  The air-gap macro element is reformulated such that rotation, rotor or stator
skewing and rotor eccentricity can be incorporated easily. The air-gap element
is evaluated using Fast Fourier Transforms which in combination with the
Conjugate Gradient algorithm leads to highly efficient and memory inexpensive
iterative solution scheme. The improved air-gap element features beneficial
approximation properties and is competitive to moving-band and sliding-surface
technique.
