There are three different kinds of electrical power. There is ?real power,? and there is ?reactive power,? and the two add up (in a trigonometric sort of way) to give you ?apparent power.? What causes ?reactive power? to come into the picture are things that involve magnetic fields (such as the windings of motors, ballasts of fluorescent lights, and welding machines) and things that involve electric fields (such as capacitors and synchronous generators).
I don?t really like the ?beer and foam? analogy for power factor. But here it is anyway. Real power is like the liquid that fills most of a glass of beer. Reactive power is like the foam at the top of the glass. What you really want is the liquid, but the foam is there and you can?t avoid it being there and it doesn?t change the amount of liquid at the bottom. Suppose you start with a glass with some amount of liquid, and a little foam at the top. In order to contain both, the glass has to be a bit larger than necessary for the liquid alone. Suppose you can generate some extra foam, and suppose you pour it into the glass. You haven?t changed the amount of liquid, but you need a bigger glass to contain it all.
Now back to electrical stuff, and a bit of trigonometry. Take a sheet of paper, and draw a horizontal line about one inch long. Label the left hand end of the line as point ?1? and the right hand end as point ?2.? From point 2, draw a vertical line upwards one inch. Label the top of that line as point ?3.? Finally connect points 1 and 3 to form a triangle. Line 1-2 represents the real power, and it is a measure of the ?real work? being done by the load equipment. Line 2-3 represents reactive power, and is a measure of the energy being exchanged (you might even say ?wasted?) between the magnetic field of the utility generator and the magnetic field of the customer?s motors and other inductive loads. Line 1-3 represents apparent power, and is a measure of the power that the utility has to provide, in order for the customer to run the motors.
Power factor is a measure of the angle located at point 1. In terms of trigonometry, PF is defined as the cosine of that angle. If that angle is zero degrees (which happens if line 2-3 has no length at all, so that line 1-2 and line 1-3 lie one on top of the other), then PF is unity. If that angle is large (which happens if line 2-3 is long), the PF is considered ?poor.?
What would happen to that triangle, if you drew the vertical line as 2 inches long, instead of 1 inch? The real work, line 1-2, (like the liquid portion of the glass of beer) does not change. But the apparent power provided by the utility (line 1-3) is now much longer. Thus, in order to allow the customer?s motors to do the same amount of work, the utility has to provide more power (i.e., you need a larger glass to hold the same amount of liquid with a larger amount of foam).
What would happen to that triangle, if you drew the vertical line as 1/2 inches long, instead of 1 inch? Here again, the real work, line 1-2, does not change. But the apparent power provided by the utility (line 1-3) is now much shorter. Thus, in order to allow the customer?s motors to do the same amount of work, the utility has to provide less power (i.e., you can get by with a smaller glass, since there is less foam to contain). You can see why the utility does not like the customer to have a lot of reactive power in their loads. You can see why the utility will charge some customers (large industrial plants, for example) a penalty for having a poor power factor.
What a power factor correction device does is to reduce the length of line 2-3. It does that by adding a reactive power load of a type that is opposite to that in the customer?s loads. Usually, a customer?s loads are motors and other inductive loads. To that, you add a capacitive load. What that does to the triangle is like starting at point 3, and drawing a line downwards towards (but not as far as) line 2. Call the end of this new line point ?4.? Here?s what you get: Line 2-3 is the customer?s inductive loads. Line 3-4 is the power factor correction capacitors. Line 2-4 is the new value of total reactive power. Line 1-4 (I didn?t tell you to draw that yet, did I?) is the new value of total apparent power. You will note that line 1-4 is shorter than line 1-3. So the amount of power supplied by the utility has gone down.
What is physically happening in the power factor correction device is that, where you used to have an exchange of energy between the between the magnetic field of the utility generator and the magnetic field of the customer?s motors and other inductive loads, you now have an exchange of energy between the magnetic field of the customer?s motors and the electric field of the power factor correction capacitors. The utility company meter does not record that energy exchange, because it takes place entirely within the customer?s facility.
One final note. The units of measure for the three lines described above are fundamentally the same. They all relate to the rate of use of energy, and can all be expressed in terms of ?joules of energy per second of time.? But to keep track of them separately, they are given separate names. The ?real power? unit (line 1-2) is expressed in ?watts (W)? or ?kilowatts (KW).? The ?reactive power? unit (line 2-3) is expressed in ?volt-amperes reactive (VAR)? or kilo-volt-amperes reactive (KVAR).? The ?apparent power? unit (line 1-3) is expressed in ?volt-amperes (VA),? or kilo-volt-amperes (KVA).?
