Rotating Off Center - A model of this type is done by creating local coordinate systems for the rotor and stator. The points and lines that define the rotating air gap (shown in yellow below) are defined in the rotor coordinate system. By changing the X and Y components of the "Origin" of the stator coordinate system, the entire stator is moved with respect to the rotor. So the rotor is off center from the stator, but it is still rotating around its physical center. The physical air gap is split into three sections, with the middle section being the rotating air gap.

Off Mechanical Center - A model of this type is also done by creating coordinate systems for the rotor and stator. In this case, the rotating air gap is defined in the coordinate system of the stator. The X and Y components of the "Origin" of the rotor coordinate system move the position of the rotor off from center (0:0), but the rotation still occurs around (0:0), since the rotating air gap is centered at that point.