The unified theory of electrical machinery should identify the inherent relationships of the electromechanical energy conversion process among different types of electrical machinery. Previously, the concepts of “Faraday reference frame” and “entrainment electromotive force” are proposed for rotational reference frames, which achieved a consistent expression of voltage equations under the winding-fixed coordinate system and the rotational coordinate system. On this basis, a four-coil unified model of electrical machinery is proposed. The electromechanical energy transformation process of the motor is uniformly characterized. Based on space vector mathematics, and its voltage and torque equation of four-coil prototype motor under space vector expressions are derived. Furthermore, the basic input and output characteristics of the unified model of electrical machinery is discussed, an equivalent physical characterization of typical electrical machinery is provided by setting additional constraint conditions through theoretical analysis. This work comes from teaching practice and is a helpful extension and supplement to the unified model of electrical machinery theory. This work also has a potential reference value for practical applications. The proposed vector control principle may potentially achieve a unified underlying algorithmic architecture for vector control of different types of electrical machineries.