Stator dc winding excited Vernier reluctance machines (DC-VRMs) are one novel kind of Vernier reluctance machines, and have doubly salient structure and additional dc field windings in their stators to generate the exciting field. These machines advantages include a wide speed range, due to the flexible exciting field by the dc winding and a robust rotor structure without permanent magnets or windings. In this paper, the nature and principles of DC-VRMs are first illustrated theoretically with winding function and harmonic theories. First, by considering the permeance modulation function, the equations and harmonics of the exciting field are obtained. Next, based on these results, the stator/rotor pole combinations and armature winding configuration methods are proposed. Additionally, the expressions for the self-inductance, mutual inductance, the back electromotive force (back-EMF) of armature windings are summarized with the winding function theories. Also, the effects of permeance, field, and armature winding harmonics on inductance harmonics are analyzed. The equation for electromagnetic torque is also given, and the design parameters that may influence the machine's torque are provided. Finally, the inductances and torque in synchronous reference frame are analyzed. All the analytical results are validated by finite element analyses and some experimental results are also given to validate the theoretical analysis.