What is a constant and alternating current definition. Permanent and alternating current. Obtaining electric current

Alternating electric current (AC, Abbreviarara from English. Alternating Current) - this is varying by its size and direction with a certain periodicity of electric current. In electrical engineering as an alphabetic designation electric current It is customary to use a tilde sign (~).

Sources of the AC electrical current are alternating current generators that create a variable power force, a change in the value and direction of which occurs at certain intervals.

Basic AC Parameters

For its description, the following parameters use (see chart):

• Period (T) - the duration of time during which the electric current makes one full cycle of changes, returning to its initial value;
• Frequency (F) - a parameter determining the amount of complete electrical current oscillations in one second, the unit of measurement is 1 hertz (Hz). So, for example The current frequency standard adopted in domestic power systems is 50 Hz or 50 oscillations per second.
• Current amplitude (IM) - The maximum achieved instantaneous value of the current value for the period, as can be seen from the graph presented - the height of the sinusoid;
• Phase - The state of the variable sinusoidal electric current: instantaneous value, direction changes, increase (decrease) in the chain. AC can be both single-phase and multiphase.

Three-phase systems are the greatest distribution, which are three separate emails. chains with the same frequency and EMF, with a shift angle φ \u003d 120 °. In more detail, the concept can be found in the article the principle of creating a three-phase AC circuit.

AC application

A variable sinusoidal electric current is used in almost all sectors of the economy. The widespread use of alternating current is due to the economic efficiency of its use in power supply systems, simplicity in conversion from energy low voltage In the energy of a higher voltage and vice versa.

This feature allows you to reduce the loss of electricity when it is transmitted over long distance along the wires, significantly reducing the area of \u200b\u200btheir cross-section.

Electricity B. modern world exists in two types. One impossible is a constant current, and the second is variable. The difference between them is the principal and what is available in the same type of electricity is not available to another. So, permanent current is known to people for a very long time, and the variable was put by a person to the civilization service literally today by historical standards. This article is devoted to considering differences and places for the use of electricity with a constant and variable component.

Permanent current, its origin and application

With sources direct current We are confronted every second. When you read this article from the screen of your monitor, in that you distinguish the letters, there is a merit of DC. It is from DC sources a computer and all its chips. It is the drops between the levels of the signal corresponding to zero and one, we must exist a digital universe. Permanent current flows in a flashlight and mobile phone, in the car and many other household devices and special purposewhere there is at least one transistor or diode.

At the same time, methods for obtaining and using direct current were known in times Ancient Mira. The archaeologists producing excavations in the Valley of the Euphrates were found strange ceramic vessels in the dwelling of some jewelers. The vessels had a device similar to the galvanic battery and combined with a copper wire. What was the surprise of archaeologists, when they filled one of the acid vessels for the experiment and received the potential equal to one and a half volt on his poles! It turned out that the battery blocks ancient jewelers were used for galvanic coating of jewelry with various metals, which was confirmed by ready-made samples of products that often came across the scientists earlier.

There are hypotheses that are in favor of the fact that during the construction of the pyramids in Egypt, electricity was used to illuminate halls and corridors in those places where the basins were applied. Scientists are arguing so far on this matter, since there is an assumption that the light was served using a system of mirrors from the surface. Be that as it may, but the traces of the soot on the walls of the ancient halls with paintings were not detected and this is the fact that remains inexplicable so far. One thing is clear that the Sumerians knew how to use electricity, and they also lived before Egyptian civilization.

In a modern sense, the permanent current occurs in closed chainconsisting of a DC source, such as a battery or chemical battery, conductors and loads. The material can act as a load with electrical resistance, much more, rather than resistance of conductors closed electric chain. This may be a light bulb with a tungsten spiral or a deostat of nichrome wire or any other load, the resistance of which has a value other than zero.

Get permanent current different ways. The most ancient of them is a chemical, based on the occurrence of the difference of potentials between conductors from different materials placed in an acidic or alkali environment. Chemical batteries and batteries are used by people not one millennium and today they are in the go, only in a very improved form compared to their ancient ancestors. More modern sources of DC - photo cells, allowing to obtain the difference in potentials during the irradiation of their Sun and DC generators, which are powered by mechanical energy attached from the outside. Today, direct current generators are most common in windings with a voltage converter.

Permanent current moves trains on railway. Electrified sections today make up a significant amount in the extent in our country. Permanent currents are used for transmission over long distances of significant capacity electrical Energy Under ultrahigh potentials.

With the entire latitude of DC application, there are significant limitations that prevent its use in everyday activity for the nutrition of household appliances and industrial installations. This is due to large losses on ohmic resistance in conductors, which affects the most negative effect on the work of lighting and other equipment. In order to reduce the loss, it is necessary to use the conductors of a larger cross section, and the alternatives of copper here are practically no. And the copper wires are very expensive.

This obstacle forced scientists to search for other ways to obtain and transmission of electricity for any distance practically without loss. Now in this field of human activity playing a major role alternating current.

AC Current - Origin and Application

The emergence of alternators and transmission systems of AC energy has become one of the most important achievements of the nineteenth century. At the same time, scientific research in this area was conducted from the very beginning of the century. The research was based on theoretical calculations that showed that the alternating magnetic field should cause an alternating electric field, which in turn causes a variable magnetic field and this process can flow to infinity. With a significant frequency of oscillations, electromagnetic waves are formed, capable of freely distributed in space, and with a slight frequency, almost all the energy remains in the conductor through which its transmission occurs.

The easiest way to excite electrical oscillations with a variable voltage amplitude is to move a permanent magnet inside the frame with isolated wire. At the same time, the greater the number of turns in the frame and the more powerful magnet, the higher the maximum value of the voltage amplitude that the voltmeter can register on the clips of the frame winding.

An important feature of alternating voltage is the change of polarity during the passage of the magnet in the opposite direction. As well as the passage of the zero mark the value of the amplitude of the voltage when changing polarity. This is the behavior of the voltage, which means the current when connecting the load, allows you to easily convert aC voltage to other values \u200b\u200busing transformers, which opens excellent prospects for transmission to virtually no loss of significant capacities for any distances, which is unattainable for DC installations, except those operating high voltages.

The first alternating current generators were developed by Tesla and Edison. Tesla developed three-phase diagram production and transmission of electricity over long distances. He also proposed the principle of voltage transformation depending on the tasks solved. So, to consume electricity by finite installations, it proposed to enter an alternating voltage of a frequency of 50 or 60 Hz with an amplitude of 110, 127 or 220 volts, and for transmission over long distances, it recommended raising the voltage to 10 thousand volts and above. At high voltages for transmission over conductor, the same power requires a smaller current, and the smaller, the smaller the loss in the conductor. Therefore, today in the power line, an alternating voltage with an amplitude of up to 330 square meters is supplied.

Simple voltage conversion opens up very wide opportunities for direct use of alternating current. So, existing asynchronous three-phase and single-phase engines, lightingHeaters and many other household appliances can work directly from the network, and more complex radio engineering and automation devices that require constant voltage to work are adapted to obtain it right on the spot from the AC power voltage. So minimize DC loss in conductors.

Prospects for the joint existence of AC and DC

Scientists and practitioners from electrical engineering has long been occupying a question of compounding the positive qualities of AC and DC. Such solutions have become possible, due to the appearance of powerful pulsed semiconductor valves. Today, no one has surprise inverter devices transforming constant pressure in variable, industrial frequency, and vice versa. Pulse power supplies in radio electronic equipment and computer equipment have become compact and powerful, ten times more efficient compared to power sources on ordinary transformers.

Today it can be argued about the real revolution in the welding case, which occurred due to the emergence of inverters, significantly facilitating in direct and figurative sense welders and processes. Now even those types of welding, which were considered the prerogative of closed defense enterprises became accessible to any welder, and the cost of manufacturing such works as argon-arc welding and semi-automatic welding significantly decreased. Available at the price, light portable welding machines that can be powered by a regular outlet in any apartment, gave the opportunity to show your creative potential to many lovers and professionals working with metal.

No less impressive achievements of impulse equipment can boast manufacturers of sources uninterrupted power, network pulse voltage stabilizers, electricity production systems from alternative sources with the possibility of accumulating and subsequent conversion of stored energy when needed. The possibilities of impulse technology are studied and used far from completely. We are at the very beginning of this path of unity of direct and alternating current. Not far from the mountains, cars on electricity and other miracles that will be the introduction of new discoveries and developments in the field of impulse sources of electricity.

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Obtaining alternating current

AC generators produce generators, electric cars - How to call them in electrical engineering. Do not forget that, depending on their application, generators are both alternating and DC. Depending on their device, generators produce:

• three-phase current with output voltage 380 volts;
• single-phase current with output voltage 220 volts.

Where exactly can three-phase generators apply? Yes, let's say for the power of a three-phase heat gun for 6 kW 380 V for heating the warehouse room.

Then other phase generators can be used? Single-phase generators, as well as three-phase, are applied to be admitted to the hospital - with emergency shutdown of electricity.

The generator, as we know, must be given mechanical rotation Anchor. How can I give an anchor of the generator mechanical rotation? Such sources serve internal combustion engines:

• gas;
• petrol;
• diesel

and other sources to lead the generator anchor in motion. Other sources of electrical energy are:

• wind power stations;
• water power plants;
• turbine power plants.

The figure shows a schematic representation of an alternator device \\ Fig.1 \\. The frame in this example can be represented as an anchor consisting of one turn of the wire. The frame is indicated by the sides of the A, B, B, the two conductors \\ a and b \\ with the rotation of the frame, intersect the magnetic power lines of the permanent magnet C, Y. when crossing the conductor silest lines, in conductors guessed electromotive force - EDF. EMF of two conductors in their meanings are opposed to each other at the moment when they crossed these power lines.

The value of EDC \\ r.3 \\, flowing current in the frame, will depend:

from the vector of magnetic induction of a permanent magnet \\ n, s \\;

the length of the conductor;

the speed of intersection by the conductor of magnetic power lines

and the angle of tilt the conductor \\ Fig.4 \\ with respect to the power lines of the permanent magnet \\ sin an angle of alpha between the direction of the conductor movement and the direction of magnetic power lines of the field \\.

When the frame is rotated in a magnetic field, it is guided by the EMF of two opposite values \u200b\u200band the current, as we can notice on the graph \\ Fig.5 \\ is performed with pulsating. One period T consists of two opposing current ripples, the upper half-period is a positive and lower half-period - negative. Half-period is designated on the graph as 1/2 T.

Therefore, the current in this example is considered as:

throbbing;

sinusoidal

or how else is called - alternating current.

Getting DC

Permanent current we get from the following sources, it is:

• primary sources \\ ordinary, simple batteries \\;
• electrochemical batteries;
• dC generators.

The principle of the device of electrochemical batteries is shown in Figure 6. Electrochemical batteries can be returned to the initial state under the influence of electric current - in the process of their charging or recharging.

Primary sources \\ Elements \\, a variety of types of batteries \\ Fig.7 \\, cannot be returned to its initial state in the process of charging by electrical current, then-there, such sources only disposal is subject to expiration.

The difference between the AC generator and the DC generator is that in the DC generator placed large quantity turns in the grooves of anchor \\ compared to the alternator \\, as well as the even number of main and additional poles on the inner bed of the generator strengthened.

The following drawing of itself is a load connection diagram to a DC generator \\ Fig.8 \\, the current in this chain is closed through the load.

The graph \\ Fig.9 \\ shows the pulsation of the current issued by the DC generator. Compared to the alternator, these pulsations look more smooth.

Application of DC

automotive generator

automobile generator device

dC Welding Power Station