 Hi, I'm Zor. Welcome to Inizor Education. Today I would like to talk about AC motors, alternating current motors. Well, they're very important actually. I mean our whole society actually is moved by electrical motors and primarily they are AC, alternating current motors. So this lecture is about their idea actually, not about all the minute details of how AC motors are created. However, I think it's very important to really talk about design, about principles of their work, etc. Well, this lecture is part of the course called Physics for Teens, presented on Unizor.com. I suggest you to watch this lecture from the website because it has, as any other lecture, it has very important notes, very detailed notes, with some nice pictures, much better than whatever I will try to draw. And also it's arranged as a course. So basically we have a sequence of lectures and some logical order and references, etc. So I do suggest you to just take the whole course from Unizor.com called Physics for Teens. Now there is a prerequisite course on the same website. It's called Maths for Teens. Without Maths, you cannot really learn Physics. The website is completely free. There are no ads. You don't even have to login. So I mean if you want you can. So it's all for general consumption. Okay, now AC motors were invented by absolutely brilliant man Nicola Tesla at the very end of 19th century. It's something like 18, 80 something, 18, 90 something. Obviously they were improved since, but it's the same idea. So it's very important that this particular person worked in this field and he created a lot of different things. But I think AC motors is probably something which was the most important impact on our lives. So AC motors are used everywhere wherever AC alternating current exists and it's in everywhere. We are all primarily using alternating current. And motors, AC motors are really very, very smartly designed and very economically designed. I mean something like losses of power due to certain whatever, frictions, whatever else, they are actually at minimum in electric motors. That's what makes electric cars very attractive. In any case, so let's go into the details. Well, by the way, where can we see it? Well elevators, for instance, are working up the electric motors. Water pumps, I mean you wouldn't have water in the big buildings without the pumps which are pumping the water up. Well, there are many different ways. I mean the whole industries actually are powered by AC motors. Now, there was a lecture about DC motors, direct current motors. And basically I did explain the design and I would like you to well recall it or read again the notes for that lecture or read or listen to that lecture again. It's important to understand the main principle we started from. So what was the main principle? We started the electric engine, electric motors. Well, if you have two permanent magnets and you have some kind of a wire loop here, and it's it has a direct current in it. Well, what happens? Well, if there is a direct current running in this wire loop, it's an equivalent to a permanent magnet because there is a magnetic field around around the wire. So it will be actually like north and south magnetic poles with this thing. Now, since we have north and south here, my wire, if there is an axis it can rotate around. It will turn in such a way that its cells will point to north. And its north will point to south and stop. That's it. We wanted to make it like a DC motor and for that we had to really invent the commutator, if you remember, which switched the polarity of the pluses and minuses, which means switch the north and south orientation of the magnet, which is magnetic field, actually, which is created by this thing, and then it will turn and then turn again because we're switching polarity every time, etc. So this is the principle. It's based on the Lorentz force. The Lorentz force basically says that if you have a magnetic field and you have an electron, which is moving across the magnetic field lines, then there is a force, which this force is actually directed perpendicularly to both direction of the magnetic lines and direction of the electrons. And in this particular case, if electrons are moving up and north-south moving across, then the force will be perpendicular to the board and that would turn this part behind the board and this part because the electrons are moving into a different direction. Right? It will go this way and that's why it will turn. And then if it will switch the polarity, it will continue turning. So it's all based on Lorentz force. And the same Lorentz force is actually used for AC motors. So we have to basically use similar design and I would like you to think about designing AC motors from scratch. I'm not sure Tesla was actually thinking in exactly this way, but I think it's a natural way of progressing from one to another. So you have the Lorentz force and from this you have to build AC motor. This is your task, so to speak. So let's just think about how you would think about creating your AC motors. Okay, now you don't want to have a commutator here, which basically is brushes and it's friction, etc. It's not a good design. We don't want to have permanent magnets because permanent magnets are expensive and heavy and not as practical. I mean whenever we have to use it, like in DC motors, we have to use it, but AC motors are different. AC is an alternating current and alternating current has its own magnetic field which is kind of moving and maybe we can use this idea in some way and then we can. So let's just improve this model and we'll try to use AC. My first improvement is not really practical, but it's educational. I will replace the rotor, the thing which is rotating with a permanent magnet and I will replace these two with electromagnets connected to AC. So we have an alternating current and we have a permanent magnet here on the axis. Let's say axis is here so it can rotate within the board plane. So what happens in this case? Well, first of all, the magnets should not be very long because otherwise it will just hit my electromagnets. So we will make it shorter and make more room here and it's actually continuation. It's winding and continue winding. So if direction of the current is in one direction, we will have north-south, north-south and if direction is in another way, it's an AC, right? So we are changing direction like 50 or 60 times per second, then it would be north here, south, north, south. So it will alternate. So what happens? If I will just turn AC on, I have a switch, I'll turn it on. So my electric current will go this way and it will magnetize my electromagnets, let's say south, north, south, north. What happens in this case with this magnet? Well, it seems that it should actually turn in such a way that if this is north, then south will turn here and north will turn there, right? So let's call this x, this is y. So if x is equal to n and y is equal to s, then what happens? My magnet would be in this position, but then when my direction of the current will be in this, in opposite, my this pole will be south and this pole will be north and in theory my magnet should align this way, right? Great, but it won't happen. Now why would it not happen? Well, this magnet is relatively high, I mean it's a permanent magnet, piece of metal, whatever, and we are actually changing direction like 50 or 60 times a second. It will just not move by itself because just sheer inertia will not actually allow it to move. It might actually vibrate because of that, but no more than that, it will not turn. But what happens if I will just manually push it to rotate with relatively high speed? Well, once it's pushed, that's much easier actually because what happens is that, well, at some moments when, as it rotates, the forces between the magnetic field of the electromagnets and its own magnetic field, sometimes they will help to rotate it faster, sometimes it will slow it down, but after a little while it will synchronize in such a way that whenever, now, these are two extremes actually because there is something in between when the direction is changing, it means at that particular moment there is no current at all, so current goes this way, then it stops, well, then it goes stronger and then it's got weaker then it stops, and then it goes again weak and then stronger in another direction, and then weaker again and stops. It's a sinusoidal change of the current. Gradually to the plus, gradually down to zero, gradually to the minus, then again up to zero, etc. So what happens if my rotation is in sync in such a way that whenever it's aligned, let's say, well, let's say we are in this position, south north, okay? Now, if whenever we are in this position, now let's start from a different point, let's start from the perpendicular position, here, south north. What happens in this case? Well, south will gravitate, will attract to north, north will attract to south. As it goes this way, and let's assume that at this moment, when it's perpendicular, the strength is maximum on both sides. Now as it goes this way, we are in sync, right? This strength of the electric current is diminishing, so this N is gradually becoming zero, and this becomes zero. Well, we go to the upper side, zero. This becomes zero and this becomes zero. At that point, my magnet should be in this position, because this south was already here. North attracted it, but at the time south comes to the magnet itself, now it's zero. So it does not prevent it to go by inertia further, just a little bit further. As soon as it's further, let's say in this position, south north. The polarity is switched, now this is north. This is south. This is south. This is north. But now it's moving this way and again north attracts south, so it will continue going this way. And again, by the time it reaches this position, south north, I will switch again to zero my current. And again it will pass this. It will rotate a little bit further because of inertia. South, north. And then I switch again, so now it's north again. South, south, north. And south will start attracting to this and it will continue rotation. So my point is I will have to push it initially in such a way that it will start rotating in sync with rotating of the alternating current. If this is 60 times a second, then I have to do 60 times a second here. Now if it's north, if it's like much, much faster, for instance, I initially push it faster, then it will actually gradually, because of disparity between the frequencies, it will start slowing down. But then when it hits the sync, when it hits exactly the same thing, it will continue indefinitely. This is a good first step to create our AC motor. It has two advantages, two disadvantages, sorry. Well, first disadvantage is this is a permanent magnet. Permanent magnet is a difficult thing. It's expensive, it's heavy, it's difficult to deal with, etc. We are having AC current. AC current means that we have the source of energy. Why can't we have induced electricity inside of this sync? So it will have polarity in a proper way. That's an idea. I mean, to replace the permanent magnet with something which has induced electricity. I mean, I don't want to have, instead of this permanent magnet, the same thing as we used to have in DC motors, like a frame and the source of direct current and commutator, etc. I don't want that design. I don't want any kind of commutator brushes, etc. So that's my first disadvantage, which I would like to think about how to use induced electricity and therefore induced electromagnetic field instead of permanent magnet. And the second one, obviously, it does not start by itself. I have to push it. That's obviously not a good thing. Whenever you have a fan, you turn on the electricity, the switch and it starts rotating. How? Well, it's an AC, right? So that's what we are going to do. We will improve this design to make it working. So no more electric magnet. And my obvious choice is just to have a frame. So this is a wire loop. It does not connect to anything. It's just a wire loop. And I'm thinking that maybe my electricity, which is actually changing, AC is changing, my electromagnetic field is changing. Maybe in this particular case it will help me to induce some electricity here. And maybe with induced electricity it will start rotating. Well, that's just not so simple. So let's just think about what actually is happening. My magnetic field has magnetic field lines, right? So what are magnetic field lines? From north to south usually, right? So it's from here to here. We can almost consider it like a uniform. At any particular moment it's uniform. However, what happens if this is AC? Well, it's uniform if it's a permanent magnet, but if it's an AC, if it's an electromagnetic magnet, then the direction of these magnetic lines would be from north to south when this is north and this is south, right? Then when AC is actually diminishing in magnitude, it becomes zero to zero. Magnetic field almost disappears. And then the same magnetic lines appear again but in a different direction. So they're stronger and they're weaker. Then they are going into a different direction, they're weaker again and going to a different direction. So they're moving like this all the time. If they are moving like that, my wire doesn't cross anything. If it doesn't cross the magnetic field lines, electrons are not moving actually relative to magnetic field. I don't have current here. Current will be induced only if my electrons are moving relative crossing the magnetic field lines. Here they're just switching still basically doing classic. There is no induced electricity, so the frame will be basically dead. However, and let's do it the same way as we did with magnets. If I will start turning it, okay, when I start turning, these wires with electrons in them will cross the magnetic field lines. The induced electricity will go in this direction or that direction. It will create my north and south poles correspondingly and again they will alternate all the time. And if my speed is, if I will push it the same way so it can align its rotation synchronously with AC oscillation, then it will start moving. And here is a very practical example. I have a fan and something happened with the fan. It no longer worked when I turned on the switch, but if I will turn the rotor, it will start rotating and it will work. That's exactly what happens here. So something should be added here and that something actually got broken in my fan and that's why I really had to do it. So what can I do to make this design working? Well, here is a very important trick. What happens if I have another pair of electromagnets here at 90 degrees and they are connected also to AC, to the same AC. What happens now is that my magnetic field actually will be like bi-directional. They will have magnetic field this way and magnetic field this way and they will be in sync with each other. So basically it means that my magnetic field will be probably this way at 45 degrees because this one will be the same thing as this one and that would create or rather this rather than this together and that would be this direction and then they will change it would be this direction. So it will be diagonally. So it doesn't really solve the problem because magnetic field lines will be steady in the same direction. I mean it's two opposite directions but along the same line. So this thing will not cross any lines. Okay, I have to really do it like a view from the top which means I will have just this way. Okay, the frame goes this way. So these are my four electromagnets. But what happens if I will introduce a delay here? I'll use the word delay without specifically telling what it is. What does it mean delay? Delay means that if this AC, this is time, goes this way, AC in these electromagnets would be, not exactly like this, like this, shifted by 90 degrees. So whenever this is top, this is bottom. Whenever this is zero, this is top. Whenever this is bottom, this is zero. Whenever this is zero, this is top, etc. So they are shifted by 90 degrees. 90 degrees, I mean rotation of 90 degrees. So if this is 60 times, for instance, per second, then this is also 60 times per second but for a little bit shifted in time. So what happens in this case here is the following. Let me do it again here. So whenever this is north, this is zero. This is south, this is zero. Whenever this is zero, this is north, this is zero, and this is south. Then this is south, this is zero, this is north, this is zero, and then zero, south, zero, north. What happens here? First they have north, north, south, south, south. So basically it's this direction, right? Well, no, actually it's from north to south, right? So it's this direction. Then when I have zero, zero north south, I have this. Then I have north south, I have this from north to south. These are zeros. And then I have from zero to zero that would be north south. That would be this. So you see the magnetic field is turning in this case. This is what this smart delay gives us. So if I will have to arrange, if I will be able to arrange my AC in these electromagnets with the same frequency but shifted by 90 degrees, I will have a rotating magnetic field. And that's what makes the whole difference. Because now, since magnetic field is rotating, and this is the frame, well, it's basically the same thing as magnetic field, the stage, and the frame is moving, right? Which means electricity will be produced in it. Because now it will cross the magnetic field blinds. And it will produce the electricity in such a way that it will, because of the Lorentz force in the magnetic field, it will start turning. So as magnetic field is circulating, my, this frame will start circulating again. Now, if it will circulate with exactly the same speed, then again it will miss the crossing the magnetic field line. So there is a little slip. So this thing is rotating a little bit with a little delay. So my magnetic field is a little faster than my rotor. So the stator magnetic field is a little faster than the rotation of the rotor. So the difference is, but it's just called a slip, is what actually makes electricity to be generated, to be induced inside this wire loop. And that's what makes it rotating. And this is a very important, actually, feature. The only question is how to introduce delay. Well, actually, that's a solved problem. We know the solution. In the previous lecture, I was talking about capacitors, and some couple other lectures we were talking about transformers. And if you remember, the result of the AC line, this is AC, and this is capacitor. This is capacitor. The result of the capacitor is that frequency in the current is the same as frequency of the EMF produced by AC. But the current will be, well, if this is sine omega t, if you remember, this will be cosine omega t, which is exactly 90 degrees shifted. Why? Because it depends on the magnetic flux here. And magnetic flux should be differentiated to create the EMF. And differentiating sine will give you a cosine. It's all in that lecture about the capacitors. Same thing with transformers. So if you have, instead of this, if you have a transformer, so this would be one, and this would be another. From here to here. If number of turns is the same, we have the same voltage, the same EMF. But this will be of the same frequency, but shifted by, again by 90 degrees, because this will be, if this is sine, this is cosine. This is cosine, this is sine. Because of this derivative, EMF is derivative of flux. So flux goes in sync with this thing, but the derivative of the flux would be shifted by 90 degrees because from sine to cosine. So that's exactly, this is the sine graph, this is the cosine graph. So that's how we produce this. So let's say we use a capacitor. Now this is idea. The real electrical motors, real AC motors are significant and more complex technically. But from the idea standpoint, this is it. Now little improvement. Instead of one particular wire, we can use many wires which are on the same and make it like a squirrel wheel or squirrel cage, whatever. So there are many wires here. They are all connected through these end rings. And there is a circuit which goes into each wire here. And each wire actually is experiencing the same rotating effect, rotating force of the rotating magnetic field. And the whole wheel would be stronger because there are more than one wire which is experiencing the same force. And then there are other improvements which can be done. And again, I'm not talking about this. It's all technical details which is only for professionals. But for you, I would like to understand the very important thing. We have created a rotating magnetic field. And that's what makes rotor rotating. If you remember from the DC motors, when our rotor was a permanent magnet, we have created this rotating magnetic field as well. In that case, we were turning the opposite electromagnets on and off using some kind of smart electronics. So whatever it is, smart electronics for DC case and permanent magnet as a rotor, something like a capacitor or a transformer in this case for AC case and something like this squirrel wheel in the squirrel cage, whatever it's called, in case of a rotor. That's all technical details. The most important thing, you have a rotating magnetic field. Rotating magnetic field creates the movement necessary to rotate the rotor. Well, that's it for today. I do suggest you to read notes for this lecture. There are some nice pictures there. And good luck!