5.1. WHY IS AC CURRENT NEEDED?

In the first electrical installations, only direct current was used. However, it soon became clear that it is much more profitable to use not constant, but alternating current, i.e., one that periodically changes its value and direction.

First of all, it is more convenient to generate alternating current at power plants. Alternators are simpler and cheaper than similar generators direct current.

It also turned out that it is more profitable to transmit electric current through wires at high voltage. Changing the AC voltage is very simple - for this you need to use a transformer. On direct current, this is much more difficult to do.

Simple and reliable AC electric motors have been constructed and are widely used in industry.

But these are all applications where AC can compete with DC. Generators, transmission lines and electric motors can be made on both direct and alternating current. However, there are such physical phenomena that appear only when the current changes.

These phenomena are widely used in radio engineering, automation, electronics, etc.

It can be said that if there were no alternating current, many of these branches of electrical engineering would not exist.

In radios, televisions, tape recorders, alternating currents are used and it is fundamentally impossible to replace them with direct current.

Many technological processes in industry are also based on alternating current.

Instruction

First, let's figure out what an electric current is. The directed movement (flow) of charged particles is called electric current. In an alternating electric current, a different number of charged particles pass through the conductor cross section for equivalent time intervals. In a constant, the number of these particles for the same time intervals is always equivalent.

Alternating current constantly changes its strength, magnitude or direction. And these changes always occur periodically, that is, they repeat at regular intervals. For example, using the variable current the battery cannot be charged or it cannot be used for such technical purposes.

Unlike permanent current, the variable has several additional meanings: - period - the time value of the completion of a full cycle of indicators of the variable current; half-cycle and frequency (number of cycles for a specific period of time); - amplitude - the highest value of the variable current;- instant value - value current at this point in time.

Alternating current is more common and widely used. It is easier to convert it into alternating current of a different voltage, to change the voltage in electrical networks, depending on the necessary needs. This can be done with a transformer. A transformer is a device that converts alternating current of one voltage into the same current, but of a different voltage at the same frequency. current.

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On planet Earth today, 98% of all electricity is generated by alternators. Such a current is quite easy to produce and transmit to long distances. In this case, the current and voltage can repeatedly rise and fall - transform. Work is done by current, not voltage. Therefore, the smaller its value, the lower the losses in the wires.

Many users believe that only alternating current with a voltage of 220V and a frequency of 50Hz is used at home. This is only true for incandescent lamps, electric motors in vacuum cleaners, refrigerators.

In any complex home appliance powered by AC power, there are components that operate at constant voltage With different meanings. It is virtually impossible to predict what these values ​​might be. Therefore, all consumers in the outlet have alternating current of the same frequency and voltage.

D.C

Despite the fact that the share of DC generation is only 2%, its value is quite large. Direct current is generated by galvanic cells, batteries, thermocouples, solar panels.

Solar batteries are becoming a very promising area of ​​​​energy today, when the issue of using renewable energy sources is acutely raised.

Direct current feeds the engines of locomotives in railway transport, is used in the on-board network of aircraft and cars.

There are more and more electric and hybrid vehicles on the roads of modern cities. To recharge their batteries, stations are being built that provide their needs for direct current.

What should be the sockets

The dimensions of the sockets, their type, the material from which they are made, depend primarily on the purpose of the sockets, the currents and voltages for which they are designed. DC voltage devices have polarized plugs. Therefore, the sockets for them must be polar. Then even an inexperienced user will not be able to confuse where the "+" and "-" are.

An alternating current in a circuit is an electric flow of charged particles, the direction and speed of which periodically changes in time according to a certain law.

Instruction

Refer to the general concept of alternating current in electrical circuit described in school textbook. There you will see that alternating current is an electric current, the value of which varies according to a sinusoidal or cosine law. This means that the magnitude of the current in the AC network varies according to the sine or cosine law. Strictly speaking, this corresponds to the current that flows in the household electrical network. However, the sinusoidality of the current is not a general definition of alternating current and does not fully explain the nature of its flow.

Draw a graph of a sine wave on a piece of paper. This graph shows that the value of the function itself, expressed by the current strength in this context, changes from a positive value to a negative one. Moreover, the time after which the sign changes is always the same. This time is called the period of current oscillations, and the reciprocal of time is called the frequency of the alternating current. For example, AC frequency home network is 50 Hz.

Pay attention to what the sign change of the function means physically. In fact, this only means that at some point in time the current begins to flow in the opposite direction. Moreover, if the law of change is sinusoidal, then the change in the direction of movement does not occur abruptly, but with gradual deceleration. Hence the concept of alternating current, and its main difference from direct current, which always flows in the same direction and has constant value. As you know, the direction of the current is given by the direction of the positively charged particles in the circuit. Thus, in an alternating current circuit, charged particles through certain time change the direction of their movement to the opposite.

In our time, there is no such branch of the national economy in which electricity would not be used. And each of them presents electric machines and devices certain requirements, on which not only the design of these machines depends, but also the type of current used. Although both alternating and direct currents are widely used in engineering and industry, their areas of application are very clearly delineated.

For the first time, people received electric current from galvanic cells. These elements created a stream of electrons in the electrical circuit, moving all the time in one specific direction. This current is called "constant".

The first rotating generators, electric motors and appliances also ran on direct current. And when, at the end of the last century, the Russian electrical engineer M. O. Dolivo-Dobrovolsky proposed the use of three-phase alternating current, many scientists reacted to this with distrust. Even the famous American electrical engineer Edison considered alternating current an invention that did not deserve attention. However, very soon alternating current began to be used in many areas of electrical engineering. Electric alternators create a flow of electrons in the electrical circuit that continuously changes the direction of its movement. Yes, in chains light bulb illuminating your room, the electrons have time in one second

100 times to change the direction of their movement: 50 times they move in one direction and 50 in the opposite direction. Such a current is said to have a frequency of 50 cycles per second.

This feature of the movement of electrons gives the alternating current a number of properties that have determined its dominant position in modern electrical engineering.

One of the most important properties of alternating current is its ability to transform. As we know, transmission electrical energy over long distances is possible only at very high voltages, reaching 110, 220 and even 500-800 thousand V. So high voltage cannot be obtained directly from generators. At the same time, for various electrical machines and devices need an electric current with a voltage of several tens or hundreds of volts. This is where his ability to transform came in handy - it allowed using transformers to change the alternating current voltage in any range.

Little of. The connection of the generator windings into a three-phase system made it possible to obtain a three-phase alternating current. This is a system of three alternating currents that have the same frequency, but differ in phase by one third of the period. Three-phase current has important advantages. Firstly, three-phase power lines are more profitable than single-phase ones: more electrical energy can be transmitted through them at the same cost of wires and insulation than with single-phase alternating current. And secondly, due to the property of a three-phase alternating current to create a rotating magnetic field, it was possible to build very simple and reliable asynchronous electric motors without a collector and brushes.

These qualities of alternating current were the reason that today all industrial power plants produce only three-phase alternating current.

More than half of the electrical energy generated by these power plants is consumed by electric motors. So that they can perform a variety of work, they are made different both in design and in size.

In addition to simple induction motors, which are widely used to drive machine tools, there are motors with windings and slip rings on the rotor. They develop great effort when starting off and are therefore successfully used on cranes. Is there some more synchronous motors having a constant rotation speed. In terms of size, electric motors are small - like a spool of thread - and huge, like a carousel.

The use of several electric motors to drive machine tools at once made it possible to simplify the mechanisms of the machine, facilitated their control and made it possible to create automatic machine lines.

The small size of electric motors made it possible to use electrical energy where only manual labor was previously used. Electric drills, saws, planes and other electrified tools made the work of workers much easier and more productive.

Electric polishers, vacuum cleaners, washing machines and refrigerators came to the aid of housewives.

Alternating current is a good heat source. Metal is melted and boiled in powerful electric arc furnaces. Electric resistance furnaces are widely used for air conditioning, heating ovens and various rooms.

Light bulbs give off light no matter how much current flows through their filaments. But since AC transmission is more economical, and transformers make it easy to maintain the voltage they need, the entire lighting network of cities and villages is served by alternating current.

The continuous change in the direction of movement of electrons in alternating current, its ability to transform opened a wide path for him into many areas of technology. But the current is not always good, changing its direction all the time. So you got into a trolleybus, a subway train, or an “electric train” car on the railway. Here you are in the realm of direct current.

The fact is that simple and convenient AC electric motors do not allow smoothly changing the speed of their rotation over a wide range. And remember how many times the driver has to change the speed of the trolleybus; only a DC motor can handle such hectic work well. These motors are powered from traction rectifier substations. The alternating current coming to them from power plants is converted to direct current with the help of mercury rectifiers, and then fed into contact network- in wires and rails.

The use of DC traction motors in transport vehicles has proven to be so beneficial that they can be found on diesel locomotives and motor ships.

Their main engines are diesel engines that drive generators that produce direct current. And he, in turn, makes electric motors turn wheels or propellers.

However, the high cost and complexity of converter substations have forced scientists and engineers to think about the use of alternating current in transport. Now there are sections railways using single-phase alternating current. It is also successfully used on many diesel-electric ships.

Further electrification of railways in our country will be carried out mainly using alternating current with a voltage of 25 thousand V. This current will be converted into direct current directly on electric locomotives using rectifiers.

Good adjusting ability of DC electric motors made it possible to successfully apply them also on hoisting and transport mechanisms. Conventional cranes you see on construction sites run AC motors. But on powerful cranes of large metallurgical plants, DC motors are installed. After all, here it is necessary to smoothly lift and carry huge ladles with molten metal, pour it into molds or feed red-hot ingots to rolling mills.

These engines also drive the mechanisms of giant walking excavators.

DC motors can develop very high rotation speeds - up to 25 thousand rpm. This allows you to get a lot of power with very small engine sizes. Therefore, they are indispensable as control motors used on aircraft for turning rudders, ailerons and flaps, for raising and lowering the landing gear and other mechanisms.

The constant direction of movement of electrons in a direct current circuit has determined a large and important area of ​​\u200b\u200bits application, in which alternating current cannot compete with it. We are talking about electrolysis - a process associated with the passage of current through liquid solutions - electrolytes. Under the influence of a direct current passing through the electrolyte, it decomposes into separate elements, which are deposited on certain electrodes - on the anode or cathode. This property is widely used in non-ferrous metallurgy - to obtain aluminum, magnesium, zinc, copper, manganese. AT chemical industry electrolysis produces fluorine, chlorine, hydrogen and other substances.

In electroplating, electrolysis is used to deposit metal on the surface of various products. In this way, protective coatings are applied to metal products (nickel plating, chromium plating), metal monuments, printing plates, etc. are made. Galvanization is used in medicine to treat certain diseases.

The constant direction of the electrons helps the direct current to compete with the alternating current in welding and some types of lighting. When welding with direct current, metal particles are transferred from the electrode to the product more correctly and the seam is better than when welding with alternating current.

Go to the film studio. Powerful arc film projectors flood the shooting pavilion with light. On alternating current, the arc burns less steadily, gives less light and emits a hum that interferes with sound recording when filming. Therefore, film projectors are fed with direct current, which gives a silent, stable arc. Powerful military searchlights and arc film projectors also use direct current.

To obtain alternating current, you need to continuously rotate the alternator, and direct current can be provided by stationary batteries or galvanic cells. These source properties electric current also in some cases determine the scope of direct current.

The car is in place. How to start his engine? A rechargeable battery is at your service. You press the starter button and the DC motor, powered by the battery, starts the engine. And when the motor is running, it rotates the generator, which charges the battery, restores the energy expended. Such a reversible process is not available for alternating current.

What would happen if train lighting was powered by alternating current? The train stopped - the wheels of the cars stopped rotating, and with it the electric generators would stop and the lights in the cars would go out. But this does not happen, because DC generators are installed under the cars, operating in parallel with the batteries. There is a train - the generators rotate, provide energy for lighting and at the same time charge the battery. The composition has stopped - the battery sends current to the lighting network.

Imagine that an accident has occurred at a power plant: all the turbo or hydro generators have stopped and the power lines connecting it with other power plants have been disconnected. In such cases, the direct current obtained from large batteries helps out. With its help, auxiliary mechanisms are set in motion, switched off switches are turned on, and the main turbo or hydro generators are put into operation again. Battery power is very reliable, so all control, automation and alarm protection circuits in large power plants operate on direct current.

Can a submarine float without direct current? On the surface of the water, maybe. In this case, its propellers are rotated by diesels. But diesels stop under water - there is not enough air. It has a DC motor powered by batteries. When the boat resurfaces and the diesels kick in, the electric motor turns into a generator and recharges the batteries.

In mines, it is not possible to hang a contact wire for electric locomotives everywhere. How can they move? And here again the battery comes to the rescue. In many mines, battery-operated electric locomotives haul coal from the most remote mines. Electric carts with batteries - electric cars - you often see at train stations. They are also found in the workshops of large plants and factories.

Pay attention to how the cameraman is filming some important event. He has a light film camera in his hands, and a battery on his belt. I pressed the button and the device started working. Such lightweight batteries are widely used for portable radio stations, signaling devices, electrical measuring instruments.

Of course, the examples listed here do not exhaust all areas of application of electrical energy. We have not said anything about its use for telegraph and telephone communications, for radio and television and other purposes - you will read about this in the relevant articles on our site.