Basic definitions. Electrical circuits of single-phase alternating current

What is a phase in electricity - definition of the concept

Phase in electricity is the colloquial name for a wire that is energized relative to another, which is called zero. This name comes from the fact that the current generated at substations and supplied to houses is variable, that is, the EMF created at substations have the same frequency (for Russia and the CIS countries it is 50 Hz), but are shifted relative to each other friend in time at a certain phase angle. Houses are usually supplied with all three phases and it does not matter which phase your apartment is connected to.

Figure 1. Electrics and electricity - schematic representation of phase, zero and ground

In Fig. 1 is a schematic diagram of the flow of electric current into the apartment from the general system. The letters $L1$, $L2$, $L3$ denote phases 1-3, and the letter $N$ denotes the neutral wire.

In Fig. Figure 2 shows a schematic connection of current to the apartment from the transformer, the letter $L_T$ denotes the phase on the transformer, the letter $L$ denotes the phase in the apartment, and the letter $R_H$ denotes a connected electrical appliance that has some resistance $R_H$.

There are 2 wires coming from the transformer, one is the so-called phase wire with voltage, and the other is the neutral wire, from which the grounding is carried out by placing the contact in the ground. There are other sources of grounding besides the ground itself; in these figures, grounding is indicated by the letters $Zml$.

In Fig. Figure 3 shows the case when the neutral grounded wire is not routed into the apartment from the substation, but is grounded directly in the apartment. The voltage $L_T$ between zero and phase will be the same for Figures 2 and 3, however, it is not recommended to ground the voltage from the transformer directly in the apartment.

Disadvantages of three-phase input

A three-phase connection also has its significant disadvantages. Let's take a closer look at them.

Operating principle of AC network

To understand what a phase is in electricity, you need to understand the features of alternating current. It differs from constant by periodic changes, both in value and direction. Its characteristics are voltage at a given time and frequency (the ratio of the number of cycles to a unit of time). Alternating current is found in outlets and direct connections to the electrical panel.

Single phase current

It is directed from the distribution panel along two wires (phase and neutral), between which there is a 220-volt voltage. In electricity, a phase is a wire that carries electric current to an outlet or appliance. What is zero in electricity? This, in turn, is a cable coming from the outlet through which the current is directed back.

Sometimes the question of what zero is is asked in the context of grounding. Physically these are different wires, although their potentials are the same. Single-phase current can be supplied to the consumer using either two wires (without grounding) or three (with it). Grounding is carried out to drain leaks, protect residents from electric shock and devices from overloads.

Two-phase current

This is a combination of two single-phase ones, offset by 90° relative to each other. Structurally, it looks like a combination of two phase wires (with the indicated shift) and two neutral wires.

Three phase current

Here the design already consists of three current phases, each of the subsequent ones is shifted relative to the previous one by 120 °. In residential buildings, such current is distributed by four wires (three phases and zero) or five (indicated plus grounding). After passing through the distribution board, the sockets in the apartment are fed through one phase and zero.

Types of current

The current can be constant or alternating. A current that does not change in value over a period of time is a current of constant value. A current, the magnitude of which, like the direction, changes over time, is called variable.

Constant current sources - batteries, batteries, and so on. Alternating current is “suitable” for household and industrial sockets in homes and businesses. The main reason for this lies in the fact that this type of current is much easier to obtain physically, convert to different voltage levels, and transmit through electrical wires over vast distances without significant losses.

Single phase current

Alternating current, which is obtained by rotating a conductor or a system of conductors connected into one coil in a magnetic flux, is called single-phase alternating current . As a rule, 2 wires are used to transmit single-phase current. They are called phase and zero, respectively. The voltage between these wires is 220 V.

Single phase power supply. Single-phase current can be supplied to the consumer in two different ways: 2-wire and 3-wire. In the first (two-wire) one, two wires are used to supply single-phase current. One carries phase current, the other is intended for the neutral wire. Thus, power supply is supplied to almost all houses built in the former USSR. In the second method, to supply single-phase current , another wire is added. This wire is called grounding (PE). It is designed to prevent electric shock to a person, as well as to drain leakage currents and prevent devices from breaking.

Two-phase current

Everyone understands the concept of two-phase electric current - the merging of two single-phase currents that have a phase shift to each other. The shift angle can be Pi2 or 90°.

Cable color marking

This is one of the simplest methods. To determine what phase and zero are by color, you need to clearly know which shades correspond to what. You can use information about the standards adopted in the country.

It's no secret that each wire has an individual color. Therefore, recognizing zero should not be a particular problem. The knowledge gained will allow you to easily cope with the installation of a lighting fixture or installation of an outlet. This method is especially relevant for new buildings. After all, there, as a rule, wires are laid by experienced specialists who strictly comply with norms and standards. The IEC 60446 standard, adopted on the territory of the Russian Federation in 2004, strictly regulates the separation of phase, grounding and zero by color.

It is worth considering that:

• if the wire has a blue or blue-white tint, we can safely say that this is a working zero
• the protective zero is represented by cables in a yellow-green sheath
• other colors are characteristic of the phase. It can be red, brown, white or black. Other options are also possible.

This designation is successfully used in most cases. But if the wiring is old, or there are doubts about the professionalism of electricians, it is more advisable to use additional methods.

How to distinguish phase, zero, ground

The easiest way to determine the purpose of the conductors is by color marking. In accordance with the standards, the phase conductor can have any color, the neutral can be blue, and the ground can be yellow-green. Unfortunately, when installing electrics, color coding is not always observed. We must not forget the likelihood that an unscrupulous or inexperienced electrician can easily confuse phase and zero or connect two phases. For these reasons, it is always better to use more precise methods than color coding.

You can determine the phase and neutral conductors using an indicator screwdriver. When the screwdriver comes into contact with the phase, the indicator will light up, as electric current passes through the conductor. Zero has no voltage, so the indicator cannot light up.

You can distinguish zero from ground using a dial tone. First, the phase is determined and marked, then with a continuity probe you need to touch one of the conductors and the ground terminal in the electrical panel. Zero will not ring. When you touch the ground, a characteristic sound signal will be heard.

Phase and zero: their meaning in the power supply network

Electricity is supplied to consumer sockets from substations, which reduce the incoming voltage to 380 V. The secondary winding of such a transformer has a “star” connection - its three contacts are connected to each other at the “0” point, the remaining three outputs go to the “A” / “B” terminals "/"WITH".

The wires connected at point “0” are connected to “ground”. At the same point, the conductor is divided into “zero” (indicated in blue) and a protective “PE” cable (yellow-green line).

This model of wiring is used in all houses currently being built. It is called the “TN-S” system. According to this diagram, three phase cables and two indicated zeros are suitable for the distribution equipment of the house.

In houses, enterprises and buildings of old construction there is often no “PE” conductor and therefore the circuit turns out not to be five-wire, but four (it is designated as “TN-C”).

All electrical wires from the substations are connected to the panel, forming a three-phase system. Then there is a division into separate entrances. Each of the entrance apartments is supplied with only one phase voltage - 220 V (wires “O” / “A”) and a protective “PE” cable.

With this scheme, the entire load on the power supply system is distributed evenly, since on each floor of the house specific panels are wired and connected to a specific 220 V power line.

The supply voltage circuit is a “star”, which exactly repeats all the vector characteristics of the supply substation. When there are no consumers in the sockets, no current flows in this circuit.

This connection scheme has been worked out for years. It confirmed its right to use by being recognized as the optimal of all existing ones. However, in it, as in any device, mechanism or device, all kinds of breakdowns and malfunctions may periodically appear. As a rule, they are associated with poor quality electrical connections or complete cable breaks in some places in the circuit.

What happens in zero and phase when a wire breaks.

Line breaks quite often occur due to the fault of the craftsmen - they forget to connect a phase or zero. Such breakdowns are quite common. Also, quite often the process of zero burnout on the access panel occurs, for example, due to high load in the system.

If a break occurs in any part of the chain, the entire chain stops functioning, because it opens. In such situations, it does not matter at all which wire is damaged - phase or neutral. The same thing happens when there is a break between the distribution board of a high-rise building and the panel in the entrance. With such a gust, all consumers who were connected to this panel will be without electricity.

All the situations that we tried to describe above occur. They may seem complicated, but they pose no danger to humanity. After all, the break occurred in only one wire, so it is not at all dangerous.

A very alarming situation is when the contact between the ground loop at the substation and the middle point, to which all the voltage of the in-house panel is supplied, is lost.

It is in this embodiment that the electric current moves along the contours AB, BC, CA. The total voltage of these circuits is 380V. It is for this reason that a rather dangerous situation arises - one panel may have no voltage at all, because the owner will turn off all electrical appliances, and a very high voltage level will form on the other, about 380V. This can contribute to the failure of many devices, because they require a voltage of 220V.

Naturally, this situation can be avoided. There is a lot of inexpensive/expensive equipment that will protect your equipment from power surges. Such equipment also includes a voltage stabilizer. There are the following types of stabilizers:

• Single phase;
• Three-phase.

Household electrical wiring device

A standard electrical wiring diagram contains the following elements:

• multi-tariff electricity meter;
• automatic switch with a rated current of 25 A;
• a shutdown mechanism that protects against short circuits and network overloads;
• differential circuit breaker with an operating threshold of 30 mA (leakage current), it protects sockets;
• cabinet for installation with busbars (neutral and grounding) and boards for installing switches;
• several lighting machines with a rated current of 10 A;
• cables with distribution boxes leading to sockets and devices that illuminate the premises.

Often apartment owners are interested in whether a phase is plus or minus, and what is the difference between zero and ground. Since the electrical phase has a variable potential, its indicator in the phase wire becomes either positive or negative. Therefore, it would be incorrect to say that phase is a minus (or a plus) - these concepts lie on different planes.

Now let's talk about how zero differs from ground. The difference is that current passes through the neutral wire and is opened by automatic devices (for example, the input wire). To ground in an apartment building, you need to connect to a conductor located in the riser, designed specifically for this purpose. It is strictly prohibited to use any other place, including the panel housing, for grounding - this can cause serious problems for the health of residents.

Single-phase and three-phase alternating current. Definitions

Alternating current is a current whose change in value and direction is repeated periodically at regular intervals.

Electric current, the strength of which varies sinusoidally in time and changes its sign (direction) twice over a period of time, is called alternating current.

Alternating current can be single-phase or three-phase. Single-phase current is transmitted through two wires (phase and zero). To transmit three-phase current, three wires are required (one wire for each phase).

Three sinusoidal (alternating) voltages of the same frequency and amplitude, shifted in phase by 120 degrees, form a three-phase network.

Three-phase networks have become widespread: three-phase power supply saves wires; three-phase generators, electric motors and transformers are cheaper, lighter, and more economical.

The widespread use of alternating current in various fields of technology is explained by the ease of its production and conversion, as well as the simplicity of the design of alternating current generators and motors, the reliability of their operation and ease of operation. Modern energy requires the transfer of energy over long distances using electric current. Such transmission requires the ability to convert current easily and efficiently. Such a conversion is possible only with the use of transformers operating only on alternating current. A great incentive for the development of alternating current electrical devices is the possibility of obtaining high-power energy sources.

Modern thermal and hydroelectric generators have a capacity of 100 – 1500 MW. The simplest and cheapest electric motors include asynchronous AC motors, which do not have moving electric contacts. Transmitting alternating current over long distances requires wires of smaller cross-section than direct current.

Basic parameters of alternating current. Period, frequency

The main parameters of alternating current are the following quantities:

• instantaneous values ​​of current and voltage - their values ​​at any time;
• amplitude values ​​of current and voltage – maximum values ​​of instantaneous quantities;
• effective values ​​of current and voltage;
• period - a period of time during which the current makes a complete oscillation and takes on the same instantaneous value in magnitude and sign; the period is expressed in seconds (s), milliseconds (ms) and microseconds (μs).
• frequency is the reciprocal of the period f = 1/T where T is the period;

The unit of frequency is hertz (Hz), f = 1/s = 1Hz.

Frequency is also measured in kilohertz (kHz), megahertz (MHz) and gigahertz (GHz).

The industrial frequency of the power grid in Russia and in most countries of the world is 50 Hz (in the USA - 60 Hz). The reason for this choice is simple: lowering the frequency is unacceptable, since already at a current frequency of 40 Hz incandescent lamps blink noticeably to the eye; An increase in frequency is undesirable, since an increased frequency negatively affects the transmission of energy through wires and the operation of many electrical devices.

Single-phase alternating current and its parameters

Alternating current has not found practical use for a long time. This was due to the fact that the first electrical energy generators produced direct current, which fully satisfied the technological processes of electrochemistry, and direct current motors have good control characteristics.

However, as production developed, direct current became less and less suitable for the increasing requirements for economical power supply. Alternating current made it possible to effectively split electrical energy and change the voltage using transformers. It became possible to produce electricity at large power plants with its subsequent economical distribution to consumers, and the radius of power supply increased.

Currently, the central production and distribution of electrical energy is carried out mainly on alternating current. Circuits with changing - alternating - currents have a number of features compared to direct current circuits. Alternating currents and voltages cause alternating electric and magnetic fields. As a result of changes in these fields in circuits, the phenomena of self-induction and mutual induction arise, which have the most significant impact on the processes occurring in the circuits, complicating their analysis.

Alternating current (voltage, emf, etc.) is a current (voltage, emf, etc.) that varies over time. Currents whose values ​​are repeated at regular intervals in the same sequence are called periodic, and the shortest period of time through which these repetitions are observed is called period T.

Basic parameters of single-phase alternating current circuits.

Single-phase industrial frequency alternating current has 50 oscillation periods per second, or 50 Hz. It is used to power small fans, household appliances, power tools, electric welding and to power most lighting devices. AC frequency, Hz:

f= 1/T = np/60,

where n is the generator rotation frequency, minˉ 1; p—number of generator pole pairs.

Single phase AC power:

active, W, Ra = IUcosφ;

reactive, var, Q = IUsinφ;

apparent, V A, S = IU =

If only active resistance is included in the single-phase alternating current circuit (for example, heating elements or electric lamps), then the value of current and power at each moment of time is determined by Ohm’s law:

I=U/R; Pa = IU = I²R=U²/R.

Power factor in a circuit with an inductive load

Cosφ= Pa/IU= Pa/S.

Basic concepts and quantities characterizing electrical circuits

Concepts:

An electrical circuit is a set of devices intended for the passage of electric current, the electromagnetic processes in which can be described using the concepts of voltage and current. In the general case, an electrical circuit consists of sources and receivers of electrical energy and intermediate links (wires, devices) connecting sources with receivers.

Sources of electrical energy are devices (galvanic cells, batteries, thermoelements, generators) in which the process of converting chemical, molecular-kinetic, thermal, mechanical or other types of energy into electrical energy occurs.

Receivers of electrical energy (load) are devices (electric lamps, electric heating devices, electric motors, resistors, capacitors, inductive coils) in which electrical energy is converted into light, heat, mechanical, etc.

Quantities:

Electric current and voltage are the main quantities characterizing the state of electrical circuits.

Electric current in conductors represents the phenomenon of ordered movement of electric charges. The term “current” also refers to the intensity or strength of current, measured by the amount of electric charge q passing through the cross-section of a conductor per unit time:

Therefore, current represents the rate of change of charge over time. In SI, charge is expressed in coulombs (C), time in seconds (s), and current in amperes (A).

Current as a ratio of two scalar quantities is a scalar algebraic quantity, the sign of which depends on the direction of movement of charges of the same sign, namely the conventionally accepted positive charge. To unambiguously determine the sign of the current in the positive direction, it is enough to arbitrarily select one of two possible directions, which is marked with an arrow.

If the movement of a positive charge occurs in the direction of the arrow, and the movement of a negative charge occurs towards it, then the current is positive. When the direction of movement of charges changes to the opposite, the current will be negative.

It is possible to uniquely set the current in the form of a certain function of time only after specifying the selected positive direction of the current. Therefore, before starting the analysis, it is necessary to note the positive directions of currents in all sections of the circuit, the choice of which can be arbitrary.

The passage of electric current or the transfer of charges in a circuit involves the conversion or consumption of energy. To determine the energy spent on moving a charge between the two points of the conductor under consideration, a new quantity is introduced—voltage.

Voltage is the amount of energy expended to move a unit of charge from one point to another:

, where w is energy.

When measuring energy in joules (J) and charge in coulombs (C), voltage is expressed in volts (V).

Stress as a ratio of two scalar quantities is also a scalar algebraic quantity. To unambiguously determine the sign of the voltage between the two terminals of the section of the circuit under consideration, one of the terminals is conventionally assigned a positive polarity, which is marked either with an arrow directed from the terminal or with the signs “+”, “-”

The voltage is positive if its polarity matches the selected one; this means that the potential of the terminal with the “+” sign, from which the arrow comes out, is higher than the potential of the second terminal.

Before starting the analysis, the selected positive voltage polarities must be indicated - only under this condition is it possible to unambiguously determine the voltages.

Although the conditionally positive voltage polarity can be chosen arbitrarily, it is usually convenient to choose it consistent with the selected positive current direction when the arrows for current and voltage coincide or the “+” sign of voltage polarity is at the tail of the arrow indicating the positive direction of current. With a consistent choice of polarity, it is obviously sufficient to limit yourself to indicating only one arrow in the positive direction of the current.

If there is a need to select a positive voltage polarity that is not consistent with the positive direction of the current, then you have to indicate two counter-directed arrows: for current and for voltage. This is not very convenient. Therefore, to denote conditionally positive polarity, we will use the signs “+.”, “-” at the terminals of a section of the circuit.

Power in an electrical circuit, equal to the product of voltage and current, is also an algebraic quantity. Its sign is determined by the signs of voltage and current: if these signs coincide, the power is positive, which corresponds to energy consumption in the section of the circuit under consideration; when the signs of voltage and current do not match, the power is negative, which means it is released from a section of the circuit (such a section is a source of energy).

Electrical circuits

An electrical circuit is a collection of devices through which electric current flows.

Let's consider the simplest electrical circuit. What does it consist of? It has a generator - a current source, a receiver (for example, a light bulb or an electric motor), and a transmission system (wires). For a circuit to become a circuit, and not a set of wires and batteries, its elements must be connected to each other by conductors. Current can only flow through a closed circuit. Let's give one more definition:

An electrical circuit is a current source, transmission lines and a receiver connected to each other.

Of course, source, receiver and wires are the simplest option for a basic electrical circuit. In reality, different circuits include many more elements and auxiliary equipment: resistors, capacitors, switches, ammeters, voltmeters, switches, contact connections, transformers, etc.

Electrical circuit

By the way, read about what a transformer is in a separate article on our blog.

By what fundamental characteristic can all electric current circuits be divided? Same as current! direct circuits , and there are alternating current circuits . In a direct current circuit, it does not change its direction; the polarity of the source is constant. Alternating current changes periodically over time, both in direction and in magnitude.

Nowadays alternating current is used everywhere. Read about what Nikola Tesla did for this in our article.

Receiving alternating current

Current generation is based on the phenomenon of electromagnetic induction, which was discovered by Michael Faraday. Its essence is this: in a conductor located in a magnetic field with changing characteristics, an electromotive force (EMF) arises.

The magnetic field parameters mean:

• field line density;
• the angle of their direction relative to the conductor.

There are several ways to ensure changes in magnetic field parameters:

1. move (rotate) the conductor in the field of a permanent magnet;
2. rotate a permanent magnet around a conductor;
3. place a current-carrying element in the field of an electromagnet (a wire wound in the form of a coil) with alternating current flowing through it.

The first two methods are used in electric generators, the last one is used in current transformers. Moving a magnet or conductor requires mechanical energy. It is converted by the generator into electric power. The direction of the EMF is determined by the right hand rule.

In this position, when the field lines enter the palm, and the thumb moved to the side coincides with the vector of movement of the conductor, the other fingers point to the direction of the EMF. The simplest alternating current generator is a wire frame rotating between permanent magnets and connected to an electrical circuit.

The contact between the moving frame and the stationary conductive elements of the chain is sliding: rings are attached to the ends of the frame, and graphite brushes (have a low coefficient of friction) pressed against these rings are attached to the ends of the chain.

The rotating part of a generator or electric motor, in our example this is a frame, is called a rotor. Fixed - stator.

The EMF induced in the frame is determined by the formula: E = B*S*ω*sinα, where B is the magnetic induction, S is the area of ​​the frame, ω is the angular frequency, A is the angle of rotation of the frame.

Only the angle α changes, therefore, the graph of the EMF changes has the form of a sinusoid. Since the current, in accordance with Ohm's law, is equal to the ratio of the emf to the load resistance (I = E/R), it is also sinusoidal.

The sinusoidality of the EMF and current variables means that they periodically change not only the magnitude, but also the direction to the opposite.

Schematic diagrams of alternating current generators

Calculation of electrical circuits

To calculate a circuit means to find all the currents in it. There are different methods for calculating electrical circuits: Kirchhoff's laws, the loop current method, the nodal potential method and others. Let's consider the application of the loop current method using the example of a specific circuit.

First, we select the contours and designate the current in them. The direction of the current can be chosen arbitrarily. In our case - clockwise. Then for each circuit we will compose equations according to Kirchhoff’s 2nd law. The equations are composed as follows: The circuit current is multiplied by the circuit resistance, and the products of the current of other circuits and the total resistance of these circuits are added to the resulting expression. For our scheme:

The resulting system is solved by substituting the initial data of the problem. We find the currents in the branches of the original circuit as the algebraic sum of the loop currents

Whatever circuit you need to calculate, our specialists will always help you cope with the tasks. We will find all currents using Kirchhoff's rule and solve any example of transient processes in electrical circuits. Enjoy your studies with us!

Disadvantages of AC

The most important disadvantage of alternating current is the presence of reactive power. As is known, a capacitor and an inductor exhibit their reactive properties only in alternating current circuits. Simply put, the coil and capacitor create reactance to AC current, but do not consume it. As a result of this, of the total power supplied by the alternator, part of the power is not spent on performing useful work, but only circulates uselessly between the generator and the load. This power is called reactive power and is harmful. Therefore, they try to minimize it.

However, most loads - motors, transformers and the wires themselves - are inductive elements. And the greater the inductance, the greater the proportion of reactive power from the total and this must be dealt with.

The second main disadvantage of alternating current is that it does not flow across the entire cross-section of the conductor, but is displaced closer to its surface. As a result, the area through which electric current flows decreases, which in turn leads to an increase in the resistance of the conductor and an increase in power losses in it.

The higher the frequency, the more current is displaced to the surface of the conductor and, ultimately, the higher the power loss.

Types of electric generators

Alternating current generators are divided into two types:

1. synchronous _ The frequency of the induced EMF corresponds to the rotor speed;
2. asynchronous . There is a difference between the rotational speeds and the induced emf, called slip. They are designed simpler than synchronous ones and are resistant to overloads and short circuits, which is why they are used in transport. They are not suitable for powering frequency-sensitive consumers.

According to the method of powering the winding of the inductor (electromagnet), electric generators are divided into 4 types:

1. powered from a third-party source;
2. devices with self-excitation, a part of the electric current generated by the generator is supplied to the winding, converted by the rectifier into direct current. Such a generator only needs a third-party source during startup. The battery is used for this purpose;
3. devices with an additional low-power generator installed on the same shaft as the main one. This additional generator powers the inductor winding. Only it needs a third-party source for starting; accordingly, the requirements for a low-power battery are reduced;
4. permanent magnet generators. There is no winding, no power required. The design flaws are mentioned above.

Electrification of railways using alternating current

The Russian AC passenger electric locomotive EP1P is produced at the Novocherkassk Electric Locomotive Plant.

In Russia and the republics of the former USSR, about half of all railways are electrified using single-phase alternating current with a frequency of 50 Hz. A voltage of ~ 25 kV (usually up to 27.5 kV, taking into account losses) is supplied to the contact wire, the second (return) wire is the rails. Electrification is also carried out according to the 2 × 25 kV system (two twenty-five kilovolts each), when a voltage of ~ 50 kV is applied to a separate supply wire (usually up to 55 kV, taking into account losses), and half the voltage of 50 kV is applied to the contact wire from autotransformers (i.e. 25 kV). Electric locomotives and AC electric trains do not require alteration when operating on 2 × 25 kV sections.

A policy is being pursued to further expand the AC traction range both through newly electrified sections and through the transfer of some lines from direct current to alternating current. Transferred to 1990s - 2000s:

on the East Siberian Railway: section Slyudyanka - Irkutsk - Zima; - on the Oktyabrskaya Railway: section Loukhi - Murmansk; — on the Volga Railway: Saratov and Volgograd railway junctions; - on the North Caucasus Railway: sections Mineralnye Vody - Kislovodsk and Beshtau - Zheleznovodsk.

It should be noted that dual-system electric locomotives are also produced that can operate on both alternating and direct current (see VL61D, VL82 and VL82M, EP10, EP20).

Generation and transformation

The principle of generating electricity is simple. If a magnetic field rotates along a stationary set of coils of turns of a conductor or, conversely, a coil rotates around a stationary magnetic field, then due to the phenomenon of electromagnetic induction, a potential difference arises at the ends of the windings. With each change in the angle of rotation resulting from the described circular motion, the output voltage will also change in both magnitude and direction.

The described conditional generator at a constant angular speed of rotation of the shaft produces a sinusoidal AC with a waveform that is no different from that supplied in a household network. Real generators are much more complex, but operate on the same principles of electromagnetic induction.

These same laws help not only in the production of AC, but also in its transmission and distribution. Voltage conversion by power companies cannot be accomplished without electrical machines called transformers.

This is why Tesla's invention was so important to the revolution in electricity transportation.

Any transformer consists of the following elements:

• primary and secondary windings;
• core.

The word “primary” is used for the winding to which the electrical voltage that needs transformation is applied. The induced voltage on the secondary coil is always equal to that applied on the primary, multiplied by the ratio of turns of the secondary to the primary. The transformer allows you to change the voltage step by step.

Classification of electrical circuits

According to their purpose, electrical circuits are:

• Power electrical circuits;
• Electrical control circuits;
• Electrical measurement circuits;

Power circuits are designed to transmit and distribute electrical energy. It is the power circuits that conduct current to the consumer.

Circuits are also divided according to the current strength in them. For example, if the current in the circuit exceeds 5 amperes, then the circuit is power. When you click a kettle plugged into a socket, you close a power electrical circuit.

Electrical control circuits are not power and are designed to activate or change the operating parameters of electrical devices and equipment. An example of a control circuit is monitoring, control and signaling equipment.

Electrical measurement circuits are designed to record changes in the operating parameters of electrical equipment.

Options

In the characteristics of alternating current, there are basic and additional parameters.

• frequency;
• period;
• amplitude;
• effective value.

Additional:

• angular frequency;
• phase;
• instantaneous value.

Let's look at them in detail.

Frequency

The letter designation is f, the unit of measurement is hertz (Hz). Indicates the number of complete cycles of current oscillations per second.

The frequency of the alternating current at the output of the simplest generator is determined by the speed of its rotor. In the power supply system of Russia and other countries of the former USSR, a current with a frequency of 50 Hz is used.

Change curves of sinusoidal alternating current at different frequencies

In some cases, the frequency is increased by converting the current. For example, in inverter welding machines and pulse power supplies - up to 20 - 80 kHz. At this frequency, the dimensions of the transformer and losses in it are significantly reduced. In some devices the frequency is increased to several MHz.

Period

Designation - "T", unit of measurement - second. Period is the duration of a complete cycle of current oscillations. This parameter is related to frequency by the following relationship:

T = 1/f.

Accordingly, in the electrical network the current period is 1/50 = 0.02 s = 20 ms.

Amplitude

The maximum value of current or EMF corresponds to the top of the half-wave. Denoted, respectively, Im and Um. During the period, these values ​​reach amplitude values ​​twice - with a positive and negative sign.

Effective value

This is a direct current equivalent to a given alternating current in terms of work performed. Constantly changing alternating current is inconvenient to use in calculations, so it is replaced by the effective value. Denoted by the letters I and U.

For sinusoidal current and EMF is determined by the formula:

I = 2^(-1/2) * Im = (1/1.414) * Im = 0.707 Im;

U = 2^(-1/2) * Um = 0.707 Um;

Voltage 220 V in a household electrical network is the effective value. The amplitude is 311 V. Similarly, if they say that the load consumes a current of 5A, they mean the effective value. The current amplitude is 7.07 A.

Angular frequency

Indicates the rate of change of angle α in the EMF calculation formula. Corresponds to the angular frequency of rotation of the rotor. Since during one period the angle a changes by 2π at a standard frequency f = 50 Hz, the angular frequency will be: ω = 2π * 50 = 100 π.

Phase - the nature of the change in angle α relative to the time reference point. Currents and EMF can be in phase or have a shift. The latter is measured in radians, degrees or fractions of a period. When the phase shifts by π (1/2 period), the quantities are said to be out of phase.

AC current and voltage phase shift

Inverter autonomous generators

In a large power plant, it is possible to ensure that the rotor rotates at a constant speed. In an autonomous generator with an internal combustion engine, this is almost impossible to achieve, as a result of which the current becomes unstable. The problem is solved by installing an inverter - a microcircuit-controlled electronic unit that digitally processes the current.

The alternating current produced by the generator is rectified, supplied to the inverter, and it already generates alternating current at the output with a precisely specified frequency and voltage.

The disadvantage of inverter generators is their high cost.