Three-Phase Power Explained

Welcome to this animated video that will quickly explain

three

-

phase

energy. I will also explain the mystery behind why the 3

power

lines are 120 degrees apart because it is a crucial piece to understanding

three

phase

power

. The power entering a data center is typically three-phase AC power, which means three-phase alternating current power. Let's look at a simplified example of how three-phase power is generated. This example is different than what you would use to describe how a three-phase motor uses energy. In the alternating current video, we showed how spinning a magnet through a wire caused current to flow from one side to the other.

Now we are going to spin a magnet across 3 wires and see the effect it has on the current in each wire. In this three-phase example, the positive north end of the magnet points directly toward line one. To help explain the concept more easily, let's use a clock face and say line one is at the twelve o'clock position. The electrons from line 1 will flow toward the north pole of the magnet. What happens when the magnet now swings 90 degrees? As we saw in the AC video, because the magnet is perpendicular to line 1, the electrons on line one will stop moving.

Then, when the magnet swings more than 90 degrees and the south pole of the magnet approaches line one, the electrons will reverse, meaning the direction of the current will reverse. This is described in detail in the AC video. If you clicked on this video without having a deep understanding of alternating current, please watch that video first. If you look at the graph you can see why I chose an analog clock format. A circle measures 360 degrees and the clock divides the circle into 12 sections so that each hour covers 30 degrees of the circle. Going from 12 to 3 is 90 degrees and going from 12 to 4 is 120 degrees.

When generating three-phase power, the copper lines are located 120 degrees apart. So when you're at the four o'clock position in our example, that's 120 degrees from line one. And at the eight o'clock position it is 120 degrees from the 4 and 12 o'clock positions. The 3 lines are equally spaced around the circle. If the north pole is closer to one of the 3 wires, then the electrons move in that direction. The closer the south pole gets to each wire, the further the electrons move away from the south pole. In each of these lines, as the electrons move back and forth, they do not always move in the same direction or speed as the other two lines.

Let's look at the example again. As the magnet rotates, when the north pole is at 1 o'clock it becomes perpendicular to line 2, so of course the electrons stop moving on line 2. But they still move on line 1 attracted by the nearest north pole and moving in line 3 repelled by the south pole. When the north pole of the magnet faces 2 o'clock, lines 1 and 2 are affected by the north pole, but the south pole is directly opposite line 3, so it is now at maximum current. At 3 o'clock, the magnet is perpendicular to line 1, so the electrons stop moving, but line 2 is affected by the north pole and line 3 is affected by the south pole, so current flows in lines 2 and 3.

Hopefully, this example shows you will see how at any time current always flows in at least 2 lines. It also shows the relationship between the 3 lines as the magnet rotates in a circle. As the magnet rotates around the clock face, each of the 3 lines will be affected by the north or south pole, except when the magnet is perpendicular to a line. Let's focus on line 1. It is at its peak current when the north pole points at the 12 and 6 o'clock positions. It has zero current when the north pole points at 3 and 9 o'clock. Only 1 of the 3 lines is at the peak, but because there are 3 lines, there are 3 positive peak positions and 3 negative peak positions for each cycle.

In 6 different positions on the watch face, one of the lines is at its maximum point. Positions 12 and 6 are the alternating peaks of Line 1, positions 2 and 8 are the alternating peaks of Line 3, and positions 4 and 10 are the alternating peaks of Line 2. Now let's explain those confusing waveforms that They are frequently used to represent 3 phases. If you look at the waveform example, you can see the first line in blue and it starts at zero. Which means the magnet is perpendicular to that line. As the magnet moves, you can see the current peak. Then, as the positive pole passes through that wire, the current begins to weaken until the magnet is once again perpendicular, resulting in zero current.

As the negative pole begins to approach, the current reverses and moves in the other direction toward another peak before returning to zero. This completes 1 full cycle for that line. In order for the two-dimensional graph to show the relationship between the lines, it now displays a space indicating the time it takes for the magnet to rotate 120 degrees. This is when the red line has zero current. As the magnet continues to rotate, the red line will move toward its maximum positive current and then return to zero, after which the current will change direction. The graph also shows that the third line will start at zero current 120 degrees after the second line.

So if you look at these 3 lines, you can see that when one line is at its peak, the other 2 lines are still generating current, but they are not at their maximum power, meaning they are not at their peak. So when electrons flow from a positive peak to a negative peak, current is shown to flow from positive to negative values. Remember that positive and negative do not cancel each other out. The positive and negative connotation is only used to describe how the current alternates. In a three-phase circuit, you typically take one of the 3 current-carrying lines and connect it to another of the 3 current-carrying lines.

An exception to this is described in the Delta versus Wye video. As an example, let's use a 208 volt three-phase line. Each of the 3 lines will carry 120 volts. If you look at the graph, you can easily see the power output of any 2 lines. If one is at its peak, the other line is not at its peak. That is why in a three-phase circuit it is incorrect to multiply 120 volts by 2 to obtain 240 volts. So if you're wondering why you have 110/120 volts at home for your regular outlets but you also have 220/240 volt appliances, what's wrong? Well, that's not three-phase power.

They are actually 2 single-phase lines. So how do you calculate the power of combining 2 lines in a three phase circuit? The formula is volts times the square root of 3, which rounds to 1.732. For 2 lines, each carrying 120 volts, the calculation is 120 volts multiplied by 1.732 and the result is rounded to 208 volts. That's why we call it a 208-volt three-phase circuit or a 208-volt three-phase line. A 400 volt three phase circuit means that each of the 3 lines carries 230 volts. The last topic I will talk about in this video is: why do companies and data centers use 3 phases? Now let me give you a simple overview.

For three phase, connect line 1 to line 2 and get 208 volts. At the same time you connect line 2 to line 3 and you get 208 volts. And you connect line 3 to line 1 and you get 208 volts. If the cable is capable of delivering 30 amps, then the power delivered is 208 volts times 30 amps times 1.732 for a total available power of 10.8 kVA. In comparison, for a 30 amp single phase circuit carrying 208 volts, you will only get 6.2 kVA. Basically, three phase offers more power. There are other factors why it is much better to run three phase power in the data center rack rather than using single phase power and those factors are discussed in the volts versus amps video and also in the 208 versus 400 volts video.