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

Hello, welcome to this AC Circuit Analysis Tutor lesson, the title of this lesson opens a new branch, a new topic of discussion in this section of electrical engineering about

three

-

phase

electricity

or

three

-

phase

power

, we also call it three-phase circuits, like this that 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

three-phase electricity actually is. I want you to know that you need to realize that when you get into three-phase circuits a lot of people have problems because it gets confusing, it can get confusing quickly because we've previously done analysis with sinusoidal AC signals running through the circuits, but in all of those cases. , you only had one source and therefore a stimulating source passing through and flowing through the circuit to handle three phase electricity well, we will have multiple sources and the difference between them will be the phase angle or the relationship between those input signals and there are very good reasons why we do it, we're going to discuss those reasons here in just a minute, but it can be a little overwhelming at first, so

what

we're going to do is take them a thought out or step by step process where first in This lesson I'm going to tell you what three-phase electricity is, in the next section I'm going to explain a little more why three-phase electricity is really useful so you can keep that in the back of your mind and you can figure it out as we do the math. and we get 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 really useful and then We'll go through each topic step by step, we'll talk about how to write three-phase supplies, how to analyze basic three-phase circuits and we will build our complexity from there, so that will be our roadmap, it will be quite long. process but I promise if you stick with me it won't be difficult because once you get the basic information and basic knowledge and work with me through the process every step along the way will be pretty easy so this lesson is what is three phase electricity I wanted a general section I'm not going to write a lot on the board we're going to write some things unfortunately I'm going to have to read you some things it's a little bit it's a little bit faster than just writing everything on the board three-phase energy electricity for 3 days is really good for many things, but it's really good for a certain subset of things that we use it for quite frequently.

This is the punch line here, why we use This is good for generation, transmission, distribution of large amounts of energy. Well, I say large amounts of energy. I'm talking about a power generating station like a power plant or something to carry a huge amount of power from point A to point A. B, it turns out that when you look at the theory, it's done efficiently with three-phase electricity and the details of why you'll get into that as you study this a little bit more, but you should know that it's for high power, high voltage situations. It is often much easier and more efficient to do this in a three-phase circuit arrangement.

The following is something I'll expand on a bit more in the next section. It's very, very good. Three-phase electricity. Three-phase power. Very very good. to power electric motors and what I mean by that when I say it's good at it, what I'm saying is that a motor is a rotating object, so when you feed it with a single sinusoid many times you end up getting vibrations in the motor and actually I'm going to expand on this a little bit in the next section when you feed it with a single signal when you say simple sinusoid, but if you were to arrange a three phase circuit that would draw a little bit and then feed the motor with a three phase motor with a three phase circuit, It turns out you end up with a lot less vibrations and a lot more efficient energy transfer to the motor, so that's really important if you're building, you know? anything from a compressor in a plant to a compressor in a small refrigerator that's a motor in your house or you know, anything with a rotating shaft where it's heavy machinery, not a small motor, then you save on vibration.

I mean, if you can make a car vibrate less or a spaceship vibrate less or anything that vibrates less, that's a big deal, so if there's no downside to that, then there's a very good reason to go ahead and use energy. triphasic. To do that now we're going to draw a lot of sinusoids and stuff in a minute, but in a block diagram type sense, what you have on the left side is basically a block, we're not going to talk about what's inside it yet. but we will and this is a three phase source. I want you to get used to seeing this terminology, this phi here means phase, so when I say three phases, I could write it down.

You could also say three phase, like three Fudd's, it's kind of a shorthand notation, so that's a three phase supply and then you have a block here that we don't know what it is, but it's probably a motor, for example, we've been talking about motors, so this is a three phase load so Again, it could be a motor, it could be something else and instead of the typical case when we have a single phase, you'll usually have those two lines that we've talked about and we've talked about how it works the CA in the past. So now you know the current that goes in, out, out, we've been talking about that for a long time, but in the three phase case, you're actually going to have another can, another connection between them like this, so now you have three sets of wires. .

Actually, there are other things called neutrals and other things that we will talk about a little later. I speak from the point of view of the block diagram. Basically you have three connections and they will correspond to the three different phases of your source. connected to the different receiving terminals of the load, the three phase terminals of the load and now we will call this three phase line, the reason I am drawing this Hokie diagram is not because it is an elegant circuit, it is because Let's quickly get into the situation where I talk to you about line current and line voltage.

The three-phase line current. You need to know that that represents the lines that connect the source to the load. We are also obviously going to talk about a lot of things, very soon we will talk a lot about three-phase sources, how they are connected, there are different ways to connect them, there is a delta connection, there is a star connection, there are other things you can talk about. but for now don't worry if there is a certain type of connection here, the only thing you need to worry about is three lines coming out, which is true for any three sided three line source going into a load that is always running .

To be true, we're going to delve into the details within these blocks as we progress through the lessons. Okay, now if you remember when we talked about sinusoidal sources, we use phasers to have a phase angle and usually you know when We're doing regular phasor analysis, the source only has zero phase because, and it's not because it's special , it's simply because since it's the source, it's kind of a natural reference for everything else in the circuit that you're driving the source in, so normally we'd say it's zero phase, it just means that the cosine starts at the zero point on the right and everything else is measured relative to the source, but now we don't have a single source, we actually have three sources, because we have three phases each of These are basically a separate source coming out, but there are a special relationship between these sources, they are not just random, they are special and the special relationship is that these sources have a very special relationship with their phase angles, because if you remember Back to the phases, if you have a phasor, you can have zero degrees, you can have 10 degrees, 20 degrees, 30 degrees, it just means that the cosine is shifting where it starts, but since there are only 360 degrees in a unit circle, actually your phase angle can only go between 0 and 360 degrees Of course, you can measure negative angles in the opposite direction, but they can always be translated to a positive angle from 0 to 360 degrees, so in a three-phase circuit, the punch line here is that these phases are separated because there are 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, so you get 120 degrees, so 120 degrees is the separation between phases and between three phases, that's right. that's important, so it doesn't matter if it's later we'll talk about it if it's a Delta source or an Y source or a delta load or an Y load or mix and match Delta over here And over here it doesn't matter These guys, here, the three phases connecting the source and the load are always separated by one hundred and twenty degrees because a unit circle can be separated three ways equally by 120 degrees, that is always true, so remember that, so phase a can be called in zero to the right, but that means that phase B has to be about 120 degrees away and so on.

We will do many more drawings to illustrate this point, but it is important that you know that each of these phases is separated from the others by 120 degrees equally, that is the special relationship that makes it a three-phase source if it is zero degrees and then the Phase 2 is like 25 degrees, that's not a 3, 4 phase, it's not a balanced three phase source. Should say it that way, we call it balanced as long as it's 120 degrees. I suppose you could have a three phase supply that is unbalanced in phase angles, but in practice we don't, we never use it at least not for typical applications.

We're always talking about three-phase balancing circuits, we're almost always talking about three-phase balancing circuits where the phases are 120 degrees apart from each other, so let's go here and illustrate that a little bit more so the simplest case is to label the phases we label them. phase a phase B and phase C and that will be universally true phase a phase B phase C so for example this could be called phase a this could be called phase B this could be called phase C so the voltage on phase a or the phase voltage a, the line here that we call phase a is in general form, it will be a magnitude V M multiplied by the cosine, this is not phasor notation, it is just time domain notation Omega T plus some phase angle. but normally we will always call the a phase zero degrees because we have to have some reference and we almost always take the a phase as that reference, so here, because it's zero degrees, this is implied as the reference phase, everything else is related to the other two, that, 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 space here, the VA phasor notation will be equal to the magnitude at a zero degree phase angle, nothing scientific 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 we basically just know that the frequency is going to be whatever, so we're going to have to write it into our phasor, we just keep track of the magnitude on the face, the phase for phase a is always zero, that's what it is.

We'll be in all these problems anyway, so 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 will have the same magnitude because we are talking about balanced three-phase circuits, so everything related to these phases is the same, the frequency, the magnitude of them , the only real difference is the phase angle between them, which is different, so we have the same magnitude multiplied by the cosine of Omega T and So I have to wonder what the phase angle of phase B will be.

Now we know which has to be different for phase A at 120 degrees, so we have the option. We could say that your first reaction would actually be to simply put in 120 more. 120 degrees there, but actually initially in the problems here you'll see well why in a moment we're going to take phase B to be at -120 degrees, okay and I'm going to make a drawing to help you with that. Now let me finish the rest of the lesson and then I'll explain a little bit more why we know we don't call it positive 120 here we call it negative but anyway the B phase is separated by the a phase by 120 degrees. it's just that we went in the negative direction, so this phasor will be phasor B, it's equal to magnitude and a negative angle of 120 degrees, that's what we'll have now if this is phase a and if it's phase B, So what do we do for the e face?

What do we write now for phase C? Well, it's the same magnitude for all three V M multiplied by the cosine of Omega T and then I have to wonder what the phase angle is going to be. Forget looking at this right now. just go back to the top, if this is separated here by 120 degrees, but in the negative direction, then this must also be separated from phase a again by 120 degrees, but since I have already gone in the negative direction phase B, the only otherway I can go on to get 122 separation is to make it positive 120 degrees to the right and then we write the phasor representation of that as phase C is equal to any magnitude is at a 120 degree angle like this, so I'm going to draw a phasor diagram to show you how they relate to each other, so don't be too scared, but I just want you to know that phase a is always It is taken as a reference, usually 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 120 degrees apart, but it just shows To be more graphic, I'm going to draw something on the board, I'm going to call the phasor diagram to show you that this is really simple but useful and will really come across when we are solving our three phase circuits in the future. When you have a real circuit for anything, you're trying to figure out the phase relationship, you'll come back. I'll show you how I worked these problems on, we'll draw this diagram that I'm I'm going to draw a lot in a second, so it's not a difficult diagram, it's just a useful concept, so what we're going to do is represent it, we'll call it a phasor diagram.

I'm going to write it in phase or journal. From this balanced three-phase circuit we are drawing a balanced three-phase circuit, so what we are going to do is write them all as phases. If you remember, it can be represented as a vector like. It's really like a rotating vector, but let's draw it in the static sense like this, so we'll call this guy VA and we'll call it the right reference and this is at 0 degrees, why is it here? Why do you point? Get it, imagine, you know, let me go in pink or something, when we just refresh your memory, this is like an XY plane, I guess this is something like that and I'm not going to do it.

It covers the arrow here but goes through the other side, so you could think of this as X, this is Y, right? So why is Phaser A on the axis like that? It's because when we come back here and look at It's a magnitude at a 0 degree angle, so this is a coordinate plane where the vector angle is the egg, the phase angle we're talking about and the magnitude is the length of this arrow, so I could put VM here. I guess I'll go do that in the afternoon. here it shows that it is the length of the arrow in the direction that phase a always points at 0 degrees now, when we go here we said well, phase B has the same magnitude of course, but at a negative angle of 120 degrees now if you look here this arrow points to 0 degrees this is a negative angle these are negative angles going in this direction and these are positive angles going in this direction remember all that trigonometry stuff so if you go in this direction this actually it's negative 90 degrees if you were to get this far, this would be negative 180 degrees, we don't really want to go that far, we want to get to negative 120 degrees, that's going to be right there because this would be too much, so basically it's 90 degrees plus. 30 more to the right, 30 degrees more, so it's going to be something like that, so let's go ahead and do our best to do what we can to make this a nice 30 degree angle from that axis and there and let's go ahead and draw it . be the same length, I'm not going to be able to make it perfect, but it should be about the same length, maybe a little bit shorter like this, okay, so this will be VB right and I'm going to illustrate it here just to make a different color that the angle here as a measurement is negative 120 degrees, make sure you can convince yourself of that, so VB is right here because we start for reference at the negative 120 degree phase, that's where phase B is the length of this .

I think I drew a little more, but you have to use your imagination. It is the same length of the vectors that we are going here. It's a little hard for me to see since I'm standing to the side, but I'm pretty close now. let's look at phase C that you see has the same magnitude but at a positive angle of 120 again measured in relation to the reference which is phase a, so here it would be 90 degrees to get to positive 120, only 30 degrees will pass here, like this what if If I were to draw that one that we come back to here, let me see if I'm going to do a good job, try to make it consistent at 30 degrees or something like the same length here which is pretty close and I'll call it sub C and just to remind you that this angle is positive 120 degrees, now I want you to look at this diagram and it's actually very powerful because when I look at this here I see zero negative 120 positive 120 and I can say from the math that they are all separate, well let me put it another way, It is clear that B is separated from a by 120 and it is clear that C is separated by a from 120, but until you do math and subtraction it is not.

It's totally clear how B and C are separated, for example, but when you look at the diagram you can easily see that a is separated by C by 120 degrees and a is separated by B by 120 degrees, but also if you look at the angle here if I have a protractor from this to this. You can see graphically that this is also 120 degrees, so I'm going to draw in a strange color. I'm going to go out here and do something like this. this is also 120 degrees, that is, put a double arrow, so it is not only separated by B by 120 and by C by percent 20, but also by B and C by the implicit nature of the unit circle and 360 degrees divided by 3 and when in reality, draw it B and C are also separated again by 120 degrees, that is why it is called balanced three-phase circuit because the balanced three-phase circuit because the magnitudes of these vectors are all equal, I tried to draw them equal in the phase.

The angles are constructed so that they are all equally spaced from each other, that is what balances them and that is ultimately what gives us their usefulness now. If you wanted to look at it graphically, you can see that here it's 120, but mathematically sometimes I know we like to try things, although I know you know this is 120, you have to be a little careful, although if you want to find out what the angle is here between these vectors, your first instinct might be to take minus 120 and then subtract something like 120, but it gets a little confusing because some of the angles are negative and some are positive, so if you really want to prove to yourself that it's actually 120 degrees , that's how you do it.

I measured it as negative. 120 that's the convention we use, but I also know that negative 120 degrees is fine if you add a hundred and if you add 360 degrees, in other words, if you start from here and go back to where it is adding 360 degrees to get a positive angle, you're going to 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 If I measured it like this from this direction it's 240, if you take 240 and you subtract 120 from here, then you're dealing with positive angles only.

Why do I do it this way? 240 minus 120, you can see it's this again. The separation here is um 120, so I'm just showing you that 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 are very simple things, three phase circuits. they are useful. in rotating machines to reduce vibration, they are useful for very high voltage and high power applications and basically consist of a source and a load and are connected by what we call a three-phase line, so we will have a line current voltages source current source voltages load currents and load voltages throughout this class we're going to define all of that as we go, the unit circle can be divided into equal 120 degree sections or sectors, so we define each of these phases as exactly the same magnitude, same frequency, just a separation in their phase angle, so here we have 0 as a reference and we have B as negative 120, we have C as positive 120 and when we extract that we see that each phase is separated by a balanced equal number of degrees of 120 degrees all right, one more thing I want to say before I go, before we move on to the next section and go into more detail 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 have drawn it like this where a is here B is here and C is here I'm going to go into it in more detail, it's called positive phase sequence, actually there is another The sequence of phases is called negative phase sequence, we'll talk about that later, so for those of you who already know a little bit about triphasic energy and are like, well, he hasn't talked about negative phase sequence yet, just wait.

Wait a second, I'm just getting you used to these things. I promise I'm going to introduce a negative phase sequence in a minute. It's not difficult, I promise, but you will have problems dealing with a positive phase sequence and a negative phase sequence. Plus everything else, so we'll get there when we get there. I just want to make sure you know that in most cases, in most problems, you will have this positive sequence arrangement and in some cases you will have a negative sequence. We'll get there in a moment, so follow me in the next lesson and we'll talk in a little more detail about why three-phase power is so useful for rotating devices.