High-voltage lines of direct and alternating current. Voltage generation in electrical engineering. Part 1

Technical and economic problems of switching to direct current

Although high-voltage DC transmission is now a proven and accepted technology, there are still a number of technical and economic questions, including about low-voltage networks, that need to be answered:

• Will DC be able to replace AC in a wide range of applications?
• Will both technologies continue to exist alongside each other?
• What might such coexistence look like?
• What technical and economic obstacles need to be overcome?
• What security measures will be necessary and effective at the same time?
• What changes would a switch to DC require in the grid and how would this impact consumers?

The benefits of this “switch” are so significant that there can be no doubt that a paradigm shift is approaching. With significant experience in the development of connection technologies, LAPP immediately takes a leading position here.

The company is an associated partner in the DC-INDUSTRIE project, part of the 6th Energy Research Program of the German Federal Ministry for Economic Affairs and Energy (BMWi). The DC-INDUSTRIE research project addresses the question of how DC grids with central conversion can be created as an energy-saving alternative, especially in the operation of equipment on production lines, and how to make better use of renewable energy sources.

Voltage formula

There is a formula in physics, although it has no practical application. The official formula is written like this.

voltage formula

Where

A is the work done by the electric field to move a charge along a section of the circuit, Joules

q - charge, Coulomb

U—voltage on a section of the electrical circuit, Volts

In practice, the voltage across a section of a circuit is derived through Ohm's law.

It will be interesting➡ What is the wire cross-section and how to determine it

voltage from Ohm's law

Where

I - current, Amperes

R - resistance, Ohms

Direct Current: Reviving an Old Technology

Solar Smart Grid in Haiti

Today, 86 years after Edison's death, there are signs that the great inventor was not as wrong about direct current as people once believed. Edison's ideas are becoming relevant again as a number of recent developments make direct current more attractive.

Previously, electricity was produced by alternating current in generators of large coal or nuclear power plants, as well as in hydro turbines. They distribute energy through the AC network. Transformers allow voltage to be increased to several hundred thousand volts by holding current in cables. But now a number of electricity suppliers are moving towards using DC. These include, for example, solar power plants, which are usually supported by batteries or electrochemical storage systems. Converting DC to AC inevitably involves losses, making the DC grid the best choice for these suppliers.

DC contact network

Historically, the first electric locomotives that served in the Suram Pass of the USSR were designed to be powered with a constant voltage of up to one and a half thousand volts. Accordingly, the entire transport infrastructure was created for constant voltage, and further development of electric locomotives was also carried out for a DC power system, and then the already created infrastructure played a leading role in the formation of technical requirements for locomotive-building enterprises. In the meantime, the railways were developing and, if we neglect historical accuracy, since our material is not about history, the Moscow Railway with a number of other railways of the USSR, mainly in the Central European region, acquired the infrastructure to power DC electric rolling stock. Only now the voltage was increased from 1.5 kVolt to 3 kVolt.

This increase was not done lightly. It's all about the volume of transportation, or rather its constant growth. The development of sectors of the national economy required railways to constantly increase passenger and cargo flows, and electric locomotives had to transport more and more weight, and this required high current values ​​(I, Ampere).

Contact wire profile

Based on the laws of electrical engineering, we know that electrical power is equal to the product of I and the effective voltage; to increase the power of an electric locomotive, we need to either increase the number of Volts, or the number of Amperes, or both. At an operating voltage of even 3000 Volts, I must constantly increase, and this leads to increased heating of the wires, which means the contact wire must be of sufficient cross-section. DC is also sensitive to the length of the conducting line: the greater the distance, the more noticeable the resistance of the conductor eats up part of the useful voltage. And also, based on the high Ampere values, when the wheels of a locomotive slip, there is a high risk of local heating at the point of contact of the pantograph with the contact wire, which can cause the latter to burn out. There is also a significant limitation on the number of simultaneously moving trains in the area served by one electrical substation, since it must produce a sum of already high currents.

Disadvantages of DC contact network

DC for the needs of railway traffic has complete disadvantages, and is definitely a less suitable option. Today, all electrification of railways is carried out only by AC, with the exception of historically established infrastructures under DC. Over time, I think, all railways in Russia will switch to alternating current, but for now there are a huge number of rolling stock units, and these are electric locomotives and electric trains, designed for “permanent” operation, which makes the transition to “alternating current” on such roads economically infeasible .

To summarize what has already been said, DC electrification has the following disadvantages:

• The need to use high I values ​​to obtain adequate power;
• It is necessary to place power supply substations at a distance of 50 kilometers from each other, because at large distances the resistance of the contact wire noticeably reduces the effective voltage, which immediately affects the power;
• Noticeable reduction in power in multi-train areas requiring high power;
• The high cost of infrastructure, the need to use contact wires with a large cross-section;
• High influence of Foucault currents on infrastructure elements.

The only advantages that can be noted are the simplicity of the design of electric rolling stock and the ease of regulating the operation of traction motors.

Why does the network have alternating voltage and not constant

Alternating current has many advantages over direct current. Low losses during the transmission of alternating current in power lines (power lines) compared to direct current. Alternators are simple and cheap. When transmitted over long distances along power lines, high voltage reaches 330 thousand volts with minimal current.

The lower the current in the power line, the lower the losses. Transmission of direct current over long distances will incur considerable losses. Also, high-voltage alternators are much simpler and cheaper. It is easy to get lower voltage from AC voltage through simple transformers.

Also, it is much cheaper to obtain DC voltage from AC voltage than, on the contrary, to use expensive DC-AC voltage converters. Such converters have low efficiency and high losses. Double conversion is used along the AC transmission path.

First, it receives 220 - 330 kV from the generator, and transmits it over long distances to transformers, which lower the high voltage to 10 kV, and then there are substations that lower the high voltage to 380 V. From these substations, the electricity is distributed to consumers and supplied to homes and electrical panels apartment building.

Three phases of three-phase current shifted by 120 degrees

Single-phase voltage is characterized by one sinusoid, and three-phase voltage is characterized by three sinusoids, offset by 120 degrees relative to each other. A three-phase network also has its advantages over single-phase networks. These are smaller dimensions of transformers, electric motors are also structurally smaller.

It is possible to change the direction of rotation of the rotor of an asynchronous electric motor. In a three-phase network, you can get 2 voltages - 380 V and 220 V, which are used to change the engine power and adjust the temperature of the heating elements. Using three-phase voltage in lighting, it is possible to eliminate the flickering of fluorescent lamps, for which they are connected to different phases.

Direct current is used in electronics and in all household appliances, since it is easily converted from alternating current by dividing it on a transformer to the required value and further straightening it. The source of direct current is batteries, batteries, direct current generators, LED panels. As you can see, the difference in alternating and direct current is considerable. Now we have learned - Why does our socket flow alternating current and not direct current?

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Why is alternating current more dangerous than direct current?

In the war of currents, in order not to suffer losses and financial collapse from the introduction and use of Tesla's ideas, Edison publicly demonstrated how alternating current kills animals. The case where an American citizen died from an alternating current shock was very detailed and widely covered in the press.

For humans, alternating current is, in general, actually more dangerous than direct current. Although you always need to take into account the magnitude of the current, its frequency, voltage, and the resistance of the person being shocked. Let's consider these nuances:

1. Alternating current with a frequency of 50 Hertz is three to four times more dangerous to life than direct current. If the frequency of the current is more than 1000 Hertz, then it is considered less dangerous.
2. At voltages of about 400-600 Volts, alternating and direct currents are considered equally dangerous. At voltages greater than 600 volts, direct current is more dangerous.
3. Alternating current, due to its nature and frequency, excites the nerves more strongly, stimulating the muscles and heart. That is why it poses a great danger to life.

Types of current

There are two types of current - direct and alternating. To understand the difference and determine whether the outlet has direct or alternating current, you should delve into some technical features. Alternating current has the property of changing in direction and magnitude. Direct current has stable qualities and direction of movement of charged particles.

Alternating current comes out of the power plant generators with a voltage of 220-440 thousand volts. When approaching an apartment building, the current is reduced to 12 thousand volts, and at the transformer station it is converted to 380 volts. The voltage between phases is called linear. The low-voltage section of the step-down substation produces three phases and a zero (neutral) wire. Energy consumers are connected from one of the phases and the neutral wire. Thus, single-phase alternating current with a voltage of 220 volts enters the building.

The distribution diagram of electricity between houses is presented below:

In the home, electricity is supplied to the meter, and then through automatic machines to the boxes of each room. The boxes contain wiring throughout the room for a couple of circuits - electrical outlets and lighting equipment. The machines can be provided one for each room or one for each circuit. Taking into account how many amperes the outlet is designed for, it can be included in a group or connected to a dedicated circuit breaker.

Alternating current accounts for approximately 90% of all electricity consumed. Such a high specific gravity is due to the peculiarities of this type of current - it can be transported over considerable distances by changing the voltage at substations to the required parameters.

Sources of direct current are most often batteries, galvanic cells, solar panels, thermocouples. Direct current is widely used in local networks of automobile and air transport, in computer electrical circuits, automatic systems, radio and television equipment. Direct current is used in contact networks of railway transport, as well as on ship installations.

Note! Direct current is used in all electronic devices. The diagram below shows the fundamental differences between direct and alternating currents

The diagram below shows the fundamental differences between direct and alternating currents.

Using DC Laws

The laws of direct current are remarkable in that they allow the use of standard mathematical apparatus to calculate the steady state of an electrical circuit of any complexity.

First, all known and unknown elements in the circuit are designated (as a rule, the values ​​of the emf at the current sources and the resistance of individual elements are known).

Then a system of several linear algebraic equations with several unknowns is constructed. For each node and each circuit of a branched chain, Kirchhoff's rules are written. And the voltages on the circuit elements are expressed through the resistance of the elements and the currents through them.

The resulting system (it can be very large and contain hundreds of equations with hundreds of unknowns) is solved using standard mathematical techniques.

How to check the voltage in the network with a multimeter and what current is in the outlet: alternating or direct

The flow of electrons (each electron) moves strictly in one direction from “minus” to “plus”. This is why it is so important to maintain polarity in batteries. If you connect two “minuses” or two “pluses”, the current simply will not flow.

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What kind of current is in the outlet - constant or variable, how many volts and amperes? In order to understand how current flows, it is necessary to understand its physical essence, based on the atomic-molecular theory of the structure of matter, to find out what conditions are necessary for its occurrence and existence, what types of currents are there and what characteristics they have. Ask, I'm in touch!

A Brief History of Electricity

Who invented electricity? And no one! People gradually understood what it was and how to use it.

It all started in the 7th century BC, on one sunny (or maybe rainy, who knows) day. Then the Greek philosopher Thales noticed that if you rub amber on wool, it will attract light objects.

Then there were Alexander the Great, wars, Christianity, the fall of the Roman Empire, wars, the fall of Byzantium, wars, the Middle Ages, the Crusades, epidemics, the Inquisition and more wars. As you understand, people had no time for any electricity or ebonite sticks rubbed with wool.

In what year was the word “electricity” invented? In 1600, the English naturalist William Gilbert decided to write the work “On the Magnet, Magnetic Bodies and the Great Magnet - the Earth.” It was then that the term “electricity” appeared.

One hundred and fifty years later, in 1747, Benjamin Franklin, whom we all love very much, created the first theory of electricity. He viewed this phenomenon as a fluid or immaterial liquid.

It was Franklin who introduced the concept of positive and negative charges (before that, glass and resin electricity were separated), invented the lightning rod and proved that lightning is electrical in nature.

Everyone loves Benjamin, because his portrait is on every hundred dollar bill. In addition to his work in the exact sciences, he was a prominent political figure. But contrary to popular belief, Franklin was not the President of the United States.

Next will be a list of discoveries important for the history of electricity.

1785 - Coulomb finds out with what force opposite charges attract and like charges repel.

1791 - Luigi Galvani accidentally noticed that the legs of a dead frog contracted under the influence of electricity.

The operating principle of the battery is based on galvanic cells. But who created the first galvanic cell? Based on Galvani's discovery, another Italian physicist Alessandro Volta created the Volta column in 1800, the prototype of the modern battery.

At excavations near Baghdad, they found a battery more than two thousand years old. What ancient iPhone was recharged with its help remains a mystery. But we know for sure that the battery has already run out. This case seems to say: maybe people knew about electricity much earlier, but then something went wrong.

Already in the 19th century, Oersted, Ampere, Ohm, Thomson and Maxwell made a real revolution. Electromagnetism was discovered, induced emf, electrical and magnetic phenomena were linked into a single system and described by fundamental equations.

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The 20th century brought quantum electrodynamics and the theory of weak interactions, as well as electric cars and ubiquitous power lines. By the way, the famous Tesla electric car runs on direct current.

Of course, this is a very brief history of electricity, and we have not mentioned very many names that influenced progress in this field. Otherwise, a whole multi-volume reference book would have to be written.

Alternating current and its properties

Alternating current cyclically changes direction and strength, characterized by the following parameters:

1. frequency. The number of cycles (periods) per second. For example, the frequency of the current in the network is 50 Hz;
2. amplitude. Maximum deviation of voltage and current from zero. Thus, the mains voltage changes from -311 V to 311 V 50 times per second;
3. effective value. This is the voltage or strength of an equivalent direct current, that is, one that causes the same heat generation in the conductor as a given alternating current. The effective value is used to simplify calculations: working with constantly changing values ​​is extremely inconvenient. For example, if you write in the formula the actual value of the alternating mains voltage, varying from -311 V to 311 V according to a sinusoidal law, you will get an equation with trigonometric functions or complex numbers. It is much easier to operate with a constant effective value of 220 V;
4. form. The mains current produced by mechanical generators has a sinusoidal shape. At the inverter output it can be acute-angled, stepped, etc.

Alternating current is inferior to direct current in the following ways:

1. it is of lower quality. Thus, the weld is stronger and more reliable if welding was carried out with direct current. The electronics also work better;
2. at a frequency of 50 Hz it is more dangerous. It causes disturbances in the body already at a force of 50 mA, while constant - at a force of 300 mA. However, as the frequency increases, alternating current becomes less dangerous. Thus, the outstanding inventor Nikola Tesla, in public experiments, passed a high-voltage alternating current through himself (a lamp clutched in his hand glowed), having previously raised its frequency to several megahertz;
3. The resistance of conductors to alternating current is higher than to direct current. An explanation of this will be given below.

But alternating current also has a useful feature: the magnetic field it creates is also variable, which means it can induce EMF (the law of electromagnetic induction) in conductors.

Alternating current makes it possible to operate the following devices:

1. transformers. By increasing the voltage, losses in power lines are significantly reduced;
2. induction heaters;
3. throttle filters. Choke - coil. The alternating magnetic field it creates counteracts the alternating current, that is, the inductor acts as a resistance. The frequency of the current that it most strongly opposes depends on the inductance of the coil. This feature allows the choke to suppress high-frequency interference in the network.

The presence of an alternating magnetic field also explains the above-mentioned increase in conductor resistance. In it, the field also induces an emf that counteracts this alternating current. This EMF is higher in the center of the conductor, where the field lines are concentrated, and accordingly, charge carriers are forced out (surface or skin effect).

As a result, instead of the entire cross-section of the conductor, only a certain part of it passes current, which is why the resistance increases. Another difference between alternating current and direct current is the ability to flow through a circuit with a capacitor connected in series. For direct current, the gap between the plates is insurmountable, while alternating current flows almost freely, charging plates with one or the other sign.

A capacitor, like a coil, stores energy each time and then returns it to the circuit, so it also provides alternating current resistance, which depends on the capacitance of the capacitor.

Electric current and electron flow

Having understood that in most cases the carriers of electric charges are electrons, it is necessary to understand why they move. To do this, it is necessary to look into the microworld of particles - atoms and understand their structure, the physical processes occurring with them.

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Nucleus and electrons

In dialectics, there are no particles capable of carrying an electric charge - they have few electrons at external levels, so they cannot break loose, turning first into chaotic, then into directed motion.

An intermediate position between dielectrics and conductors is occupied by semiconductors, the electrical conductivity of which depends on external factors (temperature, illumination, etc.).

What is the working principle of alternating current

The English abbreviation AC (Alternating Current) denotes a current that changes its direction and magnitude over time periods. The sinusoid segment “~” is its conventional marking on devices. Applying after this icon and other characteristics is also used.

Below is a figure with the main characteristics of this type of current - nominal frequency and operating voltage.

It should be noted the features of the change in the left graph, made for a single-phase current, in the magnitude and direction of the voltage with the transition to zero over a certain period of time T. For one third of the period, three sinusoids are shifted for a three-phase current on another graph.

O and “b” indicate phases. Any of us has an idea of ​​the presence of 220V in a regular outlet. But for many it will be a discovery that the maximum or otherwise called amplitude value is greater than the acting value by an amount equal to the root of two and is 311 Volts.

Obviously, in the case of direct current, the parameters of direction and voltage remain unchanged, but for alternating current, a transformation of these quantities is observed. In the figure, the opposite direction is the area of ​​the graph below zero.

Let's move on to frequency. This concept means the ratio of periods (full cycles) to a conventional unit of time for a changing current. This indicator is measured in Hertz. The standard European frequency is 50, in the USA the applicable standard is 60G.

This value shows the number of changes in the direction of the current in one second to the opposite and return to the original state.

Alternating current is present when consumer devices are directly connected to electrical panels and sockets. For what reason is there no direct current here? This is done in order to be able to obtain the required voltage in any quantity by using transformers without any significant losses. This technique remains the best way to transmit power on an industrial scale over significant distances with minimal losses.

The rated voltage, which is supplied by powerful generators of power plants, at the output is about 330,000-220,000 Volts. At a substation located in the consumption area, this value is transformed to 10,000V with a transition to a three-phase version of 380 Volts. A separate house is supplied and single-phase voltage reaches your apartment. The voltage between zero and phase will be 220 V, and in the shield between different phases this figure is 380 Volts.

What is alternating current and alternating voltage?

There are two main types of current - direct and alternating. To understand these terms, it is necessary to remember that current is the ordered movement of electrons. And when these electrons move in the same direction all the time, then such a current is called constant. But the concept of ordered movement should also be understood as the fact that at one moment the electrons move in one direction and at the second moment - in the opposite direction, and so on without stopping. This current is already called alternating. If they talk about constant and alternating voltage, then they mean that for constant voltage the + and - are always “in the same place.”

An example of constant voltage is an ordinary battery; on its body you will always find the symbols + and -. And for a variable, + and - change after a certain period of time. Consequently, constant voltage creates direct current , and accordingly alternating voltage creates alternating current. An example of alternating voltage is an ordinary electrical network. Direct current is indicated by one straight line, and alternating current by one wavy line.

I think you have often seen the inscription 220V, in front of which there is a horizontal wavy line. This is the designation for alternating current.

Please note that devices that use direct current in overwhelming quantities do not allow the + and - contacts to be mixed up with each other when connecting power to them, because if they are mixed up, the device can simply “burn out”. But for alternating voltage this is no longer relevant, let’s say you plug into a socket... or whatever, and it doesn’t matter which side you insert the plug into the socket, the device will still work smoothly. Surely, you also had to notice an inscription similar to 50Hz near the inscriptions 220V. This is the AC frequency. And it means how many times per second “plus and minus” changes places. The inscription 50Hz (Hertz) means that in one second the polarity of the voltage changes 50 times.

Charts

In order to imagine exactly how the polarity of an alternating voltage changes, it is necessary to understand the graphs that show the voltage at different times. Let's look at the graph showing constant voltage (it's on the left). Let's assume that this graph shows the voltage across the contacts of a flashlight bulb.

Starting from point 0 to point “a” the graph shows that the voltage is zero. Or in other words, it is not there at all (the flashlight is turned off). At time “a” (in our version, a voltage equal to U1 appears at the contacts of the light bulb, which remains unchanged during the time from “a” to “b” (the flashlight is on). At time “b” the Voltage disappears again (becomes equal to zero). If you look at the second graph, which displays alternating voltage, then I think it is not difficult to figure out what exactly is happening with alternating voltage at different points in time. At the zero point it is zero. During the time from “0” to “a” the voltage smoothly increases to the value U1 and at the same moment begins to decrease. As a result, at time “b” reaches zero. But as you can see in the graph, the voltage continues to drop and becomes negative. At point “g” it reaches a minimum and begins to increase again. This phenomenon is repeated throughout the existence of the voltage (until the light is turned off. It should be noted that the alternating voltage can be not only of this shape. It can be, for example, rectangular or almost any other shape. Now take another look at these two graphs, and remember , as the designation of direct and alternating current (voltage).

Based on materials from the site: https://radio-electro.narod.ru/kurs/2peremen.htm

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Direct and alternating current

Any phenomenon that cannot be seen or “touched” directly is easier to understand using analogies. In the case of electricity, we can consider water in a pipe as the closest example. Water and electricity flow through their own conductors - wires and pipes.

• The volume of flowing water is the current strength.
• The pressure in the pipe is the voltage.
• Pipe diameter is conductivity, which is the reciprocal of resistance.
• Volume per pressure - power.

The pressure in the pipe is created by the pump - the pump pumps harder, the pressure is higher, more water flows.
The diameter of the pipe is larger - the resistance is less, more water flows. The source produces more voltage - more electricity flows. Wires are thicker - less resistance, higher current. For example, you can take any chemical power source - a battery or an accumulator. Its terminals have pole designations: plus or minus. If you connect a corresponding light bulb to the battery, through the wires and the switch, it will light up. What happens? The negative terminal of the source emits electrons - elementary particles carrying a negative charge. Along the wires, through the switch connectors and the lamp spiral, they move towards the positive terminal, trying to equalize the potential of the terminals. As long as the circuit is closed across the switch connectors and the battery is not dead, electrons flow in a spiral and the light bulb is on.

For many reasons, using only constant voltage is impractical: take, for example, the inability to use transformers. Therefore, to date, a system for supplying and consuming alternating voltage has been developed, for which household appliances are created. There is a simple answer to what is the difference between direct and alternating current. In this light bulb example, the voltage on one terminal of the power supply will always be zero. This is a neutral wire, but on the other - phase wire - the voltage changes. And not only in size, but also in direction - from plus to minus. Electrons do not flow in orderly rows in one direction, on the contrary, they rush back and forth, the same particles run back and forth along the incandescent spiral and do all the work. Changing the direction of movement of electricity gives the very concept of “variable”.

Converting AC to DC

Electrical devices around the world use direct and alternating current. Therefore, there is a need to convert one current into another or vice versa.

Direct current can be obtained from alternating current using a diode bridge or, as it is also called, a “rectifier”. The main part of the rectifier is a semiconductor diode, which conducts electric current in only one direction. After this diode, the current does not change its direction, but ripples appear, which are eliminated with the help of capacitors and other filters. Rectifiers are available in mechanical, electrovacuum or semiconductor versions.

Depending on the quality of manufacture of such a device, the output current ripples will have different meanings; as a rule, the more expensive and better the device is made, the less ripples and the purer the current. Examples of such devices are power supplies for various devices and chargers, rectifiers for electric power plants in various types of transport, DC welding machines and others.

Alternating and direct current: what is the difference, history of development, application

Children are taught that they should not stick their fingers into electrical sockets! And why? Because it will be bad. There are often problems with a more detailed explanation: there is some kind of voltage, current, something is flowing somewhere. So that in the future you can explain to your children what’s what, we will now explain to you. This article is about alternating and direct currents, their differences, applications and the history of electricity in general. Science needs to be made interesting, and we modestly try to do this to the best of our ability.

For example: what current is in our sockets? Variable, of course! Voltage 220 Volts and frequency 50 Hertz. And the network through which the current is transmitted is three-phase. By the way, if at the words “phase” and “zero” you fall into a stupor, read what it is, and the day will be lived doubly not in vain! But let's not get ahead of ourselves. First things first.

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Outlet current

The frequency standards in Russia and European countries are 50 Hz, in America - 60 Hz. The current strength in apartments is limited to 16 Amperes; in private country houses this value can reach 25 A.

Current measurements are carried out in various ways. You can try it experimentally - plug the device into an outlet, and if it functions, there is electricity. There are multimeters that measure values, test lamps, testers and voltage indicators.

220 V

The nominal voltage in the home network is 220V, but in practice this value may vary.
Deviations up to 20-25 Volts. This indicator is influenced by:

• technical condition,
• network load,
• workload of power plants.

Voltage surges damage devices, so it is better to connect to the network through special stabilizers.

More than 220 V

For power electrical equipment, three-phase networks are used, which are powered by a voltage of 380 Volts and higher. Most often they can be found in electric transport - trams, trolleybuses, electric trains. For this voltage, the current load is up to 32 A.

DC Characteristics

Direct Current or DC in English means a similar variety, which has the inherent property of not changing its parameters over any period of time. A small horizontal line or two parallel lines with a line drawing of one of them is a graphic representation of direct current.

Scope of application: most models of household electrical appliances and electronic devices, including computer equipment, televisions and gadgets, use in home networks and cars. To convert alternating current into direct current in the outlet area, voltage transformers with rectifiers or specialized power supplies are used.

A common example of DC consumption is almost all power tools that operate with batteries. The battery device remains in any case a constant power source. Conversion to variable is achieved, if necessary, with the help of inverters - special elements.

What is the current in the batteries?

A variable current comes out of the outlet as the direction of the flow of electrons changes. This kind of current has different frequency and voltage values. Therefore, in sockets - 220V at 50Hz. It looks more clearly like this: in one second the flow of electrons changes 50 times, while the charges also change from positive to negative.

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This is especially noticeable when turning on or supplying electricity to fluorescent lamps. When electrons accelerate, the lamp flickers, which means that the flow is changing. The maximum voltage potential is 220V, at which electrons move.

Batteries

The charge changes with alternating current. It turns out that the voltage is either 100% or 0%. With an indicator of 100%, it is necessary that the wire be of large diameter, and if the charge is not constant, then a wire of small cross-section is sufficient. A large number of volts can be transported through such a conductor, after which the transformer takes in the excess, leaving 220V at the output.

Attention! In batteries or accumulators, the current is constant because the direction of the electrons does not change. Charging is designed to transform it from alternating to constant, in this form it is supplied by batteries.

Galvanic cell

Electric batteries

This is a reusable DC source that does not operate continuously, but until the next charge. Based on their chemical nature, they are divided into types:

• lead-acid;
• lithium-ion (lithium);
• nickel-cadmium;
• nickel-iron.

Lead-acid models are used in automobiles, uninterruptible power supplies, transport, industry, communications and telecommunications.

Lithium-ion batteries are widely used in mobile communications, power tools, telecommunications systems, as well as autonomous and emergency power supplies. Here is just a small list of the range of their compositions:

• lithium titanate;
• thionyl chloride;
• lithium cobalt;
• lithium manganese;
• lithium iron phosphate;
• lithium polymer;
• lithium sulfur dioxide;
• lithium manganese dioxide.

Lithium-ion power sources

Nickel-cadmium batteries

Nickel-iron alkaline is a very reliable type of source. Deep discharges and frequent undercharging, which are detrimental to lead-acid batteries, do not damage them. They are used in traction transport circuits and backup power circuits.

Alternating current

Most often, however, this is what is used. Here the average value of force and stress over a certain period is equal to zero. It constantly changes in size and direction, and at regular intervals.

To generate alternating current, generators are used in which an electromotive force is generated during electromagnetic induction. This is done using a magnet rotated in a cylinder (rotor) and a stator made in the form of a stationary core with a winding.

Alternating current is used in radio, television, telephony and many other systems due to the fact that its voltage and power can be converted with almost no loss of energy.

It is widely used in industry, as well as for lighting purposes.

It can be single-phase or multi-phase.

Alternating current, which varies according to a sinusoidal law, is single-phase. It changes over a certain period of time (period) in magnitude and direction. AC frequency is the number of cycles per second.

In the second case, the three-phase version is most widespread. This is a system of three electrical circuits that have the same frequency and emf, shifted in phase by 120 degrees. It is used to power electric motors, furnaces, and lighting fixtures.

Humanity owes many developments in the field of electricity and their practical application, as well as the impact on high-frequency alternating current, to the great scientist Nikola Tesla. Until now, not all of his works left to descendants are known.

Current and voltage

Oddly enough, most people do not understand the difference between the definitions of current and voltage and do not strive to do so. Although even during my school years in physics, these concepts were chewed in some detail. So, let's start by defining the concepts of electric current, current and voltage. Electric current is the ordered movement of charged particles (or electrons). The current has a strictly defined direction - from plus to minus. The main condition for the presence of current in an electrical circuit is the presence of a closed circuit. As soon as the circuit is broken, the current stops flowing. Current strength is the number of electrons passing through the cross-sectional area of ​​a conductor in a certain unit of time. Current strength is usually measured in Amperes and denoted by the letter A. Andre Ampere is a French physicist, after whom this unit of measurement is named.

Origin

The difference between AC and DC lies in their origin. Direct current can be obtained from galvanic cells, such as batteries and accumulators.

It can also be obtained using a dynamo - this is an outdated name for a direct current generator. By the way, with their help, energy was generated for the first electrical networks. We talked about this in an article about the discoveries of Nikola Tesla, in notes about the war of ideas between Tesla and Edison. Later this was the name given to small generators used to power bicycle headlights.

Alternating current is also produced using generators, nowadays mostly three-phase.

Also, both voltages can be obtained using semiconductor converters and rectifiers. So you can rectify alternating current or get the same by converting direct current.

Power factor

The active power of an AC powered load can be calculated using the simple formula P = U × I × cos (φ), where φ is the angle between voltage and current, cos (φ) is also called power factor. This is how direct and alternating current differ: for the first, cos (φ) is always equal to 1. Active power is needed (and paid for) by household and industrial consumers, but it is not equal to the complex power passing through the conductors (cables) to the load, which can be calculated using the formula S = U × I and measured in volt-amperes (VA).

The difference between direct and alternating current in the calculations is obvious - they become more complex. Even the simplest calculations require at least a mediocre knowledge of vector mathematics.

Currently, Russian Railways uses two types of current on electrified sections of railways:

• Direct current (DC) voltage 3,000 Volts;
• Alternating current (AC) with a frequency of 50 Hz and a voltage of 27,000 Volts.

On Russian railways, historically, and as a legacy from the Soviet Union, most sections of the track for train traffic are electrified.
The length of tracks already electrified in our time is steadily increasing, Russian Railways has already ensured a complete transition to electric locomotive traction on the Trans-Siberian Railway, and now movement from Vladivostok to Moscow is carried out only by electric energy. However, if you try to travel this route using the traction of only one single electric locomotive (with the exception of dual-system electric locomotives), nothing will work out as planned. Such a locomotive will simply stand in places where the current voltage and current in the contact network change. Contact network DC 3 kVolt

Current as flow

Current is the flow rate of electrons, indicating how many electrons are moving through the cable. The higher it is, the more electrons pass through the conductor. Just as large amounts of water require thicker pipes, large currents require thicker cables.

Using the water circuit model allows you to explain many other terms. For example, power generators can be thought of as water pumps, and electrical loads can be thought of as water mills that require water flow and pressure to rotate. Even electronic diodes can be thought of as water valves that only allow water to flow in one direction.

DC current

To answer the question of what kind of current is called direct current, it is enough to read the above general definition of electric current and a brief definition of direct current.
So, direct current is the ordered movement of electrical particles, during which these particles do not change their direction, and the magnitude of the current does not change. This phenomenon can also be described more broadly, based on the physical processes occurring during this process. Surely everyone remembers the concepts of “plus” and “minus” from a school physics course, that is, the concept of poles charged with opposite charges. To understand the process of the flow of our electric current, we can imagine an ordinary AA battery and a wire, which is connected at one end to the positive pole and the other to the negative pole (doing this in practice is extremely undesirable because of the possibility of damaging the power source, and in the case of large batteries, even causing burns and injuries).

So, as soon as the second end of the wire is closed, that is, connected to the pole, electron movement will immediately appear in the circuit. From the negative pole, that is, the pole at which there is an excess of elementary electrical charges, these charges will flow to the positive pole, where, on the contrary, there is a deficiency. We can say that this movement is designed to balance the number of charges on both sides. When this happens, the electrons will stop moving, meaning the battery will run out.

What is current and Ohm's law?

If we consider the example with the battery described above from the point of view of physics, then it will involve three components - current, voltage and resistance.
When talking about how direct current is designated, it is the current strength that is meant. It is designated by the letter I. Voltage by the letter U, and resistance by R. These three characteristics formed the basis of the most famous law in electrical engineering and irreplaceable in almost any calculation of electrical circuits, called Ohm’s law, in honor of its creation. By the way, the units of resistance have the same name - Ohms.

This law sounds like this: current I is directly proportional to voltage U and inversely proportional to resistance R: I=U/R.

There are special instruments to measure all of the above values. For current - an ammeter, for voltage - a voltmeter, for resistance - a voltmeter. For example, we can measure the current strength if we connect an ammeter in series with the element on which we must find the indicated characteristic. There are devices that combine all of the above measuring instruments - multimeters.

What is the current in the car network, direct or alternating? Auto repair

• The presence of charge carriers - particles moving along a conductor, gas or electrolyte;
• An electric field created by a certain power source - without this force field, the movement of free charge carriers will be chaotic, without a specific direction;
• Closed circuit - directional movement of charges is possible only in closed circuits. So, for example, consisting of a key power source (switch) and an incandescent light bulb, current will flow only when the switch, located in the conductor gap between one of the power poles and the lamp, is in the on state, allowing charge carriers to move along a closed circuit from the negative battery poles to positive.