and at low motor speed. The accuracy of the rotor position detection depends strictly on the rotor saliency, that is, on the geometry of the PM rotor. In fact both saturation and cross-coupling have a heavy influence on the correct rotor position detection. The aim of this paper is to compare the two sensorless control techniques, in conjunction with some rotor geometries. In order to highlight the effectiveness of the sensorless technique, the tests are carried out at various operating conditions. It is found that the effectiveness of the sensorless rotor position detection strongly depends on the choice of the sensorless control technique together with a proper PM rotor geometry.