Although electrical devices we use every day Everyday life, not everyone can answer how it differs alternating current from permanent, despite the fact that this is discussed within school curriculum. Therefore, it makes sense to recall the basic tenets.

General definitions

The physical process in which charged particles move in an orderly (directional) manner is called electric current. It is usually divided into variable and constant. For the first, the direction and magnitude remain unchanged, but for the second, these characteristics change according to a certain pattern.

The above definitions are greatly simplified, although they explain the difference between direct and alternating current. For better understanding what this difference is, it is necessary to give graphic image each of them, and also explain how the variable is formed electromotive force in the source. To do this, let us turn to electrical engineering, or rather its theoretical foundations.

EMF sources

Sources of electric current of any kind are of two types:

• primary, with their help, electricity is generated by converting mechanical, solar, thermal, chemical or other energy into electrical energy;
• secondary, they do not generate electricity, but convert it, for example, from variable to constant or vice versa.

The only primary source of alternating electric current is a generator; a simplified diagram of such a device is shown in the figure.

Designations:

• 1 – direction of rotation;
• 2 – magnet with poles S and N;
• 3 – magnetic field;
• 4 – wire frame;
• 5 – EMF;
• 6 – ring contacts;
• 7 – current collectors.

Principle of operation

Mechanical energy is converted by the generator shown in the figure into electrical energy as follows:

Due to such a phenomenon as electromagnetic induction, when the frame “4” rotates, placed in the magnetic field “3” (arising between the different poles of the magnet “2”), an emf “5” is formed in it. Voltage is supplied to the network through current collectors “7” from ring contacts “6”, to which frame “4” is connected.

Video: direct and alternating current - differences

As for the magnitude of the EMF, it depends on the speed of intersection of the power lines “3” by the frame “4”. Due to the features electromagnetic field minimum speed intersection, and therefore the lowest value of the electromotive force will be at the moment when the frame is in a vertical position, respectively, the maximum - in a horizontal position.

Taking into account the above, in the process of uniform rotation an emf is induced, the characteristics of the magnitude and direction of which change with a certain period.

Graphic images

Thanks to the application graphic method, you can get a visual representation of the dynamic changes of various quantities. Below is a graph of voltage changes over time for galvanic cell 3336L (4.5 V).

As you can see, the graph is a straight line, that is, the source voltage remains unchanged.

Now we present a graph of the dynamics of voltage changes during one cycle (full revolution of the frame) of the generator.

The horizontal axis displays the angle of rotation in degrees, the vertical axis displays the magnitude of the emf (voltage)

For clarity, we will show the initial position of the frame in the generator, corresponding to the starting point of the report on the graph (0°)

Designations:

• 1 – magnet poles S and N;
• 2 – frame;
• 3 – direction of rotation of the frame;
• 4 – magnetic field.

Now let's see how the EMF will change during one cycle of rotation of the frame. At the initial position, the EMF will be zero. During the rotation process, this value will begin to increase smoothly, reaching a maximum at the moment when the frame is at an angle of 90°. Further rotation of the frame will lead to a decrease in the EMF, reaching a minimum at the moment of rotation by 180°.

Continuing the process, you can see how the electromotive force changes direction. The nature of the changes in the EMF that has changed direction will be the same. That is, it will begin to increase smoothly, reaching a peak at the point corresponding to a 270° rotation, after which it will decrease until the frame completes a full rotation cycle (360°).

If the graph is continued for several rotation cycles, we will see a sinusoid characteristic of alternating electric current. Its period will correspond to one revolution of the frame, and its amplitude will correspond to the maximum value of the EMF (forward and reverse).

Now let's move on to another one important characteristic alternating current - frequency. For its designation it is accepted latin letter“f”, and its unit of measurement is hertz (Hz). This parameter displays the number of complete cycles (periods) of EMF change within one second.

The frequency is determined by the formula: . The "T" parameter displays the time of one full cycle(period), measured in seconds. Accordingly, knowing the frequency, it is easy to determine the time of the period. For example, in everyday life an electric current with a frequency of 50 Hz is used, therefore, its period time will be two hundredths of a second (1/50 = 0.02).

Three-phase generators

Note that the most cost-effective way to obtain alternating electric current is to use a three-phase generator. A simplified diagram of its design is shown in the figure.

As you can see, the generator uses three coils, placed with an offset of 120°, connected to each other by a triangle (in practice, such a connection of the generator windings is not used due to low efficiency). When one of the poles of the magnet passes by the coil, an emf is induced in it.

What is the reason for the variety of electric currents?

Many may have a well-founded question - why use such a variety of electric currents if you can choose one and make it standard? The thing is that not every type of electric current is suitable for solving a particular problem.

As an example, we give the conditions under which to use constant pressure will not only be unprofitable, but sometimes impossible:

• the task of transmitting voltage over distances is easier to implement for alternating voltage;
• it is almost impossible to convert direct electric current for heterogeneous electrical circuits that have an uncertain level of consumption;
• maintaining the required voltage level in direct current circuits is much more difficult and expensive than alternating current;
• motors for alternating voltage are structurally simpler and cheaper than for direct voltage. At this point it should be noted that for such motors (asynchronous) high level starting current, which does not allow them to be used to solve certain problems.

Now we give examples of problems where it is more appropriate to use constant voltage:

• to change the rotation speed asynchronous motors it is required to change the frequency of the power supply network, which requires complex equipment. For motors running on direct current, it is enough to change the supply voltage. That is why they are installed in electric vehicles;
• nutrition electronic circuits, electroplating equipment and many other devices are also carried out by direct electric current;
• DC voltage is much safer for humans than alternating voltage.

Based on the examples listed above, there is a need to use various types voltage.

Electric current is the directional movement of charged particles. The quantitative characteristics of the current are its current strength (the ratio of the charge transferred through the cross-section of the conductor per unit time) and its density, determined by the ratio. The unit of current measurement is ampere (1A is the characteristic value of the current consumed by household electric heating appliances). Necessary conditions the existence of current are the presence free media charges, a closed circuit and an EMF source (battery) that supports directional movement.

Electricity can exist in various environments: in metals, vacuum, gases, in solutions and melts of electrolytes, in plasma, in semiconductors, in the tissues of living organisms. When current flows, charge carriers almost always interact with environment, accompanied by the transfer of energy to the latter in the form of heat. The role of the EMF source is precisely to compensate for heat losses in the circuits. Electric current in metals is caused by the movement of relatively free electrons through the crystal lattice. The reasons for the existence of free electrons in conducting crystals can only be explained in the language of quantum mechanics.

Experience shows that the strength of the electric current flowing through a conductor is proportional to the potential difference applied to its ends (Ohm's law). The constant proportionality coefficient between current and voltage for a selected conductor is called electrical resistance. Resistance is measured in ohms (the resistance of the human body is about 1000 ohms). Magnitude electrical resistance conductors increases slightly with increasing temperature. This is due to the fact that when heated, the nodes of the crystal lattice enhance chaotic thermal vibrations, which prevents the directional movement of electrons.

In many problems, direct consideration of lattice vibrations turns out to be very labor-intensive. To simplify the interaction of electrons with oscillating units, it turns out to be convenient to replace them with collisions with gas particles of hypothetical particles - phonons, the properties of which are selected so as to obtain a description as close as possible to reality and can turn out to be very exotic. Objects of this type are very popular in physics and are called quasiparticles. In addition to interactions with vibrations of the crystal lattice, the movement of electrons in a crystal can be hampered by dislocations - violations of the regularity of the lattice. Interactions with dislocations play a decisive role at low temperatures, when thermal vibrations are practically absent.

Some materials at low temperatures completely lose electrical resistance, turning into a superconducting state. Current in such media can exist without any EMF, since there are no energy losses during collisions of electrons with phonons and dislocations. The creation of materials that maintain a superconducting state at relatively high (room) temperatures and low currents is very important task, the solution of which would make a real revolution in modern energy, because would make it possible to transmit electricity to long distances without heat loss.

Currently, electric current in metals is used mainly to transform electrical energy into thermal (heaters, light sources) or mechanical (electric motors). In the latter case, electric current is used as a source of magnetic fields, the interaction with which of other currents causes the appearance of forces.

1. Alternating current

As is known, the current strength at any time is proportional to the emf of the current source (Ohm’s law for complete chain). If the emf of the source does not change over time and the parameters of the circuit remain unchanged, then some time after the circuit is closed, the changes in current strength stop, and the circuit flows D.C..

However, in modern technology, not only direct current sources are widely used, but also various electric current generators, in which the EMF changes periodically. When an alternating EMF generator is connected to an electrical circuit, forced electromagnetic oscillations or alternating current occur in the circuit.

Alternating current is periodic changes in current and voltage in an electrical circuit that occur under the influence of alternating emf from an external source.

Alternating current is an electric current that changes over time according to a harmonic law.

In the future, we will study forced electrical oscillations that occur in circuits under the influence of a voltage that varies harmoniously with frequency u according to a sinusoidal or cosine law:

where u is the instantaneous voltage value, Um is the voltage amplitude, u is the cyclic oscillation frequency. If the voltage changes with frequency u, then the current in the circuit will change with the same frequency, but the current fluctuations do not necessarily have to be in phase with the voltage fluctuations.

Therefore, in the general case:

where is the phase difference (shift) between current and voltage fluctuations.

Alternating current ensures the operation of electric motors in machines in plants and factories, powers lighting fixtures in our apartments and outdoors, refrigerators and vacuum cleaners, heating appliances, etc. The frequency of voltage fluctuations in the network is 50 Hz. The alternating current has the same oscillation frequency. This means that within 1 s the current will change its direction 50 times. A frequency of 50 Hz is accepted for industrial current in many countries around the world. In the USA, the frequency of industrial current is 60 Hz.

2. Resistor in AC circuit

Let the circuit consist of conductors with low inductance and high resistance R (resistors). For example, such a circuit could be an incandescent filament electric lamp and supply wires. The value R, which we have hitherto called electrical resistance or simply resistance, will now be called active resistance. There may be other resistances in an AC circuit, depending on the inductance of the circuit and its capacitance. Resistance R is called active because only it releases energy, i.e.

The resistance of an electrical circuit element (resistor), in which electrical energy is converted into internal energy, is called active resistance.

So, there is a resistor in the circuit, active resistance which R, and the inductor and capacitor are missing (Fig. 1).

Let the voltage at the ends of the circuit change according to the harmonic law:

As with direct current, the instantaneous value of the current is directly proportional to the instantaneous value of the voltage. Therefore, we can assume that the instantaneous value of the current is determined by Ohm’s law:

Consequently, in a conductor with active resistance, current fluctuations in phase coincide with voltage fluctuations (Fig. 2), and the current amplitude is equal to the voltage amplitude divided by the resistance:

At low frequencies of alternating current, the active resistance of the conductor does not depend on frequency and practically coincides with its electrical resistance in a direct current circuit.

1.1 Coil in an alternating current circuit

Inductance affects the strength of alternating current in a circuit. This can be discovered by simple experiment. Let's make a circuit of a high-inductance coil and an incandescent lamp (Fig. 3). Using a switch, you can connect this circuit to either a DC voltage source or an AC voltage source. In this case, the direct voltage and the effective value of the alternating voltage must be the same. Experience shows that the lamp glows brighter at constant voltage. Consequently, the effective value of the current in the circuit under consideration is less than the direct current.

This is explained by self-induction. When the coil is connected to a constant voltage source, the current in the circuit increases gradually. The vortex electric field that appears as the current increases slows down the movement of electrons. Only after some time does the current reach its highest (steady) value corresponding to a given constant voltage. If the voltage changes quickly, then the current strength will not have time to reach those steady-state values ​​that it would acquire over time at a constant voltage equal to the maximum value of the alternating voltage. Hence, maximum value The alternating current strength (its amplitude) is limited by the inductance L of the circuit and will be less, the greater the inductance and the greater the frequency of the applied voltage.

Let's prove this mathematically. Let an ideal coil with an electrical resistance of the wire equal to zero be included in an alternating current circuit (Fig. 4).

When the current changes according to the harmonic law:

A self-induced emf occurs in the coil:

where L is the inductance of the coil, u is the cyclic frequency of alternating current.

Since the electrical resistance of the coil is zero, the self-induction EMF in it at any moment of time is equal in magnitude and opposite in sign to the voltage at the ends of the coil created by an external generator:

Consequently, the voltage fluctuations on the inductor lead the current fluctuations at p/2, or, which is the same, the current fluctuations lag in phase behind the voltage fluctuations at p/2.

At the moment when the voltage on the coil reaches its maximum, the current strength is zero (Fig. 5). At the moment when the voltage becomes zero, the current strength is maximum in magnitude.

The product I m ​​⋅ L ⋅ u is the amplitude of voltage oscillations on the coil:

The ratio of the amplitude of voltage fluctuations on the coil to the amplitude of current fluctuations in it is called inductive reactance (denoted by X L):

The relationship between the amplitude of voltage fluctuations at the ends of the coil and the amplitude of current fluctuations in it coincides in form with the expression of Ohm’s law for a section of a direct current circuit:

Unlike the electrical resistance of a conductor in a DC circuit, inductive reactance is not a constant value characterizing a given coil. It is directly proportional to the frequency of the alternating current. Therefore, the amplitude of current oscillations in the coil at a constant value of the amplitude of voltage oscillations should decrease in inverse proportion to the frequency. Direct current does not “notice” the inductance of the coil at all. At u = 0, the inductive reactance is zero (XL = 0).

The dependence of the amplitude of current oscillations in the coil on the frequency of the applied voltage can be observed in an experiment with an alternating voltage generator, the frequency of which can be changed. Experience shows that doubling the frequency of alternating voltage leads to a halving of the amplitude of current fluctuations through the coil.

1.2 Capacitor in AC circuit

Let's consider the processes occurring in an alternating current electrical circuit with a capacitor. If you connect a capacitor to a direct current source, a short-term current pulse will appear in the circuit, which will charge the capacitor to the source voltage, and then the current will stop. If a charged capacitor is disconnected from a direct current source and its plates are connected to the terminals of an incandescent lamp, the capacitor will be discharged, and a short-term flash of the lamp will be observed.

When a capacitor is connected to an alternating current circuit, its charging process lasts a quarter of a period. After reaching the amplitude value, the voltage between the plates of the capacitor decreases and the capacitor is discharged within a quarter of the period. In the next quarter of the period, the capacitor is charged again, but the polarity of the voltage on its plates is reversed, etc. The processes of charging and discharging the capacitor alternate with a period equal to the oscillation period of the applied alternating voltage.

As in a DC circuit, electric charges do not pass through the dielectric separating the plates of the capacitor. But as a result of periodically repeating processes of charging and discharging the capacitor, alternating current flows through the wires connected to its terminals. An incandescent lamp connected in series with a capacitor in an alternating current circuit (Fig. 6) appears to burn continuously, since the human eye high frequency fluctuations in current strength does not notice the periodic weakening of the glow of the lamp filament.

Let us establish a connection between the amplitude of voltage fluctuations on the capacitor plates and the amplitude of current fluctuations.

When the voltage changes on the capacitor plates according to the harmonic law:

the charge on its plates changes according to the law:

Electric current in the circuit arises as a result of a change in the charge of the capacitor: i = q’. Therefore, current fluctuations in the circuit occur according to the law:

Consequently, the voltage fluctuations on the capacitor plates in the alternating current circuit lag in phase behind the current fluctuations at p/2, or the current fluctuations lead in phase to the voltage fluctuations at p/2 (Fig. 7). This means that at the moment when the capacitor begins to charge, the current is maximum and the voltage is zero. After the voltage reaches its maximum, the current becomes zero, etc.

The product U m ⋅ u ⋅ C is the amplitude of current fluctuations:

The ratio of the amplitude of voltage fluctuations on the capacitor to the amplitude of current fluctuations is called the capacitance of the capacitor (denoted by X C):

The relationship between the amplitude value of the current and the amplitude value of the voltage coincides in form with the expression of Ohm’s law for a section of a direct current circuit, in which instead of electrical resistance appears capacitance capacitor:

The capacitive reactance of a capacitor, like the inductive reactance of a coil, is not a constant value. It is inversely proportional to the frequency of the alternating current. Therefore, the amplitude of current fluctuations in the capacitor circuit at a constant amplitude of voltage fluctuations on the capacitor increases in direct proportion to the frequency.

1.3 Ohm's law for an alternating current electrical circuit

Let's consider an electrical circuit consisting of a resistor, capacitor and coil connected in series (Fig. 8). If you apply to the terminals of this electrical circuit electrical voltage, changing according to a harmonic law with frequency u and amplitude Um, then forced oscillations of the current strength will appear in the circuit with the same frequency and a certain amplitude Im. Let us establish a connection between the amplitudes of current and voltage fluctuations

At any moment of time, the sum of the instantaneous voltage values ​​on the series-connected circuit elements is equal to the instantaneous value of the applied voltage:

In all series-connected circuit elements, changes in current strength occur almost simultaneously, since electromagnetic interactions propagate at the speed of light. Therefore, we can assume that fluctuations in current strength in all elements series circuit occur according to the law:

The voltage fluctuations across the resistor are in phase with the current fluctuations, the voltage fluctuations across the capacitor are p/2 behind the current fluctuations in phase, and the voltage fluctuations across the coil are ahead of the current fluctuations in phase by p/2.

Therefore, equation (1) can be written as follows:

where U Rm, U Cm and U Lm are the amplitudes of voltage fluctuations across the resistor, capacitor and coil.

The amplitude of voltage fluctuations in an alternating current circuit can be expressed through the amplitude values ​​of the voltage on its individual elements using the vector diagram method.

When constructing a vector diagram, it is necessary to take into account that the voltage fluctuations on the resistor coincide in phase with the current fluctuations, therefore the vector depicting the voltage amplitude U Rm coincides in direction with the vector depicting the current amplitude I m Voltage fluctuations on the capacitor lag in phase by p /2 from current fluctuations, so the vector

U Cm lags behind vector I m by an angle of 90°. Voltage fluctuations on the coil lead current fluctuations in phase by p/2, so the vector U Lm leads the vector I m by an angle of 90° (Fig. 9).

In the vector diagram, the instantaneous voltage values ​​on the resistor, capacitor and coil are determined by projections onto the horizontal axis of the vectors Rm, Cm, Lm rotating with the same angular velocity counterclockwise. The instantaneous voltage value in the entire circuit is equal to the sum of the instantaneous voltages u R , u C , and u L on individual elements chains, i.e. the sum of the projections of the vectors U Rm, U Cm and U Lm onto the horizontal axis. Since the sum of the projections of vectors onto an arbitrary axis is equal to the projection of the sum of these vectors onto the same axis, the amplitude of the total voltage can be found as the modulus of the sum of vectors:

From Figure 9 it can be seen that the voltage amplitude throughout the entire circuit is equal to:

By introducing the symbol for the impedance of an AC circuit:

let's express the connection between amplitude values current and voltage in the AC circuit as follows:

This expression is called Ohm's law for an alternating current circuit.

From the vector diagram shown in Figure 9, it is clear that the phase of the total voltage oscillations is equal to ut + u. Therefore, the instantaneous value of the total voltage is determined by the formula:

The initial phase q can be found from the vector diagram:

The value of cos c plays an important role in calculating power in an alternating current electrical circuit.

1.4 AC power

Power in a DC circuit is determined by the product of voltage and current:

The physical meaning of this formula is simple: since the voltage U is numerically equal to the work of the electric field to move a unit charge, the product U?I characterizes the work to move the charge per unit time flowing through the cross section of the conductor, i.e. is power. The power of the electric current in a given section of the circuit is positive if energy comes to this section from the rest of the network, and negative if energy from this section returns to the network. Over a very short period of time, alternating current can be considered constant.

Therefore, instantaneous power in an alternating current circuit is determined by the same formula:

Let the voltage at the ends of the circuit change according to the harmonic law:

In this case, the power changes over time both in magnitude and sign. During one part of the period, energy flows to a given section of the circuit (p > 0), but during the other part of the period, some portion of the energy returns to the network again (p< 0). Как правило, во всех случаях нам надо знать среднюю мощность на участке цепи за достаточно большой промежуток времени, включающий много периодов. Для этого достаточно определить среднюю мощность за один период.

To find the average power over a period, we transform the resulting formula in such a way as to highlight a term in it that does not depend on time. For this purpose we will use well-known formula for the product of two cosines:

The expression for instantaneous power consists of two terms. The first does not depend on time, and the second changes sign twice for each period of voltage change: during some part of the period, energy enters the circuit from an alternating voltage source, and during the other part it returns back. Therefore, the average value of the second term over the period is zero.

Therefore, the average power P for a period is equal to the first term, independent of time:

When the phase of current and voltage oscillations coincide (for active resistance R), the average power value is equal to:

In order for the formula for calculating the power of alternating current to coincide in form with a similar formula for direct current (P = IU = I 2 R), the concepts of effective values ​​of current and voltage are introduced. From the equality of powers we obtain:

The effective current value is a value that is √2 times less than its amplitude value:

The effective value of the current is equal to the strength of such a direct current at which the average power released in a conductor in an alternating current circuit is equal to the power released in the same conductor in a direct current circuit.

Similarly, it can be proven that the effective value of the alternating voltage is √2 times less than its amplitude value:

Note that usually electrical equipment in alternating current circuits shows the effective values ​​of the measured quantities. Turning to the effective values ​​of current and voltage, equation (10) can be rewritten:

Thus, the alternating current power in a section of the circuit is determined precisely effective values current and voltage. It also depends on the phase shift cs between voltage and current. The cos cc multiplier in the formula is called the power factor.

In the case when qc = ± p/2, the energy supplied to a section of the circuit during a period is zero, although there is a current in the circuit. This will be the case, in particular, if the circuit contains only an inductor or only a capacitor. How can the average power be equal to zero in the presence of current in the circuit? This is explained by the graphs shown in Figure 10 of the change over time in the instantaneous values ​​of voltage, current and power at cc = - p/2 (purely inductive reactance of the circuit section).

A graph of instantaneous power versus time can be obtained by multiplying the values ​​of current and voltage at each time. From this graph it can be seen that during one quarter of the period the power is positive and energy flows to this section of the circuit; but during the next quarter of the period the power is negative, and this section transfers the previously received energy back into the network without loss. The energy arriving during a quarter of the period is stored in the magnetic field of the current, and then returned to the network without loss.

Only in the presence of a conductor with active resistance in a circuit that does not contain moving conductors, electromagnetic energy is converted into internal energy of the conductor, which heats up. The reverse conversion of internal energy into electromagnetic energy in the area with active resistance no longer occurs.

When designing alternating current circuits, it is necessary to ensure that cos cc is not small. Otherwise, a significant part of the energy will circulate through the wires from the generator to consumers and back. Since the wires have active resistance, energy is spent on heating the wires.

Unfavorable conditions for energy consumption arise when electric motors are connected to the network, since their windings have low active resistance and high inductance. To increase cos cc in the power supply networks of enterprises with a large number of electric motors, special compensating capacitors are included. It is also necessary to ensure that electric motors do not run idle or underload.

This reduces the power factor of the entire circuit. Increasing cos cc is an important national economic task, as it allows maximum return Use power plant generators and reduce energy losses. This is achieved by proper design of electrical circuits. It is prohibited to use devices with cos cc< 0,85.

Constant and variable To

What is the difference between direct current from variable

In the previous article, what is electric current you learned how the ordered movement of electrons occurs in a closed circuit. Now, I will tell you what electric current is like. Electric current can be direct or alternating. How is alternating current different from direct current? Characteristics of direct current.

D.C

Direct Current or DC in English means an electric current that does not change direction over any period of time and always moves from plus to minus. In the diagram it is indicated as plus (+) and minus (-); on the body of a device operating on direct current, a designation is applied in the form of one (-) or (=) stripe. An important feature of direct electric current is the possibility of its accumulation, i.e. accumulation in batteries or obtaining it due to a chemical reaction in batteries. Many modern portable electrical devices, work using accumulated electric charge direct current, which is located in the batteries or batteries of these same devices.

Alternating current

(Alternating Current) or AC English abbreviation denoting a current that changes its direction and magnitude over a period of time. On electrical diagrams and housings of electrical devices operating on alternating current, the alternating current symbol is designated as a sine wave segment “~”. If we talk about alternating current in simple words , then we can say that in case of connection light bulb to an alternating current network, the plus and minus on its contacts will change places with a certain frequency or otherwise, the current will change its direction from forward to reverse. In the figure, the opposite direction is the area of ​​the graph below zero.

Now let's figure out what frequency is. Frequency is the period of time during which the current performs one complete oscillation; the number of complete oscillations in 1 s is called the frequency of the current and is denoted by the letter f. Frequency is measured in hertz (Hz). In industry and everyday life, most countries use alternating current with a frequency of 50 Hz.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. In other words, in electrical outlet, which is in every home and where we turn on irons and vacuum cleaners, plus and minus on the right and left terminals of the socket will change places with a frequency of 50 times per second - this is the frequency of alternating current. Why do we need such a “changeable” alternating current, why not use only direct current? This is done in order to be able to obtain the required voltage in any quantity by using transformers without any significant losses. The use of alternating current makes it possible to transmit electricity on an industrial scale over long distances with minimal losses.

Voltage supplied powerful generators power plants is about 330,000-220,000 Volts. Such voltage cannot be supplied to houses and apartments; it is very dangerous and difficult to handle. technical side. Therefore, alternating electric current from power plants is supplied to electrical substations, where transformation occurs from high voltage to the lower voltage that we use.

Converting AC to DC

From alternating current, you can get direct current, for this you just need to connect the alternating current network diode bridge or as it is also called “rectifier”. From the name “rectifier” it is perfectly clear what a diode bridge does; it rectifies an alternating current sinusoid into a straight line, thereby forcing electrons to move in one direction.

what is a diode And how does a diode bridge work?, you can find out in my next articles.

Sooner or later, every person is forced to face a situation where it is necessary to get to know electricity more closely than in physics lessons at school. A starting point for this could be: breakdown of electrical appliances or sockets, or just a sincere interest in electronics on the part of a person. One of the main questions to consider is how direct and alternating current are designated. If you are familiar with the concepts: electric current, voltage and amperage, you will easier to understand, what is discussed in this article.

Electrical voltage is divided into two types:

1. constant (dc)
2. variable (as)

The designation for direct current is (-), for alternating current the designation is (~). The abbreviations ac and dc are well-established and are used along with the names “constant” and “variable”. Now let's look at what is their difference. The fact is that constant voltage flows only in one direction, which is where its name comes from. And a variable, as you already understood, can change its direction. In particular cases, the direction of the variable may remain the same. But, in addition to the direction, its magnitude can also change. In a constant, neither magnitude nor direction changes. Instantaneous AC current value call its value, which is taken in this moment time.

In Europe and Russia, the accepted frequency is 50 Hz, that is, it changes its direction 50 times per second, while in the USA, the frequency is 60 Hz. Therefore, equipment purchased in the United States and in other countries may burn out with different frequencies. Therefore, when choosing equipment and electrical appliances, you should carefully ensure that the frequency is 50 Hz. The higher the frequency of the current, the greater its resistance. You can also notice that in the sockets in our house it is AC that flows.

In addition, alternating electric current is divided into two more types:

• single-phase
• three-phase

For single-phase, a conductor is required that will conduct voltage and a return conductor. And if we consider a three-phase current generator, it generates on all three windings AC voltage frequency of 50 Hz. Three-phase system- this is nothing more than three single-phase electrical circuits, shifted in phase relative to each other at an angle of 120 degrees. By using it, you can simultaneously provide energy three independent networks, using only six wires, which are needed for all conductors: forward and reverse, to conduct voltage.

And if you, for example, have only 4 wires, then there will be no problems either. You will only need to connect the return conductors. By combining them, you get a conductor called neutral. It is usually grounded. And the remaining external conductors are briefly designated as L1, L2 and L3.

But there is also a two-phase one, it is a complex of two single-phase currents, in which there is also a direct conductor for conducting voltage and a reverse one, they are shifted in phase relative to each other by 90 degrees.

Application

Due to the fact that the constant flows only in one direction, its use is usually limited to carriers with low energy intensity, for example, it can be found in regular batteries, batteries for electrical devices with low power consumption, such as flashlights or phones, and batteries using solar energy. But permanent source is necessary not only for charging small batteries, but high-power direct current is used to operate electrified railway tracks, in the electrolysis of aluminum or in arc welding, as well as others industrial processes.

To generate direct current of such strength, special generators are used. It can also be obtained by converting an alternating variable; for this, a device is used that uses an electron tube, it is called a kenotron rectifier, and the process itself is referred to as rectification. A full-wave rectifier is also used for this. It, unlike a simple tube rectifier, contains electronic tubes that have two anodes - dual-anode kenotrons.

If you don’t know how to determine which pole direct current flows from, remember: it always flows from the “+” sign to the “-” sign. The first sources of direct current were special chemical elements, they are called galvanic. Later people invented batteries.

Variable is used almost everywhere, in everyday life, for the operation of household electrical appliances powered from a home outlet, in factories and factories, on construction sites and many other places. Electrification of railway tracks can also be done on DC voltage. So, the voltage goes along the contact wire, and the rails are the opposite electrical conductor. About half of all people work according to this principle. railways in our country and the CIS countries. But, in addition to electric locomotives that operate only on constant and only alternating current, there are also electric locomotives that combine the ability to operate on both one type of electricity and another.

Alternating current is also used in medicine

For example, darsonvalization is a method of applying electricity at high voltage to the outer integument and mucous membranes of the body. Through this method Patients have improved blood circulation, improved tone of venous vessels and the body's metabolic processes. Darsonvalization can be either local, in a specific area, or general. But local therapy is more often used.

Thus we learned that There are two types of electric current: direct and alternating, they are also called ac and dc, so if you say one of these abbreviations, you will definitely be understood. In addition, the designation of direct and alternating current in the diagrams looks like (-) and (~), which makes them easier to recognize. Now, when repairing electrical appliances, you will no doubt say that they use alternating voltage, and if you are asked what current is in the batteries, you will answer that it is constant.

Content:

The debate has been going on for decades as to which type of current is more dangerous - alternating or direct. Some argue that it is the corrected voltage that poses the greatest threat, others are sincerely convinced that the alternating current sinusoid, coinciding in amplitude with the beating of the human heart, stops it. But, as always happens in life, there are so many opinions. Therefore, it is worth looking at this issue from a purely scientific point of view. But it’s worth doing this in a language understandable even for dummies, because... Not everyone has an electrical engineering education. At the same time, everyone probably wants to know the origin of direct and alternating current.

Where should you start? Yes, probably, from definitions - what is electricity, why is it called variable or constant, which of these types is more dangerous and why.

Most people know that direct current can be obtained from various units or batteries, and alternating current is supplied to apartments and premises through the electrical network and thanks to it, household electrical appliances and lighting operate. But few people have thought about why one voltage allows you to get another and why it is needed.

It makes sense to answer all the questions that arise.

What is electric current?

Electric current is a constant or variable quantity that arises from the directed or ordered movement created by charged particles - in metals these are electrons, in electrolytes - ions, and in gases - both. In other words, electric current is said to “flow” through the wires.

Some people mistakenly believe that each charged electron moves along a conductor from the source to the consumer. This is wrong. It only transfers charge to neighboring electrons, remaining in place. Those. its movement is chaotic, but microscopic. Well, the charge itself, moving along the conductor, reaches the consumer.

Electric current has measurement parameters such as: voltage, i.e. its value, measured in volts (V) and current, which is measured in amperes (A). What is very important during transformation, i.e. decrease or increase using special devices, one quantity affects another in inverse proportion. This means that by reducing the voltage using a conventional transformer, they achieve an increase in current and vice versa.

DC and AC current

The first thing to understand is the difference between direct and alternating current. The fact is that alternating current is not only easier to obtain, although this is also important. Its characteristics allow transmission over any distance over conductors with minimal losses, especially at higher voltage and lower power. That is why power lines between cities are high voltage. And already in populated areas the current is transformed into a lower voltage.

But direct current is very easy to obtain from alternating current, for which multidirectional diodes are used (the so-called diode bridge). The fact is that alternating current (AC), or rather the frequency of its oscillations, is a sinusoid, which, passing through a rectifier, loses some of the oscillations. Thus, the output produces a constant voltage (AC) that has no frequency.

It makes sense to specify how, after all, they differ.

Current differences

Of course, the main difference between AC and DC is the ability to transport DC over long distances. At the same time, if direct current is transported in the same way, there will simply not be any left. Due to the potential difference, it is consumed. It is also worth noting that converting to a variable is very difficult, while in reverse order such an action is quite easy to do.

It is much more economical to convert electricity into mechanical energy using motors powered by AC, although there are areas in which it is possible to use mechanisms only direct current.

Well, last but not least - after all, alternating current is safer for people. It is for this reason that all devices used in everyday life and powered by DC are low-current. But it will not be possible to completely abandon the use of a more dangerous one in favor of another, precisely for the reasons stated above.

All of the above leads to a generalized answer to the question of how alternating current differs from direct current - these are the characteristics that influence the choice of a particular power source in a certain area.

Transmission of current over long distances

Some people have a question to which a superficial answer was given above: why does very high voltage come through power lines? If you don’t know all the intricacies of electrical engineering, then you can agree with this question. Indeed, if a voltage of 380 V came through the power lines, then there would be no need to install expensive transformer substations. And you wouldn’t have to spend money on their maintenance, would you? It turns out not.

The fact is that the cross-section of the conductor through which electricity flows depends only on the strength of the current and its power consumption, and voltage remains completely apart from this. This means that with a current of 2 A and a voltage of 25,000 V, you can use the same wire as for 220 V with the same 2 A. So what follows from this?

Here it is necessary to return to the law of inverse proportionality - during current transformation, i.e. As the voltage increases, the current decreases and vice versa. Thus, high-voltage current is sent to the transformer substation through thinner wires, which ensures lower transmission losses.

Transfer Features

It is precisely in losses that lies the answer to the question why it is impossible to transmit direct current over long distances. If we look at DC from this angle, then it is for this reason that after a short distance there will be no electricity left in the conductor. But the main thing here is not energy losses, but their immediate cause, which lies, again, in one of the characteristics of AC and DC.

The fact is that the frequency of alternating current is electrical networks common use in Russia - 50 Hz (hertz). This means the amplitude of the charge fluctuation between positive and negative is equal to 50 changes per second. Speaking in simple language, every 1/50 s. the charge changes its polarity, this is the difference between direct current - there are practically or no oscillations in it. It is for this reason that DC is consumed by itself as it flows through a long conductor. By the way, the oscillation frequency, for example, in the USA differs from the Russian one and is 60 Hz.

Generating

A very interesting question is how direct and alternating current are generated. Of course, you can produce both one and the other, but here the problem of size and cost arises. The fact is that if we take an ordinary car as an example, it would be much easier to install a DC generator on it, excluding the diode bridge from the circuit. But here comes the catch.

If you remove the rectifier from a car generator, it seems that the volume should also decrease, but this will not happen. And the reason for this is the dimensions of the DC generator. In addition, the cost will increase significantly, which is why they use variable generators.

So it turns out that generating DC is much less profitable than AC, and there is concrete evidence of this.

Two great inventors at one time began the so-called “war of currents,” which ended only in 2007. And its opponents were Nikola Tesla together with George Westinghouse, ardent supporters alternating voltage, and Thomas Edison, who advocated the use of direct current everywhere. So, in 2007, the city of New York completely went over to Tesla’s side, thereby marking his victory. It’s worth going into a little more detail on this.

Story

Thomas Edison's company, which was called Edison Electric Light, was founded in the late 70s of the 19th century. Then, in the days of candles, kerosene lamps and gas lighting, incandescent lamps produced by Edison could work continuously for 12 hours. And although now this may seem ridiculously little, it was a real breakthrough. But already in the 1880s, the company was able not only to patent the production and transmission of direct current via a three-wire system (these were “zero”, “+110 V” and “-110 V”), but also to introduce an incandescent lamp with a resource of 1200 hours .

It was then that Thomas Edison’s phrase, which later became known throughout the world, was born: “We will make electric lighting so cheap that only the rich will burn candles.”

Well, by 1887, more than 100 power plants were successfully operating in the United States, which generate direct current and where a three-wire system is used for transmission, which is used to at least slightly reduce electricity losses.

But the scientist in the field of physics and mathematics, George Westinghouse, after reading Edison’s patent, found one very unpleasant detail - it was a huge loss of energy during transmission. At that time, there were already alternating current generators, which were not popular due to the equipment that would operate on such energy. At that time, the talented engineer Nikola Tesla was still working for Edison in the company, but one day, when he was once again denied a salary increase, Tesla could not stand it and went to work for a competitor, which was Westinghouse. In a new place, Nikola (in 1988) creates the first electricity meter.

It is from this moment that the “war of currents” begins.

conclusions

Let's try to summarize the information presented. Today it is impossible to imagine the use (both in everyday life and in industry) of any one type of electricity - both direct and alternating current are present almost everywhere. After all, somewhere constant is needed, but its transmission over long distances is impossible, and somewhere variable.

Of course, it has been proven that AC is much safer, but what about devices that help save energy many times over, while they can only work on DC?

It is for these reasons that the currents now “coexist peacefully” in our lives, having ended the “war” that lasted more than 100 years. The only thing that should not be forgotten is that no matter how much safer one is than the other (constant or alternating voltage is not important), it can cause enormous harm to the body, even death.

And that is why when working with voltage it is necessary to carefully observe all safety standards and rules and not forget about care and accuracy. After all, as Nikola Tesla said, electricity should not be feared, it should be respected.