01 - What is 3-Phase Power? Three Phase Electricity Tutorial

hello welcome to this lesson in the AC circuit analysis tutor the title of this lesson is opening up a new branch a new discussion topic in this section of electrical engineering on three-phase electricity or three-phase power we also call it three phase circuits so if you ever hear three phase power three-phase electricity three phase circuits we're talking about the same thing now before we get into the topic of this lesson which is what actually is three-phase electricity I want to let you know that you need to kind of realize that when you get into three phase circuits a lot of people have problems with it because it gets confusing it can get confusing quickly because previously we've done analysis with sinusoidal AC signals running around circuits but in all of those cases we really only had one source and so one you know a stimulating source going through and flowing through the circuit to deal with well in three-phase electricity we're gonna have multiple sources and the difference between them is going to be the phase angle or the relationship between those those input signals and there's really good reason why we do that we're going to discuss those reasons here in just a minute but it can get a little bit overwhelming at first so what we're going to do is take them a thought or step by step process where first in this lesson I'm going to tell you what is three-phase electricity in the next section I'm going to elaborate a little bit more on why three-phase electricity is actually useful so that you can keep that in the back your mind and you can realize as we do the math and crank through the pain a little bit for lack of a better word you'll know that when we come out the other end why it's actually useful and then we'll crank through every topic step by step we'll talk about how to write down the three-phase sources how to analyze the basic three phase circuits and we'll build our complexity from there so that's our roadmap it's gonna be quite a long process but I promise you if you stick with me it's not gonna be hard because once you get the basic information and basic knowledge and kind of work with me through the process here then every step along the way is gonna be pretty easy so this lesson is what is three-phase electricity I wanted a general section I'm not gonna write too much on the board we are gonna write a few things down I'm gonna unfortunately have to read you a few things it's a little bit it's a little bit faster than just writing it all on the board three-phase power 3-days electricity is really good for lots of things but it's really good for a certain subset of things that we use it for quite often this is the punchline here why do we use this stuff it's good for the generation the transmission the distribution of large amounts of power well I say large amounts of power I'm talking about a power generating station like a power plant or something to get an enormous amount of power from point A to point B it turns out when you work through the theory that it's efficiently done with three-phase electricity and the details of why you'll get into that as you study this stuff a little bit more but just know that it for high power high voltage situations it's often much easier and just more efficient to do it in a 3 phase circuit arrangement the next one is something I'll expand on a little bit more in the next section it's very very good three-phase electricity three-phase power very very good at powering electric motors and what I mean by that when I say it's good at it I'm saying is that a motor is a spinning object right so when you feed it with a single sinusoid a lot of times you end up getting vibrations in the motor and I'm actually going to expand on this a little bit in the next section when you feed it with a single signal when you says single sinusoid but if you were to arrange a 3 phase circuit which will draw in a little bit and then feed the motor with a three-phase motor with a 3 phase circuit it turns out that you end up with a lot less vibrations and a lot more efficient transfer of energy into the motor so that's a really big deal if you're building you know anything from a compressor in a plant to a compressor on a tiny little refrigerator which is a motor in your in your house or you know anything with a rotating shaft where it's a heavy bit of machinery not a tiny little motor then saving on vibration is going to is a huge thing I mean if you can make a car vibrate less or a spacecraft vibrate less or whatever vibrate less that's a huge deal so if there's no downside to it then there's a very good reason to go ahead and use three-phase power to do that now we're gonna draw lots of sinusoids and stuff in a minute but in a block diagram type of sense what you have on the left hand side is basically a block we're not going to talk yet about what is inside it but we will and this is a three phase source I want you to kind of get used to seeing this terminology this phi here this means phase so when I say three phase I might write it out I might also say three phase like three Fudd's it's a kind of shorthand notation so that's a 3 phase source and then you have a block over here which we don't know what it is but it's probably a motor for instance we've been talking about motors so this is a three phase load right so again it could be a motor could be something else and instead of in the typical case when we have a single a single phase usually you'll have those two lines we've talked about and we've talked about how AC works in the past so you know current going in going out going in going out we've been talking about that for a long time but in the three phase case you'll actually have another can another connection between them like this so actually have three sets of wires now there's actually other things called neutral and other things we'll talk about that a little bit later I'm talking about from a block diagram point of view you basically have three connections and they are going to correspond to the three different phases of your source connected to into the different receptor terminals of the load the three phase terminals of the load and we'll call this thing the three-phase line now the reason I'm drawing this Hokie diagram isn't so because it's a fancy circuit it's because we are going to rapidly get into the situation where I'm talking to you about the line current and the line voltage the three-phase line current you need to know that that is representing the lines connecting the source to the load we're also going to be talking about obviously loads we're gonna talk a whole lot pretty soon about three-phase sources how they're connected there's different ways to connect them there's there's a delta connection there's a Wye connection there's other things that you can talk about but for right now don't worry if it's a certain kind of connection in here all you need to worry about is there's three lines coming out which is true with any true of any three face source and three lines going into a load that's always going to be true we're gonna be drilling into the details inside of these blocks as we go through the lessons okay now if you remember when we talked about sinusoidal sources it's we use phasers right so we have a phase angle and you know typically we when we're doing regular phasor analysis the source just has a phase a a zero because and it's not because it's special it's just because since it's the source it's kind of the natural reference for everything else in the circuit you're driving the source so you typically say that's a zero phase it just means that the cosine starts at the zero point right and everything else is measured relative to the source but now we don't have a single source we actually have three sources so because we have three phases each one of these is basically a separate source driving out but there's a special relationship between these sources they're not just random they're special and the special relationship is that these sources are have a very special relationship with their phase angles right so since if you remember back to phases if you have a phasor you can have zero degree you can have 10 degree 20 degrees 30 degree it just means the cosine is shifting where it starts but since there's only 360 degrees in a unit circle really your phase angle can only go between 0 and 360 degrees of course you can measure negative angles going the other way that but they can always be translated into a positive angle 0 to 360 degrees right so in a 3 phase circuit these the punchline here is that these phases are separated by since there's only 360 degrees in a unit circle and we have three phases and we want equal separation and the phase angles that's what I haven't told you yet then you get 120 degrees so 120 degrees is the separation between phases and between three phase phases right so that's important so it doesn't matter if it's a later on we'll talk about it if it's a Delta source or a Y source or a delta load or a Y load or mix and match Delta over here Y over here doesn't matter these guys here the three phases connecting source and load are always separated by a hundred and twenty degrees because a unit circle can be separated three ways in an equal fashion by 120 degrees that's always true so just remember it so phase a you might call it at zero right but that means phase B has got to be plus or minus 120 degrees away and so on from that and we're gonna draw a lot more pictures to illustrate this point but it's just important for you to know that each of these phases are separated from the other ones by 120 degrees equally that is the special relationship that makes it a 3 phase source if this is zero degrees and then phase 2 is like 25 degrees that's not a 3 phase 4 it's not a balanced three-phase source I should say it that way we call it balanced whenever it's 120 degrees I guess you could have a 3 phase source that has unbalanced in the phase angles but in practice we don't do we don't ever use that at least not for typical applications we're always talking about balance three phase circuits almost always talking about balance 3 phase circuits where the phases are separated by 120 degrees with respect to one another so let's go over here and illustrate that a little bit more so the simplest case is we label the phases we label them phase a phase B and phase C and that's gonna be universally true phase a phase B phase C so for instance this might be called phase a this might be called phase B this might be called phase C so the voltage at phase a or the voltage of phase a the line over here that we're calling phase a is in general form it's gonna be some magnitude V M times the cosine this is not the phasor notation this is just the the time domain notation Omega T plus some phase angle but phase a were typically gonna always call it zero degrees because we have to have some reference and we always almost always take phase a to be that reference okay so here because it's zero degrees this is implied as the reference phase everything else is related to the other two that one all right now this is the time domain representation of this and this means that the phasor notation would mean let me go ahead and give myself a little bit of room here the phasor notation VA is going to be equal to the magnitude at a phase angle of zero degrees nothing rocket science here I mean we've been doing this for four you know many chapters now we have a time domain representation it's a frequency and a phase angle with a magnitude and we can transform that to the phasor domain and basically we just know that the the the frequency is going to be whatever it is so we'll have to write it in our phasor we just keep track of the magnitude in the face the phase for phase a is is always zero that's what it's going to be in all of these problems anyway right so then the question is if this is phase a what would phase B be and we already said that the phases are separated by 120 degrees so phase B is going to be the same magnitude because we're talking about balanced three-phase circuits so every everything about these phases is the same the frequency the magnitude of the same the only real difference is the phase angle between them is what's different so we have the same magnitude times the cosine of Omega T and then I have to ask myself what is going to be the phase angle of phase B now we know it has to be different for phase a by 120 degrees so we have a choice we could say your first reaction actually would be to just put 120 plus 120 degrees there but actually initially in the problems here you'll see well why in a little bit we're gonna take phase B to be at minus 120 degrees okay and I'm gonna draw a picture to kind of help you with that for now just let me get through the rest of the lesson then I'll kind of explain a little bit more why we know we don't call it positive 120 here we call it negative but anyway phase B is separated by phase a by 120 degrees it's just that we went in the negative direction so this phasor is gonna be phasor B is equal to the magnitude and an angle of negative 120 degrees right that's what we're gonna have now if this is phase a and if this is phase B then what do we do for face e what do we write now for phase C well it's the same magnitude for all three V M times the cosine of Omega T and then I have to ask myself what's the phase angle gonna be forget about looking at this right now just kind of go back up to the top well if this is separated here by 120 degrees but kind of in the negative sense then this one also must be separated from phase a again by 120 degrees but since I've already gone the negative direction for phase B the only other way I can go to still get 122 separation is to make it positive 120 degrees right and then we write the phasor representation of that as phase C is equal to whatever the magnitude is at an angle 120 degrees like that so I'm gonna draw a phasor diagram to kind of show you how these are related to each other so don't freak out too much but I just want you to know that phase a is always taken to be the reference typically phase B is a negative phase angle relative to phase a separated by 120 degrees phase C is again because everything is measured relative to phase a it's 120 degrees but up and that way everything is separated by 120 degrees but just show you more graphically I'm going to draw something on the board call the phasor diagram to kind of show you this it's really simple but it's it's helpful and you're gonna actually find yourself when we're solving our three phase circuits down the road when you have an actual circuit in for anything you're trying to figure out the phase relationship you're gonna come back I'm going to show you as I worked these problems at we're gonna be drawing this diagram that I'm gonna draw in a second a lot so it's not hard diagram it's just a useful concept so what we're gonna do is we're gonna represent we're gonna call this a phasor diagram I'm go and write that down phase or diary of this balanced three-phase circuit is what we're drawing balanced three-phase circuit so what we're gonna do is we're gonna write them all as phasers if you if you remember back can be represented as a as a vector as it really as a rotating vector but let's just draw it in the static sense like this so this guy we're gonna call VA and we're gonna call that the reference right and this is at 0 degrees why is it here why is it pointing get it kind of imagine you know let me gonna in pink or something when we just kind of like refresh your memory that this is this is like an XY plane I guess like this this is kind of goes like this and I'm not going to cover up the arrow here but it kind of goes through the other side so you could think of this as being X this is being Y right so why is phaser a lying on the axis like that it's because when we come back here and we look at it it's some magnitude at an angle of 0 degrees so this is a coordinate plane where the angle of the vector is the egg the phase angle that we're talking about and the magnitude is the length of this arrow so I could put VM here I guess I'm gonna go and do that p.m. here it's shows it's the length of the arrow in the direction that's pointing phase a always points at 0 degrees now when we go over here we said well phase B is the same magnitude of course but at an angle of negative 120 degrees now if you look over here this arrow is pointing at 0 degrees this is negative angle these are negative angles going this way and these are positive angles going this way you remember all that stuff from trigonometry so if you go this direction this is actually negative 90 degrees if you were to go all the way here this would be negative 180 degrees actually we don't want to go that far we want to go to negative 120 degrees that's gonna be right about there because this would be a little bit too far so basically it's 90 plus 30 more right 30 more degrees so it's gonna be about there something like that so let's go ahead and try our best to do what we can to make this a nice 30 degree angle from that y-axis there and let's go ahead and draw it should be the same length I'm not gonna be able to get it perfect but it should be about the same length maybe a little shorter like this okay so this is gonna be VB right and I'm going to illustrate on here only to make a different color that the angle here as a measure going like this is negative 120 degrees make sure you can convince yourself of that so VB lies right here because we start as a reference at phase a negative 120 degrees that's where phase B is the length of this I think I drew a little bit longer but you gotta use your imagination it's the same length of the vectors as we go here it's hard a little hard for me to see since I'm standing off to the side but I'm pretty close now let's look at phase C they see is the same magnitude but at a positive angle of 120 again measured relative to the reference which is phase a so that would be 90 degrees here to get to positive 120 is gonna be just 30 degrees passed here so if I were to draw that one we go back to here let me see if I'm gonna do a good job of that try to make it a consistent 30 degrees or something sort of like it sort of like the same length here that's pretty close and I'll call that the sub C and just to remind you this angle is positive 120 degrees now I want you to look at this diagram and it's actually really powerful because when I look at this over here see zero negative 120 positive 120 and I I can tell sort of from the math that they're all separated well let me put it another way it's clear that B is separated from a by 120 and it's clear that C is separated by a from 120 but until you actually do math and subtraction it's not totally clear how B and C are separated for instance but when you look at the diagram you can very easily tell that a is separated by C by 120 degrees a is separated by B by 120 degrees but also if you look at the angle here if you got a protractor from this one to this one you can see graphically that this is also 120 degrees also so I'm gonna kind of draw in a weird color I'm gonna kind of come out here and just kind of like do something like this this is also 120 degrees kind of put a double arrow so not only is a separated by B by 120 and a separated by C by hundred 20 but also B and C by implied nature of the unit circle and 360 degrees divided by 3 and when you actually draw it out B and C are also separated by again 120 degrees this is why it's called a balanced three-phase circuit because balanced three-phase circuit because the magnitudes of these vectors are all the same I tried to draw them the same the phase angles are constructed so that they're all equally separated from each other that is what makes it balanced and that's ultimately what gives us its usefulness now if you wanted to look at it graphically you can kind of see that it's 120 here but mathematically sometimes you know we like to prove things even though I know that you know this is 120 you have to be a little careful though if you want to figure out what the angle here is in between these vectors your first gut might be to take the minus 120 and then subtract something like 120 but it gets a little confusing because some of the angles are negative and some of the angles are positive so if you really want to prove to yourself that this is actually 120 degrees here's how you do it I measured this as negative 120 that's the convention we use but I also know that negative 120 degrees okay if you add a hundred and if you add 360 degrees to that you're gonna in other words if you start from here and you go all the way back to where you are by adding 360 degrees to get a positive angle you're gonna end up with 240 degrees so this angle down here this one here I label it as minus 120 that's our convention but I also know that in terms of positive angles it's 240 degrees if I were to measure it around like this from this direction is 240 if you take 240 and you subtract the 120 from here so you're dealing just with positive angles why I'm doing it this way 240 minus 120 you can see is again this separation here it's um 120 so I'm just proving that to you you don't have to do that ever again but that's what I'm showing you so what I wanted to illustrate in this section is very simple things three phase circuits are useful in rotating machines for less vibration they're useful for very high voltage high power applications and they basically consist of a source and a load and they're connected by what we call a three-phase line so we're gonna have line current line voltages source current source voltages load currents and load voltages all throughout this class we're going to define all that stuff as we go the unit circle can be divided into equal 120 degree pies or sections so we define each of these phases as being exactly the same same magnitude same frequency just simply a separation in their phase angle and so here we have 0 as a reference and we have B as negative 120 we have C as positive 120 and when we draw that out we see that every phase is separated by a balanced equal number of degrees of 120 degrees all right one more thing I want to say before I leave when before we go to the next section and get into some more details when we talk about this guy here where the phase angle of B is negative 120 and the phase angle of C is positive 20 this specific arrangement where we've drawn it like this where a is here B is here and C is here I'm gonna get into it in more detail that's called a positive phase sequence there's actually another phase sequence called a negative phase sequence we're gonna get into that later so for those of you who know a little bit about three-phase power already and you're like well he hasn't talked about negative phase sequence yet just hang on a second I'm just getting you good used to this stuff I promise I'm gonna introduce negative phase sequence in a minute it's not hard I promise you that but you will have problems that have positive phase sequence and negative phase sequence to deal with on top of everything else so we're gonna get there when we get there I just want to make sure you know that this is most cases most problems you're gonna have this positive sequence arrangement and in some cases will have a negative sequence we'll get to in a little bit so follow me on to the next lesson and we're gonna talk about one a little bit more detail about why three-phase power is so useful for rotating devices

Channel: Math and Science

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Keywords: 3 phase, 3 phase power, 3 phase electricity, three phase, three phase power, three phase electricity, 3 phase motor, 3 phase wiring, 3 phase power explained, three phase power explained, three phase circuits, three phase system, three phase generator, 3 phase circuit, 3 phase circuits, 3 phase circuit analysis, 3 phase circuits problems, 3 phase tutorial, 3 phase system, single phase, alternating current, circuit analysis, electrical engineering, what is 3 phase power

Id: 310jd_DxH2s

Length: 22min 38sec (1358 seconds)

Published: Thu Apr 12 2018

Reddit Comments

[u/UsernameHasBeenLost]

I wish this dude had been around when I was in college. I struggled with electrical shit, partly because I couldn't understand the professor's accent, partly because she was one of those smart people that can't break things down for normal people to comprehend