89-3 Apparent power and power factor We assume v=Vm cos(ot +a)i=Im cos(at+B) The phase angle by which the voltage leads the current is 9=a-B P=-v cos 9 or P=VI cos 9 2 S=vI--(A)--apparent power The ratio of the real or average power to the apparent power is PF= called the power factor(PF). In the sinusoidal case: PF=cos 9 For a purely resistive load,9=0, PF=1. For a purely reactive load, 9=+90, PF=0§9-3 Apparent power and power factor We assume  =Vm cos( t +) i = I m cos( t +  ) The phase angle by which the voltage leads the current is  = −  cos cos 2 1 P V I or P VI = m m = S =VI − −(VA) − −apparent power The ratio of the real or average power to the apparent power is called the power factor(PF). S P VI P PF = = In the sinusoidal case: PF = cos For a purely resistive load,  = 0 , PF=1. For a purely reactive load, 90 , PF=0.   = 