Generators in power systems are almost synchronous generators with few exceptions, for examples, induction generators. The principle of generators is based on the Fleming’s right hand law: e ¼lvB, where e: electric motive force, l: length of conductor, v: speed of conductor and B: magnetic induction. Then generators consist of field windings for generating magnetic induction and armature windings for obtaining electric power. For transforming rotating torque into electric power, they almost consist of cylindrical rotor with field winding and cylindrical stater with armature winding. The output power p is obtained as p ¼k0D2lN, where k0 is called output coefficient, D is a diameter of air-gap between rotor and stater, l is length of air-gap, that is, length of armature conductor, and N is rotor-speed. The output coefficient k0 is given as k0 ¼k1Bg AC, where k1 is coefficient, Bg is magnetic induction in air-gap, and AC is ampere-conductor per length of circumference of airgap. The magnetic induction Bg is limited due to magnetic saturation of material.