Here is an essay I wrote a year or so ago. It was an attempt to explain the nature of "apparent power," as opposed to "real power" and "reactive power." I intended it to be a bit more technical than the "beer and foam" analogy, without getting too technical, and without using any math.
For those who haven't hear of the "beer and foam analogy," it goes like this:
- Pour yourself a glass of beer. You will notice that most of the glass will contain liquid, but there will be some amount of foam at the top. No matter how carefully you pour, it is nearly impossible to avoid getting some amount of foam.
- "Real power" is like the liquid, the beer itself. Just as the liquid is the stuff you intended to drink, "real power" represents the useful work that you intend to get out of your machine.
- "Reactive power" is like the foam. You don't want it, but you can't get a glass of beer without it.
- "Apparent power" represents the entire contents of the glass (i.e., the beer plus the foam).
- Let's use a glass that is more than big enough to handle the bottle of beer that we will pour into the glass. Let's suppose that when you pour, you get a lot of foam. Take note of how much of the glass is used up by having about 12 ounces of liquid and a lot of foam.
- There are tricks and techniques that you can use, in order to reduce the amount of foam in the glass, and thereby reduce the total amount of space taken up within the glass.
- If you use one of these techniques, and if you end up with much less foam, take note of how much of the glass is used up by having about 12 ounces of liquid and a very little foam.
- You can see that by taking steps to reduce the foam (i.e., the "reactive power"), you also reduce the total space used up within the glass (i.e., the "apparent power"), without having any impact on the amount of liquid within the glass (i.e., the "real power").
Now let's turn technical. Motors, generators, fluorescent light ballasts, welders, and a host of other loads are "inductive" in nature. Capacitor banks and synchronous generators are "capacitive" in nature. The two have opposite effects on a power system.
The overall load on most power systems, indeed in most buildings, is inductive, rather than capacitive, in nature. It is a common practice to use banks of capacitors to reduce the inductive nature of the overall load. The ideal situation is to have the two just exactly cancel each other out. But load varies from moment to moment, so you can't achieve a perfect balance.
Here is one way of looking at the effect of adding a capacitor bank to a building:
- Wrap a wire around a nail and connect it to a battery. What you get is not just a magnet, it is also a magnetic field. A generator and a motor have this same type of magnetic field. So does any load that is ?inductive? in nature.
- Place two metal plates close together and connect them to a battery. What you get is not just a capacitor, it is also an electric field.
- When you connect a motor to a generator (or to a transformer), two things happen. The second thing is that the motor goes roundy-roundy. That is its job; that is the ?real power? being put to use; that is the ?beer.?
- But the first thing that happens is that the magnetic field of the generator exchanges energy with the magnetic field of the motor. It is in one sense a useless expenditure of energy, in that it doesn?t make the motor go any faster or drive a larger load. But the motor could not work at all, if it were not for that exchange of energy. So it is a "necessary useless" expenditure of energy. It is the ?Reactive Power,? the ?foam.?
- You get the ?apparent power? by adding the real power to the reactive power. But you add them by treating each as one leg of a right triangle, with the apparent power being the hypotenuse (i.e., use the Pythagorean Theorem).
- This is one way in which the "beer and foam" analogy falls short of accurately describing the true situation. Four inches of beer plus one inch of foam equals five inches in the glass. The math for apparent power does not work that way. Instead, three units of real power plus four units of reactive power equal five units of apparent power. This, you may recall, is the famous "3-4-5 Right Triangle."
- What happens when you add the capacitor to the load? Now the magnetic field of the motor will exchange energy with the electric field of the capacitor. It takes the magnetic field of the generator out of the picture. Note that this takes place close to the motor, and that the circuit breaker does not see the current associated with this energy exchange. Result: the current seen by the circuit breaker goes down.
- This phenomenon is commonly described by saying that the capacitor is canceling out the inductor. More precisely, it is said that the ?leading? reactive power added by the capacitor is opposite to the ?lagging? reactive power of the motor. It can be pictured by taking one leg of a right triangle and shrinking its length to nearly zero. What you will see is that the hypotenuse (apparent power) starts looking very much like the other leg (real power).
