The Power Factor Explanation

Now that you know some fundamentals, understanding the Power Factor Explanation will be easy.

The components of motor current are load current and magnetizing current (adding those instantaneous values yields the total motor current). Also, because load current is in phase with voltage and magnetizing current lags voltage by 90 degrees, their sum will be a sine wave that peaks somewhere between 0 and 90 degrees lagging which is the motor current's offset from voltage. There are negative effects associated with increased offset and thats part of the power factor explanation.

Anyhow, Power Factor represents the offset in time, or the delay, between voltage and the current being delivered and is defined as the cosine of that offset. So when you look at this example, Total Current lags the voltage by 45 degrees. That is the offset, or the Power Angle, or the delay. The cosine of 45 degrees is 0.707, and we call it "lagging" because the current lags behind the voltage.

If the delay was 0 degrees (voltage and current in phase), the power factor would be 1.0 (because cosine 0 = 1) and if the delay were a full 90 degrees (this would be all magnetizing current), the power factor would be 0.0 (cosine 90 = 0).So big deal....why is this offset so important? Fair question, very fair.

In a motor, the component of load current is the current associated with doing work (i.e. pumping fluid, compressing gas, etc.) and the magnetizing current is not doing work. I know, I know... without the magnetic field the motor wouldn't work. This is true, however, as stated earlier, the magnetic field takes some energy to get built up in one half cycle and then returns that energy to the system in the next half cycle, so its net effect is that it uses no energy.

So if we consider the cable that delivers power to the motor; when we add the magnetizing current to the load current, the cable's total current flow becomes larger. We, however, really just want to drive the load. If we could find a way to supply the magnetizing current without sending it down the cable, then the cable would only need to deliver the load current. Somebody has already figured this out! Hint....its the capacitor and I'll explain in the next section how it does this. It stores and releases energy for use by the magnetic field locally (by locally I mean at the motor-end of the cable delivering the power).

Buildings and Industrial Plants
So far I have only discussed motors, but you can think of your building, your campus or your city as a BIG MOTOR. The current delivered to a building or an industrial plant (or just about anything else) is similar to a motor's current....it can be broken down into two components, load current (which is the sum of all the individual load currents in the building) and magnetizing current (which is the sum of all the individual magnetizing currents in the building).

Disadvantages of Low Power Factor
Lower power factor on a line means that higher current is flowing through it. The following are a few disadvantages to that:

Higher current results in a greater voltage drop on the line, especially if the line is marginally sized or very long. Its like trying to push too much water flow through a pipe; the pressure drops as you move down the pipe. In the cable, this results in an energy loss due to heat dissipation.

The higher current can push equipment closer to their rated capacities. Cables are rated based on their current carrying capacity. Transformers and generators are rated by VA (Volt-Amps) which is current carrying capacity at a certain voltage.

Another disadvantage to higher current flow, and it can be a big one, is the Reactive Demand Charge. The Utility that delivers the power to you will likely charge you if your Power Factor is too low. There are many different ways to determine this charge but the rational is this. If the transmission line that delivers your power (remember all lines have a current carrying capacity) is carrying your magnetizing current (current that could be supplied locally by you), then you are eating into the capacity of that line and taking away the ability for them to sell power to other customers with that line.
The result...Surprise! You Pay!!

"Power Factor Capacitors"