This chapter aims to help the reader understand the fundamental principles of electromechanical energy conversion and their quantitative representation, independent of specific machine implementations. Starting from a linear motor model, it reveals how the Lorentz force, constrained by conductors and electric fields, enables reversible conversion between electrical and mechanical energy. The concepts of magnetic energy and co-energy are introduced to describe lossless electromagnetic systems, and a unified torque expression is derived using the principle of virtual displacement, explaining the generation mechanisms of electromagnetic torque and reluctance torque. In particular, the “space vector” concept is established from fundamental MMF wave functions—this serves as the book’s most essential mathematical analysis tool. Building on this, the vector expression for torque is derived.