Guys, today we're going to start the content on synchronous machines. And here I brought several examples so that we can understand how they work and what are the different topologies that we have on the market today. To understand this content, we must keep in mind how the rotating field works.
On the LABMAQ channel we already have two videos explaining everything about the giant field, so I 'll assume that you've already watched these videos and are already well versed in this. If you haven't watched it, pause and watch it because otherwise you won't understand anything. Well, how does a synchronous machine work?
We have a rotor field and a stator field that are in synchronism, that is, every rotation that my rotating field gives I will have, correspondingly, a mechanical rotation of the machine's rotor. So the speed of the rotating field is necessarily equal to the mechanical speed of my machine. We can have generators or engines.
In motors, the three-phase system, in association with the arrangement of the coils, which is a constructive issue, as shown in the rotating field video, this association will generate the rotating field which, once in synchronization with the rotor field, will perpetuate the movement of rotation. If we are talking about a generator, the opposite process happens. There is an external source providing a rotational force, rotating the machine axis and this rotational movement will generate a rotating field which, through the arrangement of the coils, will generate a three-phase system.
The electrical system is nothing more than the result of an association of hundreds of thousands of sources connected in parallel. And these sources, the vast majority of them, are synchronous machines. In Brazil, I believe that this reaches practically 90% of sources.
So we will have several small hydroelectric plants, large hydroelectric plants (Itaipu), thermoelectric plants and so on. Other types of plants all connected to the same electrical system providing power to consumers. Let's start with some technical terms.
I already talked about rotor and stator, let's remember, I explained this previously: rotor is the rotoric part, the part that moves, that rotates of a machine and the stator and the static part that stays still. In the synchronous machine we have two terminologies: field and armature. So, in the vast majority of machines, we will have the armature winding on the stator.
The armature winding is where we will have the power, it is where we will have the machine's three-phase connections. So if I'm talking about a motor, it's the armature winding that I'm going to connect the three-phase system to and if I'm talking about a generator, it's the armature winding that I'm going to extract the generated power from. So the power is in the armature and the field, normally, is in the rotor.
Remembering: the machine's field is that static field that, through rotational movement, will induce tensions in the armature. This is not a rule, it is not mandatory that the field is in the rotor and armature is in the stator, but for the sake of convenience in most machines it is like this, with a few exceptions that you will see throughout the video and later in the throughout the discipline. I talked about a static field, but what is a static field?
A magnet has a static field. In a magnet we will have well-defined poles, they do not vary. If I want a flux variation, a field variation, with a magnet I need an external movement, a mechanical movement.
If I want to increase the strength of the magnet I have to bring the magnet closer to the observing object, if I want to decrease it, I move it away. If I want to make a rotating field with a magnet, I have to rotate the magnet. We can say that the magnet is a static field, which is why permanent magnets are also used to form the field of synchronous machines.
Including the example that we used to create the rotating field in the rotating field videos, in all of them permanent magnets were used in the rotor. I don't precisely have to use the permanent magnet, I can make an electromagnet. The Electromagnet even gives greater versatility, why?
How do I make an electromagnet? A wire wrapped around a ferromagnetic core and I set a current to circulate there. Be careful not to get confused with the electromagnets that we manufactured in previous classes, because we manufactured those to use in the machine's armature and here we will use them in the field.
What's the difference? The armature is powered by alternating current, so we don't have a static field. If I want a static field in the electromagnet, I have to feed it with direct current, that is, the current will always be entering the reference terminal, the north will always be in the place of the North, the south will always be in the place of the south and not will alternate.
I will need a mechanical movement to do this alternation process. And I have the advantage that I can increase or decrease the current of my electromagnet, making it stronger and weaker. So here we have an engine.
This specific machine was manufactured to be a permanent magnet motor. And here we have another machine that is a machine with a wound rotor, with a field manufactured in a topology as if it were an electromagnet. Differences between these two topologies: the first is that in the coiled field, I have to make the direct current supply reach the field windings.
As the field is almost always in the rotor, we have a mechanical problem. I can't put a physical wire connected to the rotor windings because, when it starts to rotate, everything will go away, it will burst. So we have to use brushes so that there is a sliding mechanism that makes electrical contact without having problems with the rotation of the machine.
So I'm going to show you what this equipment is like. This machine is a machine that I refurbished a few years ago. You can see inside what its brush system looks like, but its rotor is exactly the same as this rotor, which is from another smaller machine, which I'm still going to renovate.
Here we have what we call slip rings. The slip rings are made of a conductive material and are where the carbon brushes will be used. Here I have two brushes to show you: it is made with graphite material in this part, here we have the cord and at the other end the source terminals are connected, in this case a direct current source, so I have a positive and a negative that's why I have to have two slip rings.
There is a whole mechanic there to put in, to make this electrical connection. So the brushes remain stationary, the rotor part moves and then electrical contact is made with the collector ring, which will slide on the carbon brushes. A system is often created with redundancy, so I can have two brushes, for the same collector ring, to ensure that there is always electrical contact, even if, by chance, one of them, due to vibration, loses contact, I always I'll have another one here.
This is a question of reliability, so there will be machines that will have many other brushes on the same slip ring. The connection system here we would use 4 brushes to have reasonable reliability, the minimum at least. I put two brushes per slip ring, so I use 4 brushes.
See that here we have four poles, four windings: one here, two, three and the fourth on this side. Mayor? As you saw in the rotating field class: the rotor with 4 poles will have four alternating poles, so I will have a North, a South, a North and a South.
In this case here, I have 4 coils that will be associated with each other in series or in parallel. The winding direction will be done so that we maintain this alternation of polarities and the association of the coils will have two terminals and these two terminals will be connected to the slip rings. Here we also have the other component of the synchronous machine, which is known as a cage or damper winding.
This entire structure that covers the rotor is made up of these aluminum bars that are short-circuited at their ends through these short-circuit rings, which are also made of aluminum. This cage is very similar to the squirrel cage of induction motors. In the synchronous machine, what is its purpose?
What is the function of this type of mechanism? It can have two distinct applications: 1) if we are talking about a synchronous machine with a motor, it will be used to start the machine, exactly like the induction motor. We can start a synchronous machine like an asynchronous machine, like an induction motor.
The rotating field will induce currents in this cage, which is short-circuited, these currents will induce a force that will initiate the rotational movement. 2) And if we are talking about a generator, this cage serves as a damper winding. It's the same thing, but for each application we call it something different, which is the damper winding.
It is used so that, during transients, the machine does not lose synchronism so easily. But we won't go into this too much because it can be a little complex, later on we will, in other videos, talk a little more about this. This machine here has exactly the same rotor structure that I just showed.
We are not going to open it because it has very complex mechanics, it takes a lot of work to disassemble and reassemble. I will include here in the video some photographs from the period when I was carrying out maintenance on the rotor of this machine. And then you can see the contrast of this cage and the coils really cool, but through the opening of the machine we can see the brush apparatus at the front and at the bottom the rotor, where we have the windings painted red and the cage, black.
In permanent magnet machines there is no field winding, so there are no brushes, it is not necessary to have an external source to power the field and, because of this, we can say that this is a relatively simpler machine. But it also has some disadvantages, starting with the issue of assembling the machine. As I cannot turn off the field, precisely because it is a permanent magnet, its magnets will always be attracting, so it is very complicated to assemble the rotor with the stator, which always requires specific tools for this.
Another problem is that it will be attracting small particles like iron filings , or anything that is iron magnetic will be attracted to the magnet and, therefore, this is a machine that we cannot dismantle anywhere. In the machine laboratory it would be impossible to disassemble this machine because I, most likely, would not be able to put it back together and it would break. Another disadvantage of the permanent magnet machine is the fact that the magnet has a fixed induction, so I cannot increase or decrease this induction, unlike the coiled machine, which I can increase or decrease the field current and so I do with whether my electromagnet is stronger or weaker.
In the case of a permanent one, no. It's always fixed, always the same. It will become clearer later that the field intensity is directly related to the terminal voltage of the machine, so in a wound rotor machine, with a wound field, I can increase or decrease the voltage that is being generated.
As for the permanent magnet machine, I'm a bit stuck. I will have to create a project for a certain operating point so that it generates a certain level of voltage. That's why permanent magnet machines are widely used for motors but for generators it's not so common, at least I've never seen a permanent magnet machine, a synchronous permanent magnet generator, connected directly to the grid.
It will normally have an intermediary in there, precisely because I cannot control the voltage level, so I will generate it at whatever voltage it is, go through a rectification system and, after alternation by power electronics, only then will it it thrown into the net. But the coiled machine, you've already seen, has all that mechanism with brushes, the brushes have a problem: they wear out. So the carbon brush, due to friction, is connected to the collector ring, it is rotating on the collector ring, so it is sliding on top of it and it will cause friction.
Time will pass, this brush will wear out, and the time will come when we will have to stop the machine and make a change. This exchange is often problematic because a stopped machine is not generating, Depending on the application, this may be the only machine providing power, an isolated system for example. So there are also some obstacles to using the brush, precisely because of maintenance.
That's why and we have this machine which is a wound rotor machine, field wound machine, brushless. It is a brushless machine, called a brushless synchronous generator, without a brush. To be able to understand how it works, because it is a very interesting mechanism, I cannot talk about it here, try to narrate it, and make you imagine, we are going to have to disassemble this machine, this one is really worth disassembling.
Dismantled the machine and what did we find? Look how beautiful this engineering is. We have the stator, inside here, the static part of the machine, the stator of our generator.
This guy is the generator's field, it's in the rotor part, which is this entire piece here is the rotor, so here we have our generator's field and see that the generator is really big and its little field is almost nothing, it's a fraction of the machine, similarly the stator package is also small. This is a very small machine, it is only a 1 kilowatt machine, one KVA, so its power is low. To give you an idea, this permanent magnet motor here, which we saw, has 15 kilowatts of power, it has 15 times more power than this machine can generate.
But why is this machine so big and generates so little? Because of the device that replaces the brushes, which is called an exciter. This here is the rotor part of the exciter and this here is the static part of the exciter.
Everything here is attached here inside this part, it is fitted back here. The rotor is the same for both the exciter and the generator. And here is the front part of the generator.
I think you understand more or less what the business is like. How does the exciter work? Look at the following: here we have the stator, armature winding, in this guy here, we have a field winding in the rotor part.
The field winding is fed with direct current, as we saw if we used the brushes, positive and negative. Remember I said it wasn't always like this, this rule that the field stays in the rotor and armature stays in the stator? So, we came to the conclusion, we said, throughout the video, that basically this is a rule that most machines follow for the sake of convenience.
But what is this convenience? If I did it in reverse, made the field in the stator and the armature in the rotor, I would have three-phase connections and the energy generated in the rotor, not in the stator. And then the stator, I would feed it with direct current and the rotor would have to have three rings if it were with brushes.
There would have to be three rings for the three phases, but I would probably want a neutral too, so I would need 4 rings to take the three phases and a neutral. More than that, the field is normally of low power. The actual power that is being performed, the power that is being generated, is in the armature.
When I make the field in the rotor I place the two brushes, I only add the amount of energy necessary to feed my field and all the power is in the static part, there is no brush, there is nothing. If it were reversed, in addition to having to have four brushes, which is twice as many brushes, I would have to have much stronger brushes to be able to transport all that power. In the case of the exciter we have exactly the opposite.
What is the exciter? It is a generator, where the field is in the stator (here) and the armature is in the rotor. And why is this interesting?
Because I don't need brushes or any mechanism to extract energy from the armature to the static part, because this energy, which I am generating, will be used to power the machine's field. I have poles here, on this machine, this one is an 8-pole machine, we can count it because it is well distributed and separated, and I feed these eight poles with direct current with these two terminals here. These eight poles will create a static field that will be crossing the rotor.
The rotor has a three-phase winding, which will be rotating, and so, if it is rotating, we have a static field crossing it, but it is moving in relation to this field and it will see a variable field that will induce voltage at its terminals. exactly the same way as the synchronous machine works. It's a synchronous machine, but the field is in the stator and the armature is in the rotor.
How do I take the energy generated here and transport it to the field supply of our generator? Remember? The field has to be fed with direct current and the generator generates alternating current, then we use diodes, which are these diodes here, which are connected like a three-phase rectifier, Graetz bridge, which will make this power, this energy which is being generated by my exciter, is rectified and through these terminals here it will feed the field coils in our generator.
In other words, I have a very complex and interesting machine here. Because it has two synchronous generators coupled, one with the logic inverted in relation to the other. This is the traditional field in the motor and this one is the opposite, the field in the stator.
From this connection I can have a machine that doesn't need brushes, which is a big advantage. In this specific machine, which is a very low power machine, it gives the impression that this is engineering that is not really worth it because I am using more material for the exciter than for my generator. But for a bigger machine, this exciter will be bigger, but it won't be that much bigger.
So for larger machines, this here is an exciter that would have the capacity to power a larger machine, in my experience I have seen exciter this size powering machines from 5 to 10 kilowatts. So this relationship becomes a little smaller. Here we can get an idea of how this machine works.
Cool, right? One last thing that may raise some doubts for you: all the machines we've seen here so far have had 4 poles, the rotors, the previous four-pole rotor and this one here is no different. You can clearly see: one, two, three and four magnetic poles.
What is the synchronous speed of a 4 pole machine? What is synchronous speed? Given a network frequency, what is the speed at which my generator shaft must be rotating for it to generate at the desired frequency?
In our case, frequency of 60Hz. What speed does the 4-pole machine have to rotate to give 60 Hertz? From the rotating field video lessons, synchronous speed is that formula: 120 times the frequency, divided by the number of poles: 1800 RPM four poles, 1800 RPM; for two poles, 3600RPM and so on.
And then you will look at our exciter here and we see that it has eight poles. But the axis is the same for both. How do I have an 8-pole machine on the same axis with the four-pole machine?
Do not fool yourself. I want the machine to rotate at 1800 RPM because what matters to me is the generator, this is just to power the field. This guy here, being an 8-pole machine, will rotate how often?
If it has 8 poles, its synchronous speed is 900RPM, but it will be rotating at 1800RPM, that is, it will not rotate at 60Hz, but at 120Hz. But does this change anything for us? It doesn't change anything.
It changes that, with a higher frequency, we will have less saturation of the iron that is used to make our exciter. Then I can make an even smaller package, so for engineering reasons this is worth it. But it is rectified, so whatever frequency it is being generated at, our diodes will do the rectification and this guy will always be receiving direct current.
So we don't need to worry about that. Pay attention not to make this mess. Finishing.
One last important distinction that you will hear throughout the course: smooth poles and protruding poles. These are constructive characteristics of synchronous machines and are related to the magnetic coupling between the rotor and the stator. Here we have examples of salient poles.
We can even see, physically, that the poles are protruding, I can count how many poles I have just by looking at the machine because the shape in which it is constructed is very simple to visualize. But the relationship between the pole being salient or not is not just a visual issue, it is a matter of magnetic coupling between the rotor and stator. This guy will be inside the stator, the armature winding will be separated from the field winding by an air gap, with the air gap, normally, very small, in this machine, which should only be 1 and a half millimeters.
And what is this magnetic coupling like? Due to the shape of our rotor, in our field winding, the pole head is very close to the stator. It will only be separated by an air gap.
But between the poles I have a bigger air gap, it's as if I had a much worse magnetic coupling here than I have here. This will result in an arrangement that is not uniform within the machine, so the flux will be leaving the stator and entering the rotor with great intensity here, there will be practically nothing coming this way. There will be a deformation of the field here to always enter the heads of the poles.
Unlike what happens here, we have smooth poles. See here. The winding does not make it clear to us which poles there are, how many poles this machine has.
Magnetically we will have the same result as we have here. There will be four poles, but the magnetic coupling of this guy here with the stator will be very different from this guy with the stator because, both in the direction of the pole and in the direction between poles, the flux will find the same resistance to cross. What is the practical result of this?
This is because, normally, we get machines with lower harmonic content when they have smooth poles. With prominent poles we often end up having some greater harmonic content, but this is not decisive at this point either. They are different topologies, now an important issue to take into consideration is that the salient pole machine is easier to produce.
I can simply wind the pole here, use a winder, I don't have any resistance to do this winding. On the smooth pole machine, winding is more complicated. If the machine is very large, then it starts to get much more complicated.
A machine, for example from a hydroelectric plant, with 78 poles will be very difficult to wind with smooth poles. I can wind with protruding poles, even make the poles separate from the machine, we are talking about a very large machine; Transport is also bad, you would have to transport the entire rotor. But if I make protruding poles, make them separate or just screw them onto the machine's rotor, we're talking about poles that are, I don't know, four meters high or even more, you can make machines much bigger than that.
I simply screw these poles onto the machine's rotor, it makes maintenance easier, it's easier to transport, it makes everything easier. This was an introduction, this was the introduction of the vast majority of types and topologies of synchronous machines that we have on the market. Over the next few classes we will bring some rehearsals and we will connect some of these guys here to see how they work in practice.
Beauty? Follow the next videos there and we will always be producing something new. Thanks guys, see you next time.
