A novel dynamic overmodulation strategy for fast torque control of high-saliency-ratio AC motor

This paper describes the dynamic overmodulation strategies for fast torque control of the high-saliency-ratio ac motor. For the high-saliency-ratio ac motor, the dynamic control of both d- and q-axes currents is required for fast torque control with optimal efficiency. However, in transient operation, the d- and q-axes current regulators are easily saturated even at low speed because the stator inductance is designed large enough to generate the desired reluctance torque. With the conventional overmodulation strategies, the current regulation performance degrades severely compared to a nonsalient motor. In this paper, the drawbacks of the conventional overmodulation strategies are investigated and a dynamic overmodulation strategy is proposed to improve the current response to fast transients. For the effective implementation of the strategy, a practical algorithm is also presented. The performance of the proposed strategy is confirmed by computer simulation and experiment with a 3.75-kW axially laminated synchronous reluctance motor and an interior permanent-magnet synchronous motor for 42-V integrated starter-generator applications.