# Electronic – non-linear load (rectifier) and power factor

power-factor-correction

I read that power factor shows how much there is the apparent power, which circulates between power lines and the load and does nothing but heats up the transmission lines and transformers in them. They say that this is due to the current being out of phase with voltage. Particularly, power flows out of the load when voltage is positive but current is negative.

Also, I see that diode rectifier conducts only at the short moments of peak voltage, when input voltage exeeds the load voltage (rectifiers have an output capacitor = load voltage). So, all the power is consumed during these short bursts. I have read one an article saying that people do not understand anything and the whole problem is not power factor but these bursts that overload the transformers in power lines (it is not allowed to make large currents in transformers because of danger of magnetization loss). But, when you consume all power during very tiny fraction of the sine, huge current happens. Obviously, the current is not proportional to the voltage as it should in the reference (resistor) load with power factor of 1. But, I do not see any negative power here! The rectifier precludes having positive input voltage and negative current. All current is positive under peak positive voltage. So, how does the non-linear load produce the apparent power?

In other words, Wikipedia says
http://en.wikipedia.org/wiki/Switched-mode_power_supply#Power_factor

Simple off-line switched mode power supplies incorporate a simple full-wave rectifier connected to a large energy storing capacitor. Such SMPSs draw current from the AC line in short pulses when the mains instantaneous voltage exceeds the voltage across this capacitor. During the remaining portion of the AC cycle the capacitor provides energy to the power supply.

As a result, the input current of such basic switched mode power supplies has high harmonic content and relatively low power factor.

How do they conclude that harmonic content produces the low power factor? Where the apparent power comes from?

I understand that current has harmonics (frequency components) which means that it oscillates forth and back while voltage remains of single polarity. It might be that these high-frequency oscillations of current produce the apparent power. However, the net flow is still positive, the current still flows only in one direction, corresponding to the voltage polarity and oscillations do not make it flowing in opposite direction to cause the apparent power.