Torque Estimation in High-Efficency IPM Synchronous Motor Drives

In modern electrical drives, the energy saving starts with the careful generation of the electromagnetic torque, which involves the electrical motor and its current control. A modern choice is represented by an anisotropic synchronous interior permanent magnet (IPM) motor combined with the maximum torque per ampere (MTPA) control technique. The MTPA locus brings along additional information that is worth deepening. In particular, this paper shows that a proper rearrangement of the torque expression in MTPA condition, merged with a simple least-square algorithm, returns an accurate online estimation of the electromagnetic torque. The effectiveness of the proposed method is proved by a worked out example, based on an IPM drive model fitted with finite-element analysis and experimental measurements, including motor saturation effects. The torque estimator extends its usability in the whole working region of the drive, even during transients outside the MTPA curve, and it comes almost for free. In this, the proposed method should be technically sound in many industrial applications.