Improved Voltage Boundary With Model-Based Control Algorithm for Increased Torque in the Field Weakening Region of Induction Machines

Abstract

At high speed, electric vehicles (EVs) have limited torque when powered by an induction machine (IM). In IM, the high-speed region is called field weakening, and the region is known for torque limitation. This article proposes a control method that increases the torque without applying discontinuous-modulation techniques, which is commonly used. A bespoke model-based voltage control method has been developed, which enables reaching the hexagonal voltage reference trajectory for the field weakening region. So far, all model-based control methods are constrained by the inscribed voltage circle, which lies within the hexagonal voltage boundary. This restriction limits the available inverter output voltage across the motor windings, which, in turn, restricts the output torque of the drive. To achieve the hexagonal voltage trajectory, this article introduces a new calculation of the d-axis current for the entire speed range in the field weakening region. This calculation is based on the hexagonal voltage boundary equations and the stator voltage vector position. This generates a new reference d-axis current that minimizes the difference between the hexagonal voltage boundary and its inscribed voltage circle. As a result, the proposed d-axis current maximizes the output torque and output power in the field weakening region. The proposed method is presented analytically. Simulation and experimentally validated results are presented to confirm its feasibility and effectiveness.