Motores de Indução (Aula 01) - Introdução

Hey guys. Today, here in the laboratory, we are going to start studying a very important machine in the field of electrical engineering: it is an induction motor. It's these guys here.

To begin with, I brought here two devices: a three-phase machine and a single-phase machine. Let's start our exposition by presenting some very important technical terms. We call this external structure of the engine the housing, at the front of the engine we have the connection box.

This connection box here we can unscrew, and then we have access to the motor terminals. This machine, in particular, is a six-point machine , so I only have access to the three coils, each one from one phase of the motor, and access the input and output of the coils through the terminals. We analyze, based on the board data, what the nominal voltage of the machine is, what connection scheme I have to make for my mains voltage and it is ready to operate.

Here we have a single-phase motor, as you remember from theory, a basic characteristic of the single-phase motor is the presence of the capacitor. This guy could be a starting-only capacitor or it could be permanent, if it's permanent we won't find the centrifugal switch. It (centrifugal key) is nothing more than a key that is connected to a set of springs with weights.

When this machine reaches a certain speed, the centrifugal force throws the weights out. These little weights being thrown out, by the spring system, will cause an electrical contact to be disconnected. This electrical contact deactivates the auxiliary winding, where the capacitor is connected, so the machine runs without the capacitor.

Here we have a teaching set that allows a more detailed visualization of each part that makes up the induction motor. The engine will be made up of three main parts: the rotor, the part that rotates and moves; the stator, the internal part that remains static, stopped; and all of this is coupled by the machine housing. Both the rotor and stator are part of the motor's magnetic circuit.

You must remember, from conversion, from electromagnetism, magnetic circuits in alternating current are made with a core of material with high magnetic permeability and laminated to avoid the emergence of induced currents, which will overheat the machine and dissipate power. Here we can raise two important terms, two important terminologies: we have the stator package, which comprises the stack of stator plates, and a rotor package, which comprises the stack of rotor plates. Let's visualize it, here in our teaching kit, which is easier to see.

In yellow we have the machine housing, in red, this red part here, is not the same material as the yellow part. The yellow one is cast iron in here we have electrical steel. It is a steel with high magnetic permeability.

This package is made by stamping several sheets of electrical steel stacked and placed, arranged in this way here, so that, after this procedure, the stator coils are wound. Here we have what we call the stator package. Put it here on the side.

Likewise, the rotor will also be manufactured from a pack of blades that have been stamped, then stacked. Here it is painted red, it is a little more difficult to visualize, but it is done in the same way: a series of blades are stacked and arranged. Here we have a difference between the three-phase induction machines that are being studied and those in the machine laboratory.

See that I have two different rotors here: one of them is a wound rotor and the other is a squirrel cage rotor. The squirrel cage rotor is exactly the same technology used in our single-phase machine. It is perhaps the technology most used for industrial machines today because it is very cheap to produce from these guys.

Basically, the rotor package is taken, placed in an aluminum injection molding machine and injected into the cage. We have a structure that comprises an aluminum ring, a short circuit with all the bars that make up the cage and meet in the other short circuit ring. The same happens with the single-phase machine.

So, we have a short-circuit ring, the injected aluminum passes through the grooves of the rotor package and meets at the bottom in another short-circuit ring. It can also be done with coiled wire. So we have a rotor package and it is arranged in such a way that, later, it is possible to wind the rotor.

They are two machines with different applications because, as we already said, the injected machine, the squirrel cage rotor machine, is much cheaper to implement. So, it basically dominated the market. When do we use the machine with a wound rotor?

In applications where there is a need for a high starting torque. We have the possibility of placing the resistances in series with the rotor coils and, when inserting these resistances, we have the displacement of the machine's torque curve. But we will study these details a little later.

The important thing to keep in mind now is that in this technology, we necessarily need to have access to the coils, if we want to place resistances in series. To access these coils, there must be slip rings. These rings are connected to the rotor coils, so it is a three-phase machine, I have three rings, and I need to have electrical contact on these three rings.

That's why brushes are used. Here we have the slip rings and carbon brushes that make electrical contact . Then, from these brushes, a cord is connected and then we take the conductors to the machine's connection box to be able to make the necessary connections.

But the objective of these laboratory classes is to present to you the tests that are carried out on induction motors, to characterize these machines. This is what we will do in the next videos for wound rotor machines, squirrel cage rotor machines and single-phase machines. To illustrate, let's initially think about a machine with, for example, nominal power, on its plate, 1CV.

The board also has, in addition to all the machine connection parameters and everything else, the performance. Let's assume a yield of 78%~79%. What does that mean?

It means that I will connect this machine, panel, connection box, connect it to the network. I will provide electrical power to it, it will lose something within itself and will give me mechanical power of 1 CV. Remember: the power value on the motor plate refers to the mechanical power delivered to the machine shaft and not the electrical power delivered to the motor.

Let's think, for example, about this machine here. From the moment it's moving, I'll have friction. Friction where?

Fan friction with air, there will be friction between the bearings and the bearings. Therefore, this friction represents the rotational losses of the machine. From the moment I am energizing the machine's stator, I placed the connection box here, fed this machine to the network, a current begins to circulate through the stator coils, I will have losses due to the Joule effect.

The same will happen in the rotor. The rotor will see a rotating field, it will induce a voltage. As the bars are short-circuited, it induces a current, this current circulating in the rotor bars will dissipate power through heating, Joule effect.

And the wound rotor? It happens in the same way and will dissipate power in the wires of the wound rotor windings. In addition to these losses, I have magnetic losses.

From the moment I am feeding a machine, any of them, electrical machines, induction machines. These machines will have induced currents, both in the stator and rotor. This is why these electric machines are made with a laminated package.

In summary: we have here, in induction machines, 3 classic types of losses: magnetic stones, losses in the rotor and stator windings and rotational losses, so friction, ventilation, etc. We will be able to identify these types of stones through laboratory tests. Trying to represent the machine in equivalent circuit form, As you can see in theory, through these same tests we are able to calculate the model parameters.

In the next video class we will start with these rehearsals. Let's start with a three-phase induction motor, squirrel cage rotor, which is the most traditional. Then we progress to the wound rotor motor, the single-phase machine and, if there is time, we will try to bring you standardized tests in accordance with the standard.

Thank you very much and until next time.