A lot of the geometry of the motor core size, winding size, dimensions, mounting dimensions, the size of a variety of other structural components. But what kinds of main dimensions: the motor electromagnetic process is mainly carried out in the air gap, the form of energy conversion is carried out through the main air gap flux. The main dimensions must be closely related to the air gap. Practice has proved that, near the airgap armature diameter (D) and the the core effective length (LEF) is the main dimensions of the motor, while the air gap can be said that the third size. From the geometric point of view, these dimensions has been determined, other dimensions largely determine the and many electromagnetic properties will basically around them or a slight change.