The basic concept of unit motor is introduced. Taking a 12-slot double-layer fractional-slot concentrated winding as an example, the winding principle is given. Under the condition of uncertain pole pairs of unit motor, the winding factor and the magnetomotive force of fractional-slot concentrated winding are derived for different harmonic orders. After decomposing the stator magnetomotive force for one coil, a group of coils, one-phase winding and three-phase winding by using the AC motor winding theory and the Fourier series expansion of function, the basic characteristics and general laws of the fractional-slot concentrated winding are derived. At last, it is pointed out that using the rotor pole pairs close to but not equal to half of the number of stator slots is the best choice for the permanent magnet motor with fractional slot concentrated winding.