Considering that the only parameters known are the input line-to-line voltage of 415V rms (f=50Hz), the firing angle (a= degrees) and the overlap angle (b=15degrees), calculate the Power Factor as seen from the input AC side I know PF= DPF X DF = cos(60) X DF my question is how to calculate DF. The solution is DF=4/pi . But I do not know why?