|
That is correct. A motor, for
example looks like an inductive load so the current lags the
voltage and introduces an imaginary component. Power factor is
the cosine of the angle formed when the real and imaginary
power are plotted at right angles and the apparent power is
the hypotenuse of that triangle.When the rectifiers and
filters of power supplies used a choke-input filter, the ac
current drawn was in phase with and continuous over nearly the
whole of the a-c sinusoid. Such "passive PFC" yields a power
factor of about 0.96 at or near rated load. With the demand
for small, lighter power supplies, the dominant design became
the off-line rectifier followed by a high frequency switch and
a small high frequency transformer. The input looks like a
rectifier feeding a large capacitor. Not only is this
capacitively reactive it is source of distortion on the a-c
mains. The reason is that the rectifier does not conduct until
the sinusoidal voltage rises above the d-c charge on the input
capacitor. The rectifier cuts off again as the sinusoidal a-c
input voltage falls below the capacitor's d-c level. Thus the
whole power must be delivered during the short interval when
the rectifier conducts. To supply the required power, the
input current takes the form of a large short-duration spike
(shown below in red). |
