 Hi, I'm Zor. Welcome to Inezor education. Today we will continue talking about alternating current about kind of specific type of alternating current, which is called three-phase alternating current. Now, this is, well, the most practical part of this AC current business. So, we will talk about how three-phase AC generated and how it's used, how it's transmitted, actually, and how it's used. Okay, now this lecture is part of the course called Physics for Teens, and I will be referring to other lectures in this one. Well, all lectures are really kind of interconnected, so I do recommend you to watch the lecture from this website from Inezor.com because it's the whole course where it's presented, and the course is arranged in some logical order. There are notes for each lecture, with nice pictures, at least wherever it's necessary. There are some problems solved. There are exams for those who want to take them. The course is completely free. The whole website is free, and there are some other courses like math for teens, for instance, which is a prerequisite for this one. You do have to know math before you study physics. Okay, back to three-phase alternating current. Okay, so first of all, I would like you to recall the process of generating AC, the most basic starting point from which we basically started talking about AC. We had permanent magnet, and we had a frame, wire frame, rotated around axis, which is perpendicular to magnetic lines, magnetic field lines. Now, so whenever it was rotating, we had AC generated in this frame. Now, why is it generated? Well, there is Lorentz force, if you remember. So, if you have two magnets, these are magnetic field lines, and you have, let's say, a wire, which is moving perpendicularly to this one, like this. The electrons inside that wire are moving with the wire, right? And Lorentz force is the force which is acting from the magnetic field onto the electrons. So, the electrons, now, if the wire is moving this way, the electrons are moving this way, and the Lorentz force is acting perpendicularly to the movement of the electrons, and perpendicular to the field. Now, what is the perpendicular to this direction and this direction? Well, that's the direction of the wire. So, electrons are moving towards one end of the wire. So, while we are moving the wire this way, electrons are moving to one end of the wire. Well, this or that, depending on whether we are moving this way or that way, right? So, this is the Lorentz force. Now, whenever the Lorentz force exists, well, if electrons are moving within the wire, that means that there is an electricity in it, there is some kind of electric current. So, if we are moving back in force, the electrons will move along the wire, up and down in this particular case. Well, in this case, we have a slightly different, mechanically different, but electrically it's exactly the same thing, because whenever this thing is rotating, this frame, this part goes that way, this part goes this way, so they're both crossing, which means that the electrons are moving in one direction in one, half of this frame, wire frame, and since the opposite end moves to a different direction, one goes this way and the other goes that way. So, electrons are moving in a different direction, so if it's up here, down here, and that's the circle. And then, when we are rotating all the time, now this thing, right now, it goes this way, but when it goes 180 degree, it will go that way, so the electrons will go to the opposite direction. So, as we are rotating the wire, the electrons are going into one end and then another end, one end and another end, depending on the speed of rotation. I mean, synchronously with the speed of rotation. So, this is how alternating current is generated in a simple basic experiment. Well, and obviously the name alternating relates to the fact that the electrons are moving along the wire in one direction when it moves this way and then another direction when this side goes on this side and goes this way. So, that's alternating current generated. This is a very, very basic thing and we can actually connect these things to outside consumer of electricity and like a lamp, for instance, this is a lamp, it will do it, right? Well, it's the proper voltage, etc., is generated. Okay, so it looks like we have converted mechanical energy rotation into AC, into alternating electric current. Great. Now, the strengths of this current or electromotive force generated, which is basically voltage generated by this rotation will depend on how we are turning that thing, right? Look at this this way. If my frame is in this position, it crosses when it's rotating, it crosses practically perpendicular to the magnetic field lines. If, however, my frame is in this position, perpendicular to the lines of magnetic field. Now, if this is moving, it's moving this way, right? When it's moving this way, the wires and this and that side and the further and the closer side are practically don't really cross magnetic field lines, right? So, in this position, we are crossing at a maximum speed. In this position, the crossing would be minimum. Well, and again, it all depends on very simple physical laws. We know the Faraday's law when it's basically when the EMF is proportional to speed of the changing of the magnetic flux. And in this case, my speed is very, very fast, actually, because right now it's practically zero. And then from zero we started increasing the area, which is crossing the magnetic field lines. So, and when I'm in this position, so all the magnetic field lines are going through it, but when I'm changing a little bit, it almost doesn't really change it, meaning the relative sense. So, basically, EMF generated by this rotation depends on the angle. And how does it depend? Well, we did discuss this issue many times. It's some kind of a maximum times sine of omega t, where omega is angular speed of rotation. Assuming that time t, for instance, we have this position, I think, right? And the EMF is not this position, I guess. Well, it doesn't really matter. I mean, as the time goes, it's still rotating. Wherever you start, it doesn't really matter. And the graph of my EMF would be like this, where this is E max, maximum. Okay, so that's done. Now, the first obvious improvement to this is, why do we have to have one rotating frame? Why don't we have two of them? So, let me show it as the top view. As the top view, it will be, this is one frame. So, frame is like this. And it's rotating this way. And another is perpendicular. So, you have two frames. Well, if we have two frames, then we apparently have twice as much electricity, right? Because each one of them generates this. Now, we have two of them. Now, the only difference is that if this is one of them, let's call it E1. E2 will also be the same sinusoidal kind. But the sinusoid will be shifted by 90 degree pi over 2. Why? Well, because whenever this E1, for instance, generated in this position, this will take this position after it turns by pi over 2, right? So, whenever the angle is turned by pi over 2, this should produce exactly the same as this one. Or minus. It depends on which direction it rotates. It doesn't really matter. Plus or minus, it's actually the same thing. Okay, fine. So, we kind of improved our design, right? And now we have twice as much electricity, which is good, right? Why stop there? Let's do three. We have three frames. One, two, and three at 120 degree from each other. So, if we will position three frames like this, we will have three times more electricity, right? So, the only difference is the shift would be, instead of pi over 2, it will be 120 degree. 2 pi over 3. And the 3 would be 4 pi over 3. Which is actually equal to, the period is 2 pi, right? If I will add 2 pi or 6 pi over 3, it would be omega t plus 2 pi over 3. So, we have three different things. Now, graphically, one of them would be like this. Now, another one would be something like this, I guess. And the third one would be like this, something like this. And we will have six wires from three different frames. Each frame has two wires. So, I will have, let's say, if this is A1, and this is B1, and this is C1, this is A2, B2, and C2. So, I have two ends, A1, A2 from one frame. And it goes along one sinusoidal EMF pair from B1, B2 from another one. It would be this one, shifted by minus 2 pi over 3. And the third one would be shifted by minus 4 over 3 or plus 2 pi over 3. We have six wires. Well, I mean, we can obviously increase it, but it's kind of too dense already. So, we shouldn't really increase anymore. Practically, people are using three. And that's why it's called three-phase AC. Now, this shift is called a phase shift. This is, this piece under the sinusoid is called the phase. So, this is the phase minus 2 pi over 3. This is the phase minus 4 over 3. Shift it. Phase is shifted. Three-phase, three different shifts. Zero to pi over 3 and 4 pi over 3. Or minus 2 pi over 3 and plus 2 pi over 3. It doesn't matter. Okay. So, we have three-phase electricity generated. This is the basic principle. And that's the end of the basic principle because the practicality is slightly different. Now, from practical reasons, it's inconvenient to have the frames inside. Why? Because they're moving. How can you take the electricity from the moving frame? You have to have some kind of a commutator, whatever it is, with brushes. It's not easy, actually, from the moving wire to take the electricity in a nice fashion. So, obviously, the design of the real thing is slightly different. And we have already thought about this when we were designing DC direct current motor. We had actually all these coils as a stator and the magnet as a rotor, as a rotating thing. So, in this case, in this basic design, our magnets are fixed and the coils are rotating. I'm talking about coils because, obviously, the frame is not just a single wire. It's a coil of wires. Why? Because every turn of the coil generates something, some electricity. And the more coils you put, the more electricity you generate. Okay, so we have three coils in this particular case. And instead of putting coils on the rotor, which is a rotating thing, and magnets on the stator, we'll do it other way around. How? Very easily. We will have one coil. This is A1 and this is A2. Then we will have another coil with B1. Well, I should actually say the third one. This would be B1, B2. This would be C1, C2. So, that's how it corresponds to this picture. And we will put the magnet here, rotating. So, if I will rotate magnet, it will be exactly the same effect relative to the coils, because what's important? Important is relative movement of the wire in the magnetic field. So, the wire should really cross the magnetic field lines. And obviously, if I'm rotating that thing, I will have exactly what's necessary. So, the relative movement between the magnetic field and the coil will create electric current in this coil. And again, it will be exactly the same thing, because magnetic field will be shifted. And from A to B, it will be 120 degrees to pi over 3. And from B to C would be another, etc. So, this is actually a design. Okay, so that's the first step which people did when they were designing. Well, people, Nikola Tesla. I mean, that's what's important. The next thing, which was actually, again, another improvement. You see, now we have six wires, A1, A2, B1, B2 and C1, C2. What happens, and this is, again, kind of an ingenious idea, what happens if you will connect together. So, this, this and this. We will connect together in one point. Well, we know what EMF is in each coil, right? So, we will just have to add them together, because whenever we are connecting them, EMF is supposed to be added. So, if I will add E1 plus E2 plus E3, E1, E2, E3, maximum times sin, the same maximum times shifted by minus pi over 3 and the same shifted by plus pi over 3. Well, in the notes for this lecture, I put very detailed calculations and the result is zero. That's extremely important thing. And quite frankly, when I first learned about this, I didn't even expect it, but then I just did the calculation and yes, indeed, if you add them together, it will be identically zero. I mean, everything cancels out. If you will open sin of two angles like sin of this times cosine of that, cosine of this times sin of that, etc., etc., you will open up everything called parentheses. Everything cancels out. Which means that what I can do is I can connect them together and have one wire coming from it to a consumer and obviously A1, B1 and C1. And this one is called zero or neutral. So, I have four wires instead of six. It's an improvement. I mean, it's, you know, more practical, right? So, I have four wires and the difference in EMF between A1 and zero is exactly like this. The difference between B1 and zero is this and the difference between C1 and zero is, again, shifted. And this is the practical implementation of this thing. Now, I would like to have a couple of words about energy conservation. Look, we started from one frame. Then we added another one. Then we added the third one. Well, we are taking more electricity basically, right? Because each pair of wires carries a certain amount of electric energy. But it looks like we are actually, you know, rotating this rotor, this magnet using whatever the force we are using, like a steam or a hydra power, the water falling from whatever, the dam or whatever. So, it looks like we are spending the same amount of mechanical energy by rotating. And the more coils we put around it, the more energy we consume. And something is wrong in this picture, right? It can be. Obviously, it cannot. And here is the very important point. If you remember when we were talking about magnetic induction, we did this experiment. We had two rails and a wire which can actually slide on these rails. Let's say this is some kind of a consumer of electricity. And we had magnetic field which goes this way. So, the wire, if we are moving the wire, what happens if we are moving the wire? Well, again, electrons are moving with the wire, right? Since electrons are moving, the Lorentz force pushes them perpendicular to their movement and perpendicular to direction of the field, which means along the wire. So, as we are moving the wire this way, my electrons are going one way. If we are moving the wire another way here, electrons moving this way. But what's important is whenever we are moving the electrons, what happens in this particular case? Well, again, moving electrons cause the force. Now, electrons are participating in two movements. One movement is this way with the wire and another is this way along the wire. So, this direction of electron movements causes the force to electrons this way. Now, but they are moving this way as well. So, what is the direction of the Lorentz force in this case? This way, against the movement. So, we cannot move just without any efforts. As we are moving, the Lorentz force, which is created by movement of electrons in the magnetic field, actually resists our movement. And the faster we are moving, the stronger the force actually is generated. The Lorentz force will be stronger. So, again, we are moving electrons this way and that causes the Lorentz force to force electrons to go this way. But as they are moving this way, the Lorentz force pushes them perpendicular to their movement and perpendicular to the direction of the field, which is opposite to their mechanical movement. So, as we are rotating the same thing, as we are rotating here, there is a Lorentz field, which is Lorentz force, which is generated in these coils, which actually prevents their relative movement. So, it slows down the magnet. So, if we want to generate more electricity, the faster we are moving, the more electricity is generated. The stronger resistance of this relative movement actually will be experienced, actually. So, if we want to have three coils, for instance, we will have to actually spend three times more mechanical energy to rotate with the same speed as if it was only one coil. And that's the law of conservation advantage. Okay, now, small, very short thing, AC motors. Now, we have learned about AC motors. If we have AC power, one phase, we can talk about the term phase. If we just have an AC connected to an AC motor, like in a fan, for instance, small power thing. If you remember, we had to use two different coils. One coil was the primary coil and another coil. We used a capacitor to shift the phase by 90 degrees. So, that actually caused the rotation. Without the second coil, the AC motor was not self-starting. We needed the rotating magnetic field around the rotor to force it to work, basically. That was in one of the previous lectures, actually maybe exactly the previous lecture. In this case, what's important, we already have rotating magnetic field. So, if these three electromotive forces will be arranged around the rotor, and this is the rotor, this is A10, this is B10, and this is C10, where zeros are all connected to this zero, and A1, B1, and C1 are these. So, what happens? Well, each one will be shifted. So, we will have a rotating magnetic field. So, we don't really need another capacitor or anything like this. If we have all three phases connected to three different coils of the AC motor, then it will just be self-starting without any problems. It's much more powerful. Obviously. So, the big AC motors, like the AC motor which pumps the water to high floors of the high-rise, for instance, the powerful motors are all three phase motors. Something small, like a fan in the room, that can be one phase. So, they have one phase from A1, let's say, in zero. But then they need this condenser or something else, or transformer to convert one phase into another phase, which is 90 degrees shifted by P over 2, pi over 2. Okay. And that basically completes this lecture. And the only thing I would like to add is that the whole business of these AC motors, generators, etc., they're designed mostly by Nikola Tesla. He was absolutely genius. And what's important is that whatever he was doing, it was before physicists discovered the electron. So, from Tesla's perspective, he didn't know about at that time when he was actually, you know, discovering these things and inventing these things. He didn't know about electrons as an elementary particle. That was a very, very end of 19th century. And he was in, like, 1880 something. So, it was like 10, 15 years before that. So, that actually just emphasizes his genius. So, before understanding all the details of this, as we understand now, he was able to come up with all these designs. Thank you very much and good luck.