Stability of open-loop operation without rotor information for Permanent Magnet Synchronous Motors

Open-loop operation mode is often applied in sensorless Permanent-Magnet Synchronous Motors (PMSM). The stator windings are supplied in a proper proportion to the desired working frequency in this mode without the knowledge of rotor position. However, the rotor synchronization will be destroyed once the ratio is inappropriate. In order to analyze the reliabilities of the open-loop operation mode, a dynamic equation is established to represent the synchronism error between the rotor and the stator. The stability of the open-loop control mode is analyzed based on the indirect Lyapunov theory. Theoretical analysis indicates that the open-loop control mode is stable only when the stator current phase lags appropriately behind the back electromotive force (EMF). The result of an experiment on a certain PMSM verifies the effectiveness of the theory.