Optimal voltage angle for maximum torque per voltage control of induction machine in deep field-weakening region

This paper investigates the optimal stator voltage angle in a rotor flux-oriented system of an induction machine drive, aiming to maximise machine torque while operating in a deep field weakening region. Here, torque maximisation leads to maximum torque per voltage control, as only voltage constraints are relevant in this region due to high back-electromotive force. Previous studies have predominantly focused on field-weakening operation and torque maximisation, assuming a constant synchronous speed. However, for a given rotor speed, the variation in the dq voltage components impacts the slip speed, thereby influencing the synchronous speed. Therefore, this paper proposes enhanced analytical expressions to address this limitation. It is shown that after a linearisation around a suitable operating point, a closed-form algebraic equation for calculating the speed and parameter-dependent optimal voltage angle for torque maximisation can be obtained. The theoretical analysis is supported by numerical and experimental results. The presented linearised expression is proven to be an effective tool for the analytical calculation of the optimal voltage angle, making it suitable for real-time control applications. It is shown that the proposed approach achieves higher drive torque and efficiency than the conventional voltage component distribution.