Efficiency, stability and reliability of interior permanent magnet (IPM) type brushless DC motor(BLDCM) are verified through a number of studies and experiment until today. In addition, IPM type BLDCM is widely used in many applications for high power and excellent control of the per unit volume. Especially, IPM type BLDCM is used as a core driving source in the factory automation and electric propulsion systems of electric vehicles due to their wide control range and high robustness. Although there is many merits, IPM type BLDC motors involve potentially risk about an irreversible magnetization. If these systems are continuously driving under the irreversible magnetization condition, the residual flux density of permanent magnet is more decreased because the demagnetizing regions are increased by overlap of inverse flux.