We established in a previous post why it is necessary to have AC power – to reduce transmission losses – but why do we have three-phase power? Why not just one phase, like the inside of a house? Wouldn’t it be cheaper to run just one set of power lines instead of three? Wouldn’t there be a third less labor involved in electrical work if there were a third the wires? What is so important about three-phase power that we go through so much cost and effort to utilize it?
It all comes down to power. Let’s look at power in an AC Circuit. If the current and voltage waves are perfectly lined up, power simply equals the current times the voltage, as seen below. (The x-axis is in radians, and shows one complete cycle, or 2 Pi radians. In the US power system, there are 60 of these cycles every second.)
See a problem here? Current and voltage cross zero twice a cycle. If there are 60 of these cycles per second, which there are in the United States, the power from a single phase AC circuit drops to zero 120 times every single second. That means everything essentially turns off 120 times every second. Your toaster briefly stops toasting 120 times a second, your dishwasher briefly stops washing 120 times per second, and your lights turn off 120 times per second.
Thankfully, most of these changes are not visible to the naked eye. The coils of wire in your toaster remain hot even if there is no power in them for a split second and the tungsten burning away in your incandescent light bulb does not cease to be red hot because there is no current flowing through it for a one-thousandth of a second. It does flicker a little bit, which is one of the reasons natural light is more pleasant than artificial. In fact, the choice of 60 hertz for the US power grid was based on the minimally accepted flicker rate of light bulbs. (The English had a high tolerance for flickering lights due to their superior beer, so the UK grid operates at only 50 hertz.)
Okay, so the power stops for a tiny fraction of a second, but none of this matters since we can’t tell. Right? Well, kind of. For most everything in your home, it doesn’t matter, and if you’ve ever noticed the utility pole next to your house, only two electrical wires connect to your home from it. (If there are more you probably have a phone or cable TV.)
Constantly accelerating and decelerating power does matter, however, for large industrial operations and even for most operations in commercial-scale buildings. Imagine for a moment that you are a piece of equipment and a motor is powering you. You could be a compressor or a pump, for instance. It would wear you down badly if that motor drove you and then stopped and then drove you again 120 times per second. If instead the motor just pushed you at a constant, steady rate, you would last much longer and be able to do your job with much higher consistency and quality. This is exactly what three-phase power accomplishes.
Each phase of three-phase power is 120 degrees apart. The 120 degree separation is cleverly chosen to make the sum of all three-phases at any given point constant. This is easier to see then to describe, so below is a graph of three-phase power. You can see the power delivered by each phase, and in the top of the graph the sum of each phase combined.
The black line is the same as the single phase power in the earlier graph, and the red and green line are power in the phase 120 degrees ahead, and 120 degrees behind. Note that the purple line, the sum of the three lines below it, is constant. That is the beauty of three-phase power! Any motor, pump, fan, refrigerator, recycling plant, office building, or manufacturing plant can easily acquire perfectly smooth and constant power from the three-phase grid. With single-phase, or even two-phase power, perfectly constant power is not achievable.
Three-phase power allows us to combine the benefits of AC and DC power: we get low losses in the transmission system due to the use of simple AC transformers, and constant, perfectly smooth power like one gets with a DC circuit! This is why practically every country in the world goes to all the trouble of using three-phase power.
For the ambitious viewer, below is a graph of everything that is going on in three-phase power: each phase of current is represented, each phase of voltage is represented, their resulting power delivery curves, and the sum of those curves is shown below.