With AC you have a voltage waveform and a current waveform. For resistive loads like heating elements etc the voltage and current waveforms overlap completely, ie when voltage is at its peak, so is current. When this is the case, the power factor is 1.

However, for more complex loads such as anything inductive (like induction motors) or capacitive (like switchmode power supplies, which have large capacitors on their inputs), the current waveform is not in sync with the voltage waveform.

With inductive loads, the current waveform lags the voltage waveform, which means it is behind it (put simply, inductors resist changes in current, so a change in voltage takes time to translate into a change in current). The amount behind is given in degrees of the cycle (a full AC cycle is 360 degrees). The more inductive the load, the more it lags, and when the lag reaches 90 degrees (current is maximum when voltage is minimum) the power factor is said to be 0.

Same with capacitive loads, except current leads the voltage (think of it like this: capacitors are like a current sink, as soon as there's any voltage, current flows rapidly into them and then tapers off as they charge, so the current is ahead of the voltage. That's simplistic, but easy to visualise). So again, when the current leads the voltage by 90 degrees, the power factor is also 0.

Power factor is calculated by taking the cosine of the angle, so if the angle were 45 degrees then the power factor would be 0.71. See http://en.wikipedia.org/wiki/Power_factor for a more detailed explanation.

Posted Wednesday 20 May 2015 @ 11:52:00 pm from IP

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