This paper develops a functional model for symmetrical multi-pulse autotransformer rectifier units (ATRUs) aimed at more-electric aircraft (MEA) applications, addressing the lack of existing models for unbalanced or faulty conditions. The model relates DC voltage and current to AC terminal parameters and has been validated through simulations and experiments, demonstrating significant improvements in efficiency, especially under balanced conditions. This work seeks to enhance the reliability and application of ATRUs in MEA systems.