Patent ID: 7459878

Claim:
A controlling method of a brushless DC motor (BLDCM) comprising an inverter, a stator having a winding, a rotor and a controller, wherein a back electromotive force (BEMF) is generated while said rotor rotates, comprising the steps of: (a) controlling said inverter to let a first current having a first direction flow to said winding so as to rotate said rotor; (b) cutting off a loop flowing therethrough by said first current in said inverter at a first moment, wherein said first current flows through a flywheel current loop and is degenerated to zero at a second moment; (c) measuring a terminal voltage of said winding, which is said BEMF, and obtaining a third moment at which said BEMF drops to zero; (d) adjusting said first moment by said controller according to a relationship between said third moment and said second moment; and (e) employing a second current having a second direction to replace said first current at a fourth moment and then repeating steps (a) to (d).