A few years ago I received as a gift a wonderfully nerdy item for any electrical engineer: the Kill-A-Watt. The device is used to measure electrical power consumption in household items and I've been using it to do just that, mainly to see how much power devices draw when they are plugged in but turned off.
One of the measurements this device makes is something called the power factor. The power factor is a measure of the ratio of real power a device consumes to the amount of power the device appears to draw from the grid (the apparent power). Devices with a low power factor are bad because they demand more power from the grid than they actually use, sending the excess back every half cycle. Some of this power that gets sent back gets burned up in losses on the wires and is in essence, wasted.
Traditionally, power factor has been caused by electric motors due to their high inductance and the fix was relatively straight-forward: adding a capacitor near the motor provides a place to temporarily store the energy that would have been sent back to the grid. When the motor then appears to demand it again, it gets pulled out of the capacitor. In fact, in these cases, the power factor can be calculated by a tiny bit of math measuring the difference between when the peak current flows and when the peak voltage is present. The larger the difference in time between these two events, the lower the power factor.
Over the past few decades, though, there has been another big power factor culprit on the rise: rectifiers. These are the circuits that change AC into DC and are in nearly every electronic goody around the house: computers, TVs, DVD players, laptops: if you buy it at an electronics shop it has a rectifier. The power factor problems these devices create is very different from motors, though. They send the power back to the grid in a very different way; they convert some of the power at the traditional 60Hz frequency to power at higher frequencies, distorting the waveform. The fix that works for motors does nothing in this case and measuring the power factor is much more complicated.
Well, in a lab I was finishing for class, we had a very fancy (and expensive) power meter we were using that measured the power factor in two ways, the motor way (displacement power factor) and the rectifier way (true power factor, the more useful and accurate of the two). Since I had access to this, I brought in my Kill-A-Watt and we set up both meters to measure the power factor on a PC (which contains a rectifier). Much to my surprise, the fancy power meter showed that my Kill-A-Watt measured the more complicated true power factor. I was very impressed that such a simple and inexpensive device was able to measure this accurately.
This made me a happy nerd for the day.
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