The paper deals with the applicability of unified machine theory to electrical machines where space harmonics can not be neglected. It is shown that there exists cases where a linear transformation can de determined to transform the set of time-depended linear differential equations for a machine at constant speed into a set of linear time-invariant differential equations, even if space harmonics are taken into considertation. A criterion for applicability is presented that involves the number of phases, the number of harmonics and the order of the harmonics that have to be considered.