Connecting wires to the ten. Calculation of the power of heating elements, explanation of connection. Electrical installation in a three-phase network

Accomplished when a test electric charge is transferred from a point A exactly B, to the value of the test charge.

In this case, it is considered that the transfer of the test charge doesn't change distribution of charges on field sources (by definition of a test charge). In a potential electric field, this work does not depend on the path along which the charge moves. In this case, the electrical voltage between two points coincides with the potential difference between them.

Alternative definition -

Integral of the projection of the effective field (including third-party fields) onto the distance between points A And B along a given path starting from a point A exactly B. In an electrostatic field, the value of this integral does not depend on the path of integration and coincides with the potential difference.

The SI unit of voltage is the volt.

DC voltage

Average voltage

The average voltage value (constant voltage component) is determined over the entire oscillation period as:

For a pure sine wave, the average voltage value is zero.

RMS voltage

The root mean square value (outdated name: current, effective) is most convenient for practical calculations, since it does the same work on a linear active load (for example, an incandescent lamp has the same brightness, a heating element releases the same amount of heat as an equal constant voltage:

In technology and everyday life, when using alternating current, the term “voltage” means precisely this value, and all voltmeters are calibrated based on its definition. However, by design, most devices actually measure not the root mean square, but the average rectified (see below) voltage value, so for a non-sinusoidal signal their readings may differ from the true value.

Average rectified voltage value

The average rectified value is the average value of the voltage modulus:

For sinusoidal voltage the equality is true:

Rarely used in practice, most AC voltmeters (those in which the current is rectified before measurement) actually measure this value, although their scale is graduated in rms values.

Voltage in three-phase current circuits

In three-phase current circuits, phase and linear voltages are distinguished. By phase voltage we mean the root mean square value of the voltage on each of the load phases, and by linear voltage we mean the voltage between the supply lines. phase wires. When connecting the load in a triangle, the phase voltage is equal to linear, and when connecting in a star (with symmetrical load or with a solidly grounded neutral), the line voltage is times greater than the phase voltage.

In practice, the voltage of a three-phase network is denoted by a fraction, the denominator of which is the linear voltage, and the numerator is the phase voltage when connected in a star (or, which is the same thing, the potential of each line relative to the ground). Thus, in Russia the most common networks are with a voltage of 220/380 V; 127/220 V and 380/660 V networks are also sometimes used.

Standards

Current and voltage are quantitative parameters used in electrical diagrams. Most often, these quantities change over time, otherwise there would be no point in the operation of the electrical circuit.

Voltage

Conventionally, voltage is indicated by the letter "U". The work expended in moving a unit of charge from a point of low potential to a point of high potential is the voltage between the two points. In other words, it is the energy released after a unit of charge moves from high to low potential.

Voltage can also be called potential difference, as well as electromotive force. This parameter is measured in volts. To move 1 coulomb of charge between two points that have a voltage of 1 volt, 1 joule of work must be done. Coulombs are measured electric charges. 1 pendant equal to charge 6x10 18 electrons.

Voltage is divided into several types, depending on the types of current.

• Constant pressure . It is present in electrostatic and direct current circuits.
• AC voltage . This type of voltage is found in circuits with sinusoidal and alternating currents. When sinusoidal current The following voltage characteristics are considered:
  - amplitude of voltage fluctuations– this is its maximum deviation from the x-axis;
  - instantaneous tension, which is expressed at a certain point in time;
  - effective voltage, is determined by the execution active work 1st half period;
  - average rectified voltage, determined by the magnitude of the rectified voltage over one harmonic period.

When transmitting electricity through overhead lines, the design of supports and their dimensions depend on the magnitude of the applied voltage. The voltage between phases is called line voltage , and the voltage between the ground and each phase is phase voltage . This rule applies to all types air lines. In Russia, in household electrical networks, the standard is three-phase voltage with a linear voltage of 380 volts and a phase voltage of 220 volts.

Electricity

Current in an electrical circuit is the speed of movement of electrons at a certain point, measured in amperes, and denoted in diagrams by the letter “ I" Derived units of ampere with the corresponding prefixes milli-, micro-, nano, etc. are also used. A current of 1 ampere is generated by moving a unit of charge of 1 coulomb in 1 second.

It is conventionally considered that the current flows in the direction from positive potential to negative. However, from the physics course we know that the electron moves in the opposite direction.

You need to know that voltage is measured between 2 points on the circuit, and current flows through one specific point in the circuit, or through its element. Therefore, if someone uses the expression “tension in resistance,” then this is incorrect and illiterate. But often we are talking about voltage at a certain point in the circuit. This refers to the voltage between the ground and this point.

Voltage is generated from exposure to electrical charges in generators and other devices. Current is created by applying a voltage to two points on a circuit.

To understand what current and voltage are, it would be more correct to use. On it you can see the current and voltage, which change their values ​​over time. In practice, the elements of an electrical circuit are connected by conductors. At certain points, the elements of the circuit have their own voltage value.

Current and voltage obey the rules:

• The sum of currents entering a point is equal to the sum of currents leaving the point (charge conservation rule). This rule is Kirchhoff's law for current. The point of entry and exit of the current in this case is called a node. A corollary of this law is the following statement: in a series electrical circuit of a group of elements, the current value is the same for all points.
• IN parallel circuit elements, the voltage on all elements is the same. In other words, the sum of the voltage drops in a closed circuit is zero. This Kirchhoff law applies to stresses.
• The work done per unit time by a circuit (power) is expressed as follows: P = U*I. Power is measured in watts. 1 joule of work done in 1 second is equal to 1 watt. Power is distributed in the form of heat and is spent to perform mechanical work(in electric motors), converted into radiation various types, accumulates in containers or batteries. When designing complex electrical systems, one of the problems is thermal load systems.

Characteristics of electric current

A prerequisite for the existence of current in an electrical circuit is a closed circuit. If the circuit is broken, the current stops.

Everyone in electrical engineering operates on this principle. They're tearing apart electrical circuit movable mechanical contacts, and this stops the flow of current, turning off the device.

In the energy industry, electric current occurs inside current conductors, which are made in the form of busbars and other parts that conduct current.

There are also other ways to create internal current in:

• Liquids and gases due to the movement of charged ions.
• Vacuum, gas and air using thermionic emission.
• , due to the movement of charge carriers.

Conditions for the occurrence of electric current:

• Heating of conductors (not superconductors).
• Application of potential differences to charge carriers.
• A chemical reaction that releases new substances.
• Impact magnetic field to the conductor.

Current Waveforms

• Straight line.
• Variable harmonic sine wave.
• A meander, similar to a sine wave, but having sharp corners(sometimes the corners may be smoothed).
• A pulsating form of one direction, with an amplitude varying from zero to the greatest value according to a certain law.

Types of work of electric current

• Light radiation created by lighting devices.
• Generating heat using heating elements.
• Mechanical work (rotation of electric motors, action of other electrical devices).
• Creation of electromagnetic radiation.

Negative phenomena caused by electric current

• Overheating of contacts and live parts.
• The occurrence of eddy currents in the cores of electrical devices.
• Electromagnetic radiation into the external environment.

Creators of electrical devices and various schemes when designing, they must take into account the above properties of electric current in their designs. For example, bad influence eddy currents in electric motors, transformers and generators are reduced by fusion of cores used to pass magnetic fluxes. Lamination of the core is its production not from a single piece of metal, but from a set of individual thin plates of special electrical steel.

But, on the other hand, eddy currents are used for work microwave ovens, ovens operating on the principle of magnetic induction. Therefore, we can say that eddy currents are not only harmful, but also beneficial.

Alternating current with a signal in the form of a sinusoid can differ in frequency of oscillations per unit time. In our country, the industrial frequency of electrical current is standard and equal to 50 hertz. In some countries, a current frequency of 60 hertz is used.

For various purposes in electrical engineering and radio engineering, other frequency values ​​are used:

• Low frequency signals with a lower current frequency.
• High frequency signals that are much higher than the frequency of industrial current.

It is believed that electric current arises from the movement of electrons within a conductor, which is why it is called conduction current. But there is another type of electric current, which is called convection. It occurs when charged macrobodies move, for example, raindrops.

Electricity in metals

The movement of electrons when exposed to them constant force compared to a parachutist who is falling to the ground. In these two cases it happens uniform motion. The force of gravity acts on the skydiver, and the force of air resistance opposes it. The movement of electrons is affected by a force electric field, and the ions of the crystal lattices resist this movement. The average speed of electrons reaches a constant value, just like the speed of a parachutist.

In a metal conductor, the speed of movement of one electron is 0.1 mm per second, and the speed of electric current is about 300 thousand km per second. This is because electric current only flows where voltage is applied to charged particles. Therefore, a high current flow rate is achieved.

When electrons move into crystal lattice There is the following pattern. Electrons do not collide with all oncoming ions, but only with every tenth of them. This is explained by the laws quantum mechanics, which can be simplified as follows.

The movement of electrons is hampered by large ions that offer resistance. This is especially noticeable when metals are heated, when heavy ions “sway”, increase in size and reduce the electrical conductivity of the conductor crystal lattices. Therefore, when metals are heated, their resistance always increases. As the temperature decreases, the electrical conductivity increases. By reducing the temperature of a metal to absolute zero, the effect of superconductivity can be achieved.

That is electric field had to “pull” electrons through the load, and the energy that was expended is characterized by a quantity called electrical voltage. The same energy was spent on some change in the state of the load substance. Energy, as we know, does not disappear into nowhere and does not appear from nowhere. This is what it says Law of energy conservation. That is, if the current spent energy passing through the load, the load acquired this energy and, for example, heated up.

That is, we come to the definition: electric current voltage is a quantity that shows how much work the field did when moving a charge from one point to another. The voltage in different parts of the circuit will be different. The voltage on a section of an empty wire will be very small, and the voltage on a section with any load will be much greater, and the magnitude of the voltage will depend on the amount of work done by the current. Voltage is measured in volts (1 V). To determine the voltage there is a formula:

where U is the voltage, A is the work done by the current to move charge q to a certain section of the circuit.

Voltage at the poles of the current source

As for the voltage on the circuit section, everything is clear. What then does the voltage at the poles mean? current source? In this case, this voltage means the potential amount of energy that the source can impart to the current. It's like water pressure in pipes. This is the amount of energy that will be consumed if a certain load is connected to the source. Therefore, the higher the voltage at the current source, the more work the current can do.

2) Dielectrics in an electric field

Unlike conductors, dielectrics have no free charges. All charges are

connected: electrons belong to their atoms, and ions of solid dielectrics vibrate

near the nodes of the crystal lattice.

Accordingly, when a dielectric is placed in an electric field, no directional movement of charges occurs

Therefore, our proofs of properties do not pass for dielectrics

conductors - after all, all these arguments were based on the possibility of the appearance of current. Indeed, none of the four properties of conductors formulated in the previous article

does not apply to dielectrics.

2. The volumetric charge density in a dielectric can be different from zero.

3. Tension lines may not be perpendicular to the surface of the dielectric.

4. Different points of the dielectric may have different potentials. Therefore, talk about

“dielectric potential” is not necessary.

Polarization of dielectrics- a phenomenon associated with a limited displacement of bound charges in a dielectric or rotation of electric dipoles, usually under the influence of an external electric field, sometimes under the influence of other external forces or spontaneously.

The polarization of dielectrics is characterized by electric polarization vector. The physical meaning of the electric polarization vector is the dipole moment per unit volume of the dielectric. Sometimes the polarization vector is briefly called simply polarization.

The polarization vector is applicable to describe the macroscopic state of polarization not only of ordinary dielectrics, but also of ferroelectrics, and, in principle, any media with similar properties. It is applicable not only to describe induced polarization, but also spontaneous polarization (in ferroelectrics).

Polarization is a state of a dielectric, which is characterized by the presence of an electric dipole moment in any (or almost any) element of its volume.

A distinction is made between polarization induced in a dielectric under the influence of an external electric field and spontaneous (spontaneous) polarization, which occurs in ferroelectrics in the absence of an external field. In some cases, polarization of a dielectric (ferroelectric) occurs under the influence of mechanical stress, frictional forces, or due to temperature changes.

Polarization does not change the net charge in any macroscopic volume within a homogeneous dielectric. However, it is accompanied by the appearance on its surface of bound electric charges with a certain surface density σ. These bound charges create in the dielectric an additional macroscopic field with intensity , directed against the external field with intensity . As a result, the field strength inside the dielectric will be expressed by the equality:

Depending on the polarization mechanism, the polarization of dielectrics can be divided into the following types:

Electronic - displacement of the electron shells of atoms under the influence of an external electric field. The fastest polarization (up to 10−15 s). Not associated with losses.

Ionic - displacement of nodes of a crystal structure under the influence of an external electric field, and the displacement is by an amount less than the lattice constant. Flow time 10−13 s, without losses.

Dipole (Orientation) - occurs with losses in overcoming coupling forces and internal friction. Associated with the orientation of dipoles in an external electric field.

Electron relaxation - orientation of defect electrons in an external electric field.

Ion-relaxation - displacement of ions that are weakly fixed in the nodes of the crystal structure, or located in the interstice.

Structural - orientation of impurities and inhomogeneous macroscopic inclusions in the dielectric. The slowest type.

Spontaneous (spontaneous) - due to this type of polarization, in dielectrics in which it is observed, the polarization exhibits significantly nonlinear properties even at low values ​​of the external field, and the phenomenon of hysteresis is observed. Such dielectrics (ferroelectrics) are characterized by very high dielectric constants (from 900 to 7500 for some types of capacitor ceramics). The introduction of spontaneous polarization, as a rule, increases the loss tangent of the material (up to 10 −2)

Resonant - the orientation of particles whose natural frequencies coincide with the frequencies of the external electric field.

Migration polarization is caused by the presence in the material of layers with different conductivities, the formation of space charges, especially at high voltage gradients, has big losses and is polarization in slow motion.

Polarization of dielectrics (except for resonant polarization) is maximum in static electric fields. In alternating fields, due to the presence of inertia of electrons, ions and electric dipoles, the electric polarization vector depends on frequency.

Voltage unit

First, we'll briefly review the concept of voltage and the units of voltage. Electric current can be thought of as the directed movement of electrons caused by an electric field.

Voltage unit

How more quantity moving electrons, the more work is done by the electric field. In addition to current, voltage also affects the operation of the electric field.

This work involves moving electrons from a point of low potential to a point where the charge on the electrons is greater. In other words, voltage can be considered as a potential difference, and it is determined by the ratio:

U = A/q where: A is expressed in joules as the work of the electric field, and q is the charge of electrons in coulombs.

Where does the voltage unit come from:

1B = 1 J/1C. That is, the unit of voltage measurement is 1 Volt.

IN electrical network residential buildings standard adopted phase voltage 220 V or linear three-phase voltage 380 V.

Measuring voltage with a multimeter

To measure voltage, you need a multimeter, tester or voltmeter. The multimeter is convenient to use when installing electrical wiring, testing cables, repairing sockets, chandeliers and switches. Thus, a multimeter has become a necessary device in every home.

There are three types of voltage - alternating voltage (ACV), direct voltage (DCV) and pulse voltage. Pulse voltage has several parameters and is best checked with an oscilloscope. You can use a multimeter to check the pulse voltage in the position of the DCV switch, but only purely conditionally. When repairing switching power supplies, use an oscilloscope.

In most apartments and houses, the electrical network has 220 V. When measuring alternating voltage, the measurement type switch is set to V ~. If the measured alternating voltage is known, then the measurement limit is set to the appropriate position, and if its value is not known, then the switch is set to the maximum limit of 750 V.

Switch position when measuring voltage

Before measuring voltage with a multimeter, the black probe is inserted into the COM socket, and the red socket into VΩmA. Do not touch with your hands when measuring metal parts probes and short-circuit them to avoid short circuit. The 10A multimeter socket is designed to measure DC current up to 10A.

In this case, the red probe is inserted into the 10 A socket, the black one remains in the COM socket, and the switch is set to the 10 A position. When measuring DC voltage, the probes are placed in the same sockets as when measuring AC voltage, and the choice of measurement mode is set to position V - the corresponding limit.

Voltage sockets used

In this case, the probes should be set to the appropriate polarity, the red probe to the plus (+) of the source being measured, and the black probe to the minus (-). If the probes are mixed up, then nothing bad will happen, only the multimeter will show a minus sign (-) in front of the number. For alternating voltage, the polarity of the probes does not matter. In everyday life, DC voltage measurements are carried out when checking batteries, accumulators, and repairing household appliances.

How to check the voltage in an outlet with a multimeter

To measure the voltage in an outlet, you need to perform the same operations with a multimeter as when measuring alternating voltage. Since an alternating voltage of 220 V is supplied to the socket, with some variation, the measurement limit is set to 750 V. The black probe should be in the COM socket, and the red one in VΩmA. Carefully, without touching the metal ends of the probes with your hands, insert them into the sockets of the socket. The display will show the mains voltage.

Measuring voltage in a socket

You can also use a multimeter to determine the phase in the socket. To do this, one probe is applied to the ground, on the third grounding contact of the socket, and the other probe is inserted in turn into the sockets of the socket until the mains voltage appears on the display. This socket will contain the phase, and the other will contain the neutral. It is possible that there will be no voltage in this outlet. This indicates a malfunction in the outlet itself or in the electrical wires connected to it.

Basic unit of measurement electrical voltage is volt. Depending on the magnitude, voltage can be measured in volts(IN), kilovolts(1 kV = 1000 V), millivolts(1 mV = 0.001 V), microvolts(1 µV = 0.001 mV = 0.000001 V). In practice, most often you have to deal with volts and millivolts.

There are two main types of stress - permanent And variable. Batteries and accumulators serve as a source of constant voltage. The source of alternating voltage can be, for example, the voltage in the electrical network of an apartment or house.

To measure voltage use voltmeter. There are voltmeters switches(analog) and digital.

Today, pointer voltmeters are inferior to digital ones, since the latter are more convenient to use. If, when measuring with a pointer voltmeter, the voltage readings have to be calculated on a scale, then with a digital one, the measurement result is immediately displayed on the indicator. And in terms of dimensions, a pointer instrument is inferior to a digital one.

But this does not mean that pointer instruments are not used at all. There are some processes that cannot be seen with a digital instrument, so arrows are more often used in industrial enterprises, laboratories, repair shops, etc.

On electric circuit diagrams a voltmeter is indicated by a circle with a capital Latin letter " V" inside. Near symbol the voltmeter indicates it letter designation « P.U." and the serial number in the diagram. For example. If there are two voltmeters in the circuit, then next to the first one they write “ PU 1", and about the second " PU 2».

When measuring direct voltage, the diagram indicates the polarity of the voltmeter connection, but if alternating voltage is measured, the polarity of the connection is not indicated.

Voltage is measured between two points schemes: in electronic circuits ah between positive And minus poles, in electrical circuits between phase And zero. Voltmeter connected parallel to the voltage source or parallel to the chain section- a resistor, lamp or other load on which the voltage needs to be measured:

Let's consider connecting a voltmeter: on top diagram voltage is measured across the lamp HL1 and simultaneously on the power source GB1. On bottom diagram voltage is measured across the lamp HL1 and resistor R1.

Before measuring the voltage, determine it view and approximate size. The fact is that the measuring part of voltmeters is designed for only one type of voltage, and this results in different measurement results. A voltmeter for measuring direct voltage does not see alternating voltage, but a voltmeter for alternating voltage, on the contrary, can measure direct voltage, but its readings will not be accurate.

It is also necessary to know the approximate value of the measured voltage, since voltmeters operate in a strictly defined voltage range, and if you make a mistake with the choice of range or value, the device can be damaged. For example. The measurement range of a voltmeter is 0...100 Volts, which means that voltage can only be measured within these limits, since if a voltage is measured above 100 Volts, the device will fail.

In addition to devices that measure only one parameter (voltage, current, resistance, capacitance, frequency), there are multifunctional ones that measure all these parameters in one device. Such a device is called tester(mostly these are arrows measuring instruments) or digital multimeter.

We won’t dwell on the tester, that’s the topic of another article, but let’s move straight to the digital multimeter. For the most part, multimeters can measure two types of voltage within the range of 0...1000 Volts. For ease of measurement, both voltages are divided into two sectors, and within the sectors into subranges: DC voltage has five subranges, AC voltage has two.

Each subrange has its own maximum measurement limit, which is indicated digital value: 200m, 2V, 20V, 200V, 600V. For example. At the “200V” limit, voltage is measured in the range of 0...200 Volts.

Now the measurement process itself.

1. DC voltage measurement.

First we decide on view measured voltage (DC or AC) and move the switch to the desired sector. For example, let's take a AA battery, the constant voltage of which is 1.5 Volts. We select the constant voltage sector, and in it the measurement limit is “2V”, the measurement range of which is 0...2 Volts.

The test leads must be inserted into the sockets as shown in the figure below:

red the dipstick is usually called positive, and it is inserted into the socket, opposite which there are icons of the measured parameters: “VΩmA”;
black the dipstick is called minus or general and it is inserted into the socket opposite which there is a “COM” icon. All measurements are made relative to this probe.

We touch the positive pole of the battery with the positive probe, and the negative pole with the negative one. The measurement result of 1.59 Volts is immediately visible on the multimeter indicator. As you can see, everything is very simple.

Now there's another nuance. If the probes on the battery are swapped, a minus sign will appear in front of the one, indicating that the polarity of the multimeter connection is reversed. The minus sign can be very convenient in the process of setting up electronic circuits, when you need to determine the positive or negative buses on the board.

Well, now let’s consider the option when the voltage value is unknown. We will use a AA battery as a voltage source.

Let’s say we don’t know the battery voltage, and in order not to burn the device, we start measuring from the maximum limit “600V”, which corresponds to the measurement range of 0...600 Volts. Using the multimeter probes, we touch the poles of the battery and on the indicator we see the measurement result equal to “ 001 " These numbers indicate that there is no voltage or its value is too small, or the measurement range is too large.

Let's go lower. We move the switch to the “200V” position, which corresponds to the range of 0...200 Volts, and touch the battery poles with the probes. The indicator showed readings equal to “ 01,5 " In principle, these readings are already enough to say that the voltage of the AA battery is 1.5 Volts.

However, the zero in front suggests going even lower and measuring the voltage more accurately. We go down to the “20V” limit, which corresponds to the range of 0...20 Volts, and take the measurement again. The indicator showed “ 1,58 " Now we can say with certainty that the voltage of a AA battery is 1.58 Volts.

In this way, without knowing the voltage value, they find it, gradually decreasing from a high measurement limit to a low one.

There are also situations when, when taking measurements, the unit "" is displayed in the left corner of the indicator. 1 " A unit indicates that the measured voltage or current is higher than the selected measurement limit. For example. If you measure a voltage of 3 Volts at the “2V” limit, then a unit will appear on the indicator, since the measurement range of this limit is only 0…2 Volts.

There remains one more limit “200m” with a measurement range of 0...200 mV. This limit is intended to measure very small voltages (millivolts), which are sometimes encountered when setting up some amateur radio design.

2. AC voltage measurement.

The process of measuring alternating voltage is no different from measuring direct voltage. The only difference is that for alternating voltage the polarity of the probes is not required.

The AC voltage sector is divided into two subranges 200V And 600V.
At the “200V” limit, you can measure, for example, the output voltage secondary windings step-down transformers, or any other in the range of 0...200 Volts. At the “600V” limit, you can measure voltages of 220 V, 380 V, 440 V or any other voltage in the range of 0...600 Volts.

As an example, let's measure the voltage of a 220 Volt home network.
We move the switch to the “600V” position and insert the multimeter probes into the socket. The measurement result of 229 Volts immediately appeared on the indicator. As you can see, everything is very simple.

And one moment.
Before measurement high voltage ALWAYS double check that the insulation of the probes and wires of the voltmeter or multimeter is in good condition, and also additionally check the selected measurement limit. And only after all these operations take measurements. This way you will protect yourself and the device from unexpected surprises.

And if anything remains unclear, then watch the video, which shows how to measure voltage and current using a multimeter.