介绍了单元电机的基本概念。以12槽双层的分数槽集中绕组为例，给出绕线原则，并在不确定单元电机极对数的情况下，用交流电机绕组理论、函数的傅里叶级数展开两种方法，从一个线圈、一组线圈、一相绕组到三相绕组详细推导了分数槽集中绕组各次谐波的绕组因数和磁动势，得出了分数槽集中绕组的一般特点和普遍规律。最后指出转子极对数接近但不等于定子槽数的一半是采用分数槽集中绕组的永磁电机的最佳选择。

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.