Abstract: The general calculation formulas of the rotation induced voltage of brushless DC motors are deduced systematically. Based on these formulas, the effect on the rotation induced voltage caused by the flat top width of air-gap magnetic density, the distributed winding, the short pitched winding, the fractional slot, the skewing slot and so on are studied detailedly. The essential factors of design of rotor magnet and stator winding are presented in order to ensure that the rotation induced voltage is trapezoid with the ideal flat top width.The research contents of this paper supply theoretical basis to the design of brushless DC motor of high performances and low torque ripple.