Sensorless Control Exploiting PWM Ripple in PMSM with Low Anisotropy

Rotor position estimation in sensorless control of PMSM at low speed or standstill is often obtained using high-frequency voltage injection. This method leads to some drawbacks, the main of which is acoustic noise. The frequency of the injected signal should therefore be increased to minimize this problem. A square-wave signal injection at the switching frequency is the subject of many studies, since this is the maximum theoretically achievable frequency. These methods are based on the exploitation of the rotor magnetic saliency and therefore they are widely used with interior PMSMs. On the contrary, the low anisotropy that characterizes surface PMSMs makes it difficult to obtain a good signal-to-noise ratio. A method that uses square-wave voltage injection at the switching frequency is proposed in this study for surface PMSMs. In particular, a new current oversampling method is developed, in order to increase the signal-to-noise ratio. Experimental results on a 2 Nm surface PMSM are showed, to demonstrate the effectiveness of this approach.