I. Introduction
The purpose of the model transformation in ac machines is to obtain a new set of equations, usually fewer in number and less complex in nature, representing the machine's behaviour exactly as the original equations [1]. Initially applied to three-phase machines, the model transformation consists in referring the equations to a new reference frame constituted by three mutually orthogonal axes. Two of them are placed in the cross-sectional plane of the machine and are usually referred to as or d-q. They represent the electromagnetic energy transfer across the airgap and constitute the so-called flux/torque producing subspace ( subspace herein after). The third axis, usually denoted as 0, represents the zero-sequence components and is seldom used, as, normally, the neutral point of the ac rotating machines is left isolated.