Developing an Approach in Calculating Reference Currents for Field-Weakening Control

Field-weakening control is an efficient way to achieve higher speed or larger torque within constraints of the supplied voltage and current. However, computing reference d-q-axes currents for controllers is a complex project due to nonlinearity among the limited dc-link voltages, stator currents, and motor torque. This study proposes an approach to calculate the reference currents based on the model of the interior permanent magnet synchronous machine (IPMSM). It dedicates to simplify computation, where maximal torque per ampere (MTPA) and maximal torque per voltage (MTPV) points are utilized. On a torque contour, the MTPA point has the maximal efficiency, and the MTPV point has the maximal voltage margin from the voltage constraint. Every MTPA or MTPV point is invariable if ignoring influence of temperature and corresponds to a maximal allowable speed within voltage and current limitations. The proposed approach linearly combines the MTPA and MTPV points on the same torque contour with respect to the speed, to reduce computational cost. The calculated working point can satisfy torque requirement accordingly. The validity of the proposed approach in field-weakening control is proven by simulations and experiments.