Saliency-based zero-sequence injection for instantaneous current ripple reduction in VSI-fed SynRM drives

This thesis presents the development, implementation, and validation of an optimal zero-sequence voltage injection strategy for Voltage Source Inverters (VSI)-fed Synchronous Reluctance Machine (SynRM) aimed at reducing instantaneous stator current ripple, with a corresponding reduction in instantaneous power losses and associated temperature rise in the machine stator. The proposed work is first simulated in the Simulink environment with a detailed simulation model of SynRM in which the machine is represented using a nonlinear flux–current relationship and controlled in closed-loop Field-Oriented Control (FOC) with standard Space Vector Pulse Width Modulation (SVPWM). The proposed formulation explicitly incorporates machine inductive characteristics in the synchronous 𝑑 − 𝑞 reference frame. A modified SVPWM approach, “Saliency-Based Space Vector Pulse Width Modulation (SBSVPWM)” is then introduced, where the zero-sequence voltage component is reformulated to influence zero vector dwell time distribution during the switching interval. The amplitude and phase shift of this optimal zero-sequence voltage component are derived from the modulation index and the ratio of 𝑑 − 𝑞 axis inductance to obtain minimal instantaneous current ripple. The standard modulation strategies are evaluated along with the proposed optimal zero-sequence voltage injection strategy - SB-SVPWM under identical operating conditions to demonstrate performance improvement with the proposed method in terms of optimal reduction in instantaneous current ripple and machine power losses. The proposed optimal modulation strategy SB-SVPWM is then implemented in a laboratory test setup consisting of 15 kW SVPWM controlled from a real-time rapid control prototyping environment to validate the practical applicability of the proposed method. The machine is operated under various speeds and load conditions, and instantaneous current ripple and machine power losses are measured under standard SVPWM and SB-SVPWM to confirm the usability of the proposed method. It is shown that the proposed SB-SVPWM significantly reduces the current ripple in certain operating points, but the effect is not that significant at the nominal operating point.