Generator- a device that converts energy various types to electric. Generators produce electricity. Examples of generators: galvanic cells, electrostatic machines, solar panels etc. Depending on the characteristics, generators are used various types.
For example, using electrostatic machines, you can create a very high voltage, but the current will be very small. And with the help of galvanic cells you can create an acceptable current strength, but they can only work for a short time.
Generator structure
Consider an induction electromechanical generator alternating current. There are many generators of this type, but each of them has common basic parts.- Permanent or electromagnet. It creates a magnetic field.
- Winding. An alternating emf is induced in it.
The amplitude of the EMF is induced in each turn of the winding. Since the turns are connected in series, the EMF values will add up. The EMF in the frame will be proportional to the number of turns in the winding. For getting of great importance magnetic flux in generators is made of a special system of two cores.
In the grooves of one core there are windings that create a magnetic field, and in the grooves of the other, windings in which an emf is induced. One of the cores rotates, it is called a rotor. The second one is stationary and is called a stator. They try to make the gap between the cores as small as possible in order to increase the flux of the magnetic induction vector.
The figure shows a model of a simple generator.
Operating principle of the generator
In the generator, the model of which is shown in the figure, a magnetic field is created permanent magnet, and the wire frame rotates inside it. In principle, you can leave the frame stationary and rotate the magnet. From nothing would change.
This is exactly what is done in industrial generators. The electromagnet rotates, and the windings in which the EMF appears remain motionless. This is due to the fact that in order to supply current to the rotor or remove it from the rotor windings, it is necessary to use sliding contacts. Brushes and slip rings are used for this purpose. The current strength that will make the rotor rotate is much less than the one that we remove from the windings.
Therefore, it is more convenient to supply current to the rotor and remove current from the stator. In low power generators, to create magnetic field They use a rotating permanent magnet, then it is not necessary to supply current to the rotor at all. And you don't need to use brushes and rings.
When the rotor rotates, an emf appears in the stator windings. This happens because a vortex occurs electric field. Modern generators are very large machines. Moreover, with such dimensions (several meters), some of the most important internal parts are manufactured with millimeter precision.
Transformers
Generators that are located at power plants produce a very powerful EMF. In practice, such tension is rarely needed. Therefore, such voltage must be converted.
Devices called transformers are used to convert voltage. Transformers can either increase the voltage or decrease it. There are also stabilizing transformers that do not increase or decrease the voltage.
Consider the transformer design in the following figure.
Transformer symbol: 
Transformer design and operation
The transformer consists of two coils with wire windings. These coils are placed on a steel core. The core is not monolithic, but is assembled from thin plates.
One of the windings is called the primary. The alternating voltage that comes from the generator and which needs to be converted is connected to this winding. The other winding is called the secondary winding. A load is connected to it. Load is all the devices and devices that consume energy.
The following figure shows symbol transformer.
picture
The operation of a transformer is based on the phenomenon electromagnetic induction. When alternating current passes through the primary winding, an alternating magnetic flux is created in the core. And since the core is common, the magnetic flux induces a current in the other coil.
The primary winding of the transformer has N 1 turns, its total induced emf is equal to e 1 = N 1 e, where e is the instantaneous value of the induced emf in all turns. e is the same for all turns of both coils.
The secondary winding has N 2 turns. EMF e 2 = N 2 e is induced in it.
Therefore: e 1 / e 2 = N 1 / N 2.
We neglect the winding resistance. Consequently, the values of induced emf and voltage will be approximately equal in magnitude: |u 1 |≈|e 1 |.
When the secondary winding circuit is open, no current flows in it, therefore: |u 2 |=|e 2 |.
Instantaneous values of EMF e 1, e 2 oscillate in one phase. Their ratio can be replaced by the ratio of the values of the effective emf: E 1 and E 2 . And we replace the ratio of instantaneous voltage values with effective voltage values. We get:
E 1 /E 2 ≈U 1 /U 2 ≈N 1 / N 2 = K
K – transformation coefficient. At K>0 the transformer increases the voltage when K<0 – the transformer reduces the voltage. If a load is connected to the ends of the secondary winding, an alternating current will appear in the second circuit, which will cause another magnetic flux to appear in the core.
This magnetic flux will reduce the change in the magnetic flux of the core. For loaded transformer, the following formula will be valid: U 1 /U 2 ≈ I 2 /I 1.
That is, when the voltage increases several times, we will reduce the current by the same amount.
Today we are all familiar with household electric generators. Depending on the fuel consumed, purpose and type of engine used, these can be gasoline, gas, diesel and even wind electric generators. These devices have become a part of our lives, and we are accustomed to using them in the countryside and on camping trips, at construction sites and in the garage. Many types of electric generators and electrical appliances do the work for us. Portable hand-held electric generators are built into flashlights, solar panels power remote instruments and sensors, space satellites and mountaineering equipment. But it was not always so. The beginning of the 19th century erupted with a whole series of discoveries related to electricity and magnetism.
After the discovery and study of electromagnetic induction and the calculations carried out, it became obvious that it was possible to create an electric generator that could convert mechanical energy into electrical energy. To obtain current in a closed coil of wire, it is necessary to change the induction flux passing through it. This can be done in two ways: either move the magnet relative to the coil of wire, or move the coil of wire relative to the magnet.
The first homemade magnetic electric current generator, built in 1832, was a very simple installation. Look at his drawing: you see that the EMF in the windings of his coils was excited by the rotation of a horseshoe magnet. The current created by such a machine was not like the current from a galvanic cell - it seemed to rush from side to side, every now and then changing its direction. This current was called alternating, in contrast to direct current produced by a galvanic cell.
The installation of another electric generator looked different: a conductor frame rotated between the fixed poles of a magnet. Its ends were connected to two rings on the axis of rotation of the frame, and an electrical circuit was connected to the rings using sliding contacts. At the contacts of the rings, either “plus” or “minus” appeared, which meant the generation of an EMF variable.
The fact that the current was alternating was considered a disadvantage and they began to look for a way to straighten it. To do this, they resorted to the so-called switch. In the second machine, for example, both ends of the frame were connected to a ring, which was cut in half, and each half was insulated with a layer of non-conducting substance. One sliding contact touched only the end of the rotating frame on which there was a “plus”, and the second contact closed on the “minus”. But although the current in the circuit became constant in direction, its magnitude changed with each half-turn of the frame.
To avoid sudden changes in the current value, the number of frames was increased. Their ends were connected to diametrically opposite sections of the cut collector ring of the electric generator. The current from such a magnetic generator is the more similar to a constant one, the more frames there are on the rotating drum - the rotor (the stationary magnets in such a machine are called a stator).
DC and AC electric generators are very similar in design to electric motors. In addition, if you rotate the armature of a DC electric motor, a potential difference appears on its windings - the motor begins to produce electric current, becoming an electric generator. However, for technical reasons, electric current generators are built somewhat differently than electric motors.
Let's take, for example, an alternating current electric generator of a large thermal power plant
Its stator has a winding inside, in which an electric current arises. The rotor is a cylinder with two magnetic poles: north and south. If you magnetize the rotor by passing direct current from an external source into the pole windings, and then begin to rotate it, alternating current will appear in the stator winding.
A separate small DC generator is usually used to excite and operate the rotor. This electric generator is placed directly on the rotor shaft. There is another design option - instead of an exciter generator, a semiconductor current rectifier operates. It takes an insignificant part of the power of the electric generator itself, rectifies the alternating current, and with the resulting current powers the rotor winding.
Our country has adopted an alternating current frequency standard of 50 cycles per second - 50 Hz. This means that within a second the current must flow 50 times in one direction and 50 times in the other. Accordingly, the rotor must make exactly 50 revolutions per second, or 3000 revolutions per minute. Electric generators of thermal stations operate at this speed: they are driven by gas turbine units specially designed for this speed.
This happens as often as in an electric generator in a thermal power plant, where the rotation speed of the gas turbine unit is 3000 rpm. Thus, the frequency of 50 periods is maintained here.
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Chapter 5. PRODUCTION, TRANSMISSION AND USE OF ELECTRIC ENERGY
Electric Energy has undeniable advantages over all other types of energy. It can be transmitted by wire over vast distances with relatively low losses and conveniently distributed among consumers. The main thing is that this energy with the help is enough simple devices easy to transform into any other forms: mechanical, internal (heating of bodies), light energy, etc.
Alternating current, unlike direct current, has the advantage that voltage and current can be converted (transformed) within a very wide range with almost no energy loss. Such transformations are necessary in many electrical and radio engineering devices. But transformation of voltage and current is especially necessary when transmitting electricity over long distances.
§ 37 GENERATION OF ELECTRIC ENERGY
Electric current is generated in generators- devices that convert energy of one type or another into electrical energy. Generators include galvanic cells, electrostatic machines, thermopiles 1, solar panels, etc. The possibilities of creating fundamentally new types of generators are being explored.
1 Thermopiles use the property of two contacts of dissimilar materials to create an emf due to the temperature difference between the contacts.
For example, so-called fuel cells, in which the energy released as a result of the reaction of hydrogen with oxygen is directly converted into electricity.
The scope of application of each of the listed types of electricity generators is determined by their characteristics. Thus, electrostatic machines create a high potential difference, but are not capable of creating any significant voltage in the circuit. amperage. Galvanic cells can produce a large current, but their duration of action is short.
The main role in our time is played by electromechanical induction alternating current generators. In these generators, mechanical energy is converted into electrical energy. Their action is based on the phenomenon of electromagnetic induction. Such generators have a relatively simple design and make it possible to obtain large currents at a sufficiently high voltage.
In the future, when talking about generators, we will mean induction electromechanical generators.
Alternator. The principle of operation of an alternating current generator has already been discussed in § 31.
There are many different types of induction generators available today. But they all consist of the same basic parts. This is, firstly, an electromagnet or permanent magnet that creates a magnetic field, and, secondly, a winding in which an alternating EMF is induced (in the considered generator model this is a rotating frame). Since the emf induced in series-connected turns add up, the amplitude of the emf induction in the frame is proportional to the number of its turns. It is also proportional to the amplitude of the alternating magnetic flux (Ф m = BS) through each turn (see § 31).
To obtain a large magnetic flux, generators use a special magnetic system consisting of two cores made of electrical steel. Windings that create a magnetic field
are located in the slots of one of the cores, and the windings in which the EMF is induced are in the slots of the other. One of the cores (usually internal) together with the winding rotates around a horizontal or vertical axis. That's why it's called a rotor. The stationary core with winding is called the stator. The gap between the stator and rotor cores is made as small as possible to increase the flux of the magnetic induction vector.
In the generator model shown in Figure 5.1, a wire frame rotates, which is a rotor (without an iron core). A magnetic field creates a stationary permanent magnet. Of course, you could do the opposite: rotate the magnet and leave the frame motionless.
In large industrial generators It is the electromagnet, which is the rotor, that rotates, and the windings in which the EMF is induced are placed in the stator base and remain motionless. The fact is that current must be supplied to the rotor or removed from the rotor winding to an external circuit using sliding contacts. To do this, the rotor is equipped with slip rings attached to the ends of its winding (Fig. 5.2). Fixed plates - brushes - are pressed against the rings and connect the rotor winding with the external circuit. The current strength in the windings of the electromagnet creating the magnetic field is significantly less strength current supplied by the generator to the external circuit. Therefore, it is more convenient to remove the generated current from the stationary windings, and through the sliding contacts to supply a relatively weak current to the rotating electromagnet. This current is generated by a separate direct current generator (exciter) located on the same shaft.
In low-power generators, the magnetic field is created by a rotating permanent magnet. In this case, rings and brushes are not needed at all.
The appearance of EMF in the stationary stator windings is explained by the occurrence of a vortex in them electric field, generated by a change in magnetic flux when the rotor rotates.
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