What is the difference between lagging and leading power factor?

Answer 1

Power factor is the ratio of watts (true power) to VA (volt-amperes, also called apparent power). Where the load is resistive only, the power factor is one, or unity, because the voltage waveform and the current waveform are in phase. Thus, for resistive loads only, true power and VA are the same. Where the load is reactive, the load stores energy, releasing it during a different part of the cycle. This shifts the current waveform so that it is offset, or out of phase with the voltage waveform. Reactive loads can be inductive (electric motors), capacitive, or non-linear (rectifier power supplies). When the load is inductive, the inductance tends to oppose the flow of current, storing energy then releasing it later in the cycle. The current waveform lags behind the voltage waveform. When the load is capacitive, the opposite occurs, and the current waveform leads the voltage waveform. So, lagging vs. leading is another way of saying the net reactance is either inductive or capacitive. This is slightly simplistic, and what we are talking about above is really DPF, or Displacement Power Factor. Non-linear loads don't really shift the current waveform, they distort it. The current waveform starts to look like a square wave, and square waves contain harmonics. So non-linear loads add harmonic distortion, and this tends to mimic a capacitively reactive load, adding some leading power factor. So when we say power factor, we really must include DPF plus harmonic distortion in total. One memory aid that may help to remember all this is: ELI the ICE man The L in ELI means inductance. The E (voltage) comes first, then the I (current) lags behind. Inductive reactance produces a lagging power factor. The C in ICE means capacitance. The I (current) comes first (leads) then the E (voltage) comes later. Capacitive reactance produces a leading power factor. Remember, it's always the current waveform that is affected by the reactive load, so you have to think about whether the current is leading or lagging. Most reactive loads are inductive, so at most sites the PF is lagging. One cool tidbit is that capacitive reactance cancels out inductive reactance. So if we have a building full of motors, we can add a bunch of capacitors to improve our power factor, meaning we get as close to unity as we can. Thus we have power factor correction capacitors that are made just for this purpose.

Answer 2

With a lagging power factor the phase of the current lags the voltage, as happens in an inductive load. With a leading power factor the current phase leads the voltage, as happens in a capacitor.

A lagging power factor is usually the problem in electricity supply. A load such as a motor with a lagging power factor can have its power factor corrected by placing a capacitor in parallel. The VA rating of the capacitor should be equal to the VAR (volt-amps reactive) taken by the load.