"I began wearing hats as a young lawyer because it helped me to establish my professional identity. Before that, whenever I was at a meeting, someone would ask me to get coffee." -Bella Abzug
Yesterday I went to a meeting carrying a Fluke 43B power quality analyzer.
I had just bought the instrument and it was the first time I had the opportunity to use it. I was asked to evaluate the wiring of a portable power distribution system, and I brought it along because I wanted to be prepared for anything. It turns out I wasn't prepared at all for the greeting I received.
"Anyone who carries around a power quality analyzer is definitely a friend of mine," the tech said.
It was an unintended but welcome consequence of spending a couple of thousand dollars on a very good tool. It was my "professional hat" of sorts, implying that I might know what I was doing. At the very least, no one was going to ask me to fetch coffee. Whether or not I know what I'm doing is debatable, but what's not debatable is that I have learned a lot about power quality since I bought the instrument and read the user manual and application guide.
Not Just a Multimeter
The power quality analyzer looks much like an ordinary multimeter, perhaps a bit larger, and it has a 3-inch-by-3-inch LCD display. It's actually a combination of a dual trace oscilloscope and multimeter. With it, you can measure voltage, current, resistance, capacitance and more. So what makes it unique? Glad you asked.
The unique thing about this instrument is that it not only measures voltage and current, but it also measures power factor, harmonic content, K-factor and more in a power system. With the LCD screen, you can actually see the waveforms and all of the accompanying distortions.
Power factor is a very important concept in power distribution. It affects the amount of current drawn by a load, which determines how you size your power distro. To understand how it works, look at the illustration of the power triangle on this page. The wattage of a particular load is shown by the green arrow at the bottom of the triangle. The length of the arrow tells you the relative number of watts. That's known as real power.
Some loads have inductance or capacitance as well as resistance. For example, a motor, like a chain motor, has internal windings that act like inductors. Those inductors store some of the electrical energy in the form of a magnetic field. At some point the magnetic energy is returned to the source (minus the losses due to the resistance of the copper, which contributes to the inefficiency of the system). So when the current flows to a motor, it draws more current than it actually uses in order to energize the magnetic field.
The red arrow at the right of the triangle represents the amount of energy that is stored in an inductive or capacitive load. All of that energy will eventually be returned to the source (again, minus the losses due to inefficiency). That's called the reactive power, or the volts-amps reactive (VAR).
The blue arrow represents the product of the voltage and the amps drawn by the load. Since the load draws more current than it actually uses (due to the reactance, which is another word for the inductive or capacitive part of the load), you can see that the volt-amps (or VA) is greater than the wattage of the load. The VA is known as the apparent power, because it's the amount of power flowing to the load, even though the load is not using all of it.
The Phase Angle
Now look at the angle between the watts and the volt-amps. That's the phase angle between the voltage and current in the system. The current waveform lags behind the voltage in an inductor and the voltage waveform lags behind the current waveform in a capacitor. The amount of lag or lead is called the phase angle. If the phase angle is big, that means that the reactive power is also big, because there is a lot of inductance or capacitance in the load. That causes lot of energy to be stored in the load and the amount of real power being used by the load compared to the apparent power is small. If the phase angle is small, that means that the reactive power is also small, because there is not a lot of inductance or capacitance in the load. That also means there is little energy being stored in the load and the amount of real power being used by the load compared to the apparent power is about the same.
If you know a little bit about the relationship between the sides and the angles of a right triangle (Warning: If you're squeamish about math, skip the last part of this sentence because another word for "the relationship between the sides and the angles of a right triangle" is "trigonometry") then you know that the cosine of the phase angle θ is the ratio of the real power to the apparent power, which is also known as the power factor.
Think about that. If the phase angle is 0°, then the real power and the apparent power are the same, and the power factor is 1. As the phase angle approaches 90°, the power factor approaches 0, meaning that the real power is also 0 watts. The ideal situation is when the system power factor is 1, because that means the system is drawing only enough current for the real power, and there is no "extra" current being drawn due to the reactive power.
Up until recently, I thought the only way to measure the power factor was to use a dual trace oscilloscope and compare the voltage waveform to the current waveform. And since most field techs I know don't carry an oscilloscope around with them, it was not practical to measure the power factor. It used to be that when people asked me about how to measure the power factor in an electrical system, I would show them pictures from my ski vacation in Colorado and then sneak out the back door. Now I just show them my Fluke 43B.
