The invention discloses an electronic 
phase change detection circuit of a 
position sensor-free brushless 
direct current motor, which is characterized in that a back 
electromotive force zero crossing point detection circuit detects a three-phase back 
electromotive force zero crossing point and sends the detect results isolated by a photoelectric isolating circuit into a 
microprocessor unit for further 
processing; a 
motor circuit, a detection circuit and the 
microprocessor unit are isolated from one another by the photoelectric isolating circuit, so that the interference generated by electric signals in the 
motor circuit and the detection circuit to an input 
signal of a 
microprocessor is prevented, the work stability and the 
system reliability of the microprocessor unit are enhanced, and a back 
electromotive force phase position is compensated through 
phase compensation information presorted in the microprocessor unit; and the back electromotive force 
zero crossing point and the 
phase change processing are determined, and further, the position of a rotor is determined without the adoption of the 
position sensor, so that the effect of the 
position sensor on the 
system reliability in the special environments of high temperature, 
high voltage, high interference, high vibration and the like is avoided, and further, and the 
system reliability is ensured.