For double-fed a.c. machines by various values of slip operation current and voltage complex values equations are developed. Moreover the equations for active, reactive power of exciter in rotor winding (for example of the converter cycles) taking into account saturation of machine magnetic circuit and high time harmonics are derived. Assumed, that the MMF high space harmonics in air gap of large double-fed machines have only the leakage components and the influence on torque at the shaft of double-fed a.c. machines is not exert.