Low-Carrier-Ratio Modulation Strategy Based on Improved Flux Ripple Model

Since the quality of the direct and quadrature axis current (dq-axis) is an important performance indicator of a motor, the technique of flux ripple analysis is necessary to determine the appropriate working conditions for various voltage sequences and reduce current fluctuations. However, the traditional flux ripple model cannot analyze dq-axis flux ripples (except under no-load conditions). Therefore, it is unable to calculate the modulation ratio and voltage angle (angle between the reference vector and the q-axis) ranges to satisfy the given q-axis (or d-axis) current fluctuation requirement based on this model. To solve this problem, in this paper, we derived an improved flux ripple model by applying coordinate transformation to the traditional flux ripple model, which can be used to calculate the RMS (root mean square) value of the q-axis and the total flux ripple distribution within sector I under different modulation ratios and voltage angle ranges. The modulation ratio and voltage angle range of low-carrier-ratio modulation strategy (0121 sequence) with a smaller RMS value of flux ripple compared to the traditional modulation strategy (0127 sequence) was determined based on the calculated results of the RMS values of the flux ripple. It was concluded that the 0121 sequence is suitable for high-modulation-ratio conditions as a result of analysis by applying the proposed method to determine the proper working conditions for the low-carrier-ratio modulation strategy, and there was an improvement effect of reducing the current ripple and THD by adopting a low-carrier-ratio modulation strategy instead of the traditional modulation strategy in high-modulation-ratio conditions. The validity of the proposed improved flux ripple analysis method and low-carrier-ratio modulation strategy has been verified by simulation and experiment.