Three-Phase Power Explained


Welcome to this animated video that will quickly explain 3 phase power. I’ll also explain the mystery behind why the 3 power lines are 120 degrees apart because that’s a crucial piece to understanding 3 phase power. The power that enters a data center is usually 3 phase AC power, which means 3 phase alternating current power. Let’s look at a simplified example of how 3 phase power is generated. This example is different from what I would use to describe how a three phase motor uses power. In the alternating current video, we showed how spinning a magnet past one wire caused the current to flow back and forth. Now we’re going to spin a magnet past 3 wires and see the effect that has on the current in each wire. In this three phase example, the north positive end of the magnet is pointing straight up at line one. To help explain the concept easier, let’s use a clock face and say that line one is at the twelve o’clock position. The electrons in line 1 are going to be flowing towards the north pole of the magnet. What happens when the magnet now swings 90 degrees? As we saw in the alternating current video, because the magnet is perpendicular to line 1, the electrons in line one will stop moving. Then as the magnet swings more than 90 degrees and the south pole of the magnet comes closer to line one, and the electrons will reverse which means the direction of the current will reverse. That was described in detail in the alternating current video. If you clicked on this video without a thorough understanding of alternating current, please view that video first. Looking at the chart, you can see why I picked an analog clock face. A circle is 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 3 phase power, the copper lines are located 120 degrees apart. So when you’re at the four o’clock position in our example here, that’s 120 degrees away from line one. And at the eight o’clock position is 120 degrees away from both 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 more the electrons move away from the south pole. In each three of these lines, as the electrons are moving back and forth, they are not always moving in the same direction or speed as the other two lines. Let’s look again at the example. As the magnet is spinning, when the north pole is at 1 o’clock it becomes perpendicular to line 2 so of course the electrons stop moving in line 2. But they are still moving in line 1 attracted by the closer north pole and they are moving in line 3 repelled by the south pole. When the magnet’s north pole faces 2 o’clock, then Line 1 and [Line] 2 are affected by the north pole but the south pole is directly opposite Line 3 so it’s now at peak current. At 3 o’clock, the magnet is perpendicular to Line 1 so electrons stop moving, but Line 2 is affected by the north pole and line 3 is affected by the south pole so current is flowing in lines 2 and 3. Hopefully this example shows you how at any time current is always flowing in at least 2 lines. It also shows the relationship between the 3 lines as the magnet spins in a circle. As the magnet goes around the clock face, each of the 3 lines will be affected by either the north or south poles, except when the magnet is perpendicular to a line. Let’s focus on line 1. It’s at its peak current when the north pole points to both 12 and 6 o’clock positions. It is at zero current when the north pole points to 3 and 9 o’clock. Only 1 of the 3 lines is ever at peak, but because there are 3 lines, there are 3 peak positive and 3 peak negative positions for every cycle. At 6 different positions on the clock face, one of the lines is at peak. 12 and 6 positions are Line 1’s alternating peaks, 2 and 8 positions are Line 3’s alternating peaks, and 4 and 10 are Line 2’s alternating peaks. Now let’s explain those confusing waveforms that are frequently used to depict 3 phase If you look at the wave form 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 go to its peak. Then as the positive pole spins past that wire, the current starts to weaken until the magnet is perpendicular again which results in zero current. As the negative pole starts to come closer, the current reverses and moves in the other direction towards another peak before returning to zero current. This completes 1 full cycle for that line. In order for the 2 dimensional chart to show the relationship between the lines, it now shows a gap that signifies the length of time it takes the magnet to spin 120 degrees. This is when the red line is at zero current. As the magnet keeps spinning, the red line will move towards its peak positive current, then come back to zero after which the current will change direction. The chart also shows that the third line will start at zero current 120 degrees after the 2nd 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’re not at full strength, meaning they’re not at peak. So as the electrons flow from a positive to a negative peak, the current is displayed as flowing from positive to negative values. Remember that positives and negatives don’t cancel each other out. The positive and negative connotation is only used to described how current alternates. In a 3 phase circuit, you usually take the one of the 3 current carrying lines and connect it to another of the 3 current carrying lines. One exception to doing this is described in the Delta versus Wye video. As an example, let’s use a 3 phase 208 volt line. Each of the 3 lines will be carrying 120 volts. If you look at the chart, you can easily see the power output of any 2 lines. If one is at peak, the other line isn’t at peak. That’s why in a 3 phase circuit it’s incorrect to multiply 120 volts times 2 to get 240 volts. So if you’re wondering why you have 110/120 volts at home for your regular outlets but you’ve also have 220/240 volt appliances, what gives? Well, that’s not 3 phase power. It’s actually 2 single phase lines. So how do you calculate the power of combining 2 lines in a 3 phase circuit? The formula is volts times the square root of 3, which happens to be rounded off to 1.732. For 2 lines each carrying 120 volts, the calculation for this is 120 volts times 1.732, and the result is rounded up to 208 volts. That’s why we call it a 208 volt three phase circuit, or a 208 volt 3 phase line. A 400 volt three phase circuit means that each of the 3 lines are carrying 230 volts. Last topic I’ll talk about in this video is: why do companies and data centers use 3 phase? Right now let me give you a simple overview. For three phase, you connect line 1 to line 2 and get 208 volts. At the same time you [can] connect line 2 to line 3 and get 208 volts. And you [can] connect line 3 to line 1 and get 208 volts. If the wire is capable of delivering 30 amps, then the power that’s being delivered is 208 volts times 30 amps times 1.732 for total power available of 10.8 kVA. In comparison, for a single phase 30 amp circuit carrying 208 volts, you will only get 6.2 kVA. Basically, 3 phase delivers more power. There are other factors why it’s far better to run 3 phase power to 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.

100 thoughts on “Three-Phase Power Explained

  1. The windings of a 3 phase generator are 120 degrees apart. It would seem reasonable that the power coming from a 3 phase generator would reflect the 120 degree locations of the generator windings. It would be an interesting experiment to wind a generator with the coils located at different degree locations and observe the phase pattern
    david

  2. Unfortunately this doesn't show a very good representation or explanation of how an AC generator moves electrons around, but there are a number of other good concepts in the video that help explain the basics of AC power.

    Electrons need a loop of wire to move around, and if that were shown (and it has to be in a specific orientation), we'd see that electrons move both towards and away from a magnetic pole in an AC circuit. If it were as simple as saying electrons were attracted to a "+" or "-" as in a DC circuit, they wouldn't be able to move around the loop at all. A better explanation is that the magnetic forces of a north pole push electrons in one direction around a loop and a south pole pushes them in the other direction. In between poles the magnetic forces are near zero so the electrons stop. Further, electrons only move in a changing magnetic field so the magnetic poles have to always be moving or the wires themselves have to be moving. In a large generator magnets rotate (spin) horizontally inside stationary coils made of copper bars that run vertically and the electrons move up or down in the bars depending on whether a north or south pole is acting on them. They also move perpendicular to both the magnetic field and direction of rotation (see Flemings Right Hand Rule for Generators).

  3. Shall I hook up a single phase dc voltage controller which is 150 amps to phase and neutral because the voltage in the workshop is 440 v phase to phase or we need a transformer to do that

  4. Very good very clear explanation, l have been looking at this stuff for years and for some reason for me it just takes one mind and l know……yippee….what a life …does it ever puzzle you why we have to search for so long before we find the right answer ….why can't we get it straight away

  5. informative and well explained.but three phase are very hazard to appliances if you dont check the balance voltage.example L1 to L2=220v to 240v,L1 to L3=220v to 240v,L2 to L3=220v to 240v.but L1 to ground active=108v to 112v,L2 to ground active=108v to112v.this is the problem L3 to ground are max 208volts.so that you may consult your appliance and equipment to well trained license technician/electrician.so that your investment would be longer.simple connect or plugged you appliances at safe with voltage regulator L1 and L2 with active ground L3 use for electric pump or heavy motors.single phase no problem

  6. Altough the information in this video is completely correct, still one question was not answered: why 3 phases? why not 2, 4, 5, 6? why 3? And there is ONE major reason why 3 phase power is the way to go and the reason is the same why 1, 2, 4 phase power sucks. 5 phase power supply would have been the next choice. why? because power output over time is constant with 3, 5, 6, 7, 9 and so on phases (uneven numbers and any nmultiples of those)

  7. Another, unmentioned, benefit of 3-phase power is that 3-phase AC induction motors are much simpler, not needing a shaded-pole device on the field windings or a special startup motor to get the principal motor started. A 2-phase AC induction motor, like an internal-combustion engine, can keep itself going but needs a separate device to get it started.

  8. Almost all of my higher voltage "Data Center" equipment (Printers, CISC based CPU and HD Racks) required 240v as standard. When we relocated and the power company could only supply 208v to our facility we had to have the manufacturer come out and make adjustments to operate on the reduced voltage. At least now I think I understand where the 208v came from, but why is 240v 3 phase standard for so much of the older, heavier equipment and household appliances such as stoves and dryers?

  9. Thank you for using kVA instead of Watts! It drives me nuts when I see people trying to use ohms law for DC circuits, in an AC circuit.

  10. Ok, so you have two phases of 120v in your house the offset between which is 180 degrees, which can be referenced to each other to create 240v. How do you get that when the power lines carry 3 phases with a 120-degree offset between each?

  11. It's just how the AC power is transmitted, it's 120 degrees out of phase from each other. The magnet is the generator that is generating the power i.e. Hydro-electric Turbine at a Dam.

  12. you talked about other videos to watch but they don't seem to be available on the website or youtube , how do you get access to these ?

  13. Mr.Raritan
    Thank you so much
    Your video is one of the most excellent explanation animation videos again I greatly appreciate your video
    I would like to upload animation videos
    Of synchronize rotating machines induction motors, Generators
    I hope so look forward to hearing from you
    Thanks again

  14. You suggest that electrons are attracted / distracted by a magnetic field. That's wrong.
    It's the changing magnetic field that causes an electric voltage in the coils.

  15. as an EE I want this video to be tough in colleges, it explains it perfectly , I want to go back to college, who would not want to be in college -all you have to do i study nothing else lol kids these days have it made with the internet, this is a great video, I watched it twice because its so well done, who ever did the animation I want them to do my power point stuff and make it like this

  16. Thanks for the professional breaking down of information to simple yet informative points. It's great to have instructors like you.

  17. At trade school 40 years ago we studied the GM delcotron alternator.. the study materials called it the six sweeps of the alternator… which btw are all three phase… great video!

  18. Well, well, well. One thing has not changed since I took my first course in basic electricity in 1960. When the instructor says, "basically" they are about to tell you a lie. This "basically" happens at 9:32 and is followed by rapid speech designed to get past the subject before you see the error. Three-phase power can be more efficient than single phase, but not because a wire in a three-phase circuit can magically carry more kva than a wire in a single-phase circuit. A wire carrying 30 amps at 208 vac delivers 3.6 kva no matter how it is connected, period. The reason he came up with 10.8 versus 6.2 is because he LEFT OUT A WIRE carrying 3.6 kva when he switched to single-phase. Most aircraft three-phase alternators are rated at 120/208 volts and operated at 115/200 volts as measured at the generator buses. Some of the loads connected to the buses are single-phase, and some are three-phase. It makes no difference how much power is used in the single-phase versus three-phase loads. A 40 kva, 3-phase alternator can deliver a continuous 40 kva to the loads, period. One advantage to three-phase connections: if the load is balanced (same current in all three phases), there is no need for a neutral wire connection to the load. In fact, many high-current, three-phase loads are connected to the associated power relay by only three wires, and no connection is made to the neutral point in the load. Look it up.

  19. You explanations are clear but I can’t get over the diagram on 0:52. It’s really really wrong. Electrons aren’t getting attracted to the the magnet in straight pieces of wire like that. Current can’t even flow in the way you’ve shown the wires. The magnet points towards coils instead, and the magnetic flux through the center of the coils generates current through the wires of the coils. The diagram you have is very misleading to beginners

  20. Ive always thought that the neutral phase wire is always repeat always hot!correct me if im wrong. Thanks for great video much appreciated

  21. In the end of the video you said that In single line is 208 v *30 amps =6kva it’s wrong , the right is , in single line 120 * 30 amps

  22. Explanation is quite good enough but the rotating magnetic field is generated after current passes we should not place magnet before to understand this that makes it complicated.

  23. hey guys, i'm wondering if 3 phase is so nice. why not going to 4 phase electricity? i mean 90deg phase difference is much nicer than those 120… 4 quadrants are much more natural than 3. for real. using 4 would distribute the load among more wires! imagine a 4 phase light bulb!

  24. How do you get 180 degree out of phase, single phase 240v out of the 3 120 degrees out of phase lines than? is the power company before it gets to your house converting it to 180 degrees out of phase? Is the transformer or some device doing this in residential areas? I am guessing residential homes are only getting 1 of the 120 degree phase voltage lines .

  25. Most of the negative comments are dealing in symantics While they may be correct technically the method of showing the process is successful. Besides, who gets thier electrical engineering degree from youtube videos? Chill tf out. Nobody's gonna get killed because the video didn't follow every rule of science in the illustration.

  26. The clockface was both unnecessary and confusing. Why not print degrees when talking about degrees?

  27. Had to watch 9 mins to find out why a data center would want 3 phase power. But hey, it's cool, I learned something along the way.

  28. I'm Very Pleased To Watch This Video And That I Can More Understand Its Explanation With The Writing if, My Advice.

  29. hi all is it true ? use a servo motor to run yr house? but gt get sum thing to turn th servo motor over hagd1 here it is 240v all

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