Electric current

Alternating Current

Alternating current

Alternating current (AC) is a type of electrical current that changes over time. The variation can be in current intensity or in direction at regular intervals.

The voltage varies between the maximum and minimum values ​​cyclically. The voltage is positive half the time and negative the other half. This means that half the time the current circulates in one direction and the other half in the opposite direction.

The most common form of undulation follows a sine-like trigonometric function. This is the most efficient and practical way to produce electrical energy using alternators. However, there are certain applications in which other waveforms are used, such as the square wave or the triangle wave.

Direct current is one that maintains its voltage value constant and without polarity change.

What is the frequency of an alternating current?

Frequency is the number of cycles of the sine wave that occur in a unit of time.

The sine curve represents the variation of the voltage in an alternating current. The frequency of the same current is the number of turns or cycles given by the radius of the trigonometric circle in a unit of time.

Usual frequencies

The frequency of the electrical system varies by country and sometimes within a country.

Most of the electrical energy is generated at 50 or 60 Hertz. Some countries have a mix of 50Hz and 60Hz supplies.

A low frequency facilitates the design of electric motors. Especially for lifting, crushing and rolling applications. These motors require a high turning speed. It is also beneficial for commutator type traction motors for applications such as railways.

However, the low frequency also causes noticeable flickering in arc lamps and incandescent bulbs. The use of lower frequencies also provided the advantage of lower impedance losses, which are proportional to the frequency.

Effects of high frequencies

A direct current flows uniformly through the cross section of a uniform cable. In an alternating current of any frequency, the electrical charge is forced away from the center of the cable, towards its outer surface. This is because the acceleration of an electric charge in an alternating current produces waves of electromagnetic radiation. 

These waves cancel the spread of electricity to the center of materials with high conductivity. This phenomenon is called the film effect.

At very high frequencies, current no longer flows in the cable. Current flows on the surface of the cable within a shallow thickness of the crust.

The depth of the crust is the thickness to which the current density is reduced by 63%. Even at relatively low frequencies used for power transmission (50 Hz - 60 Hz), uneven current distribution still occurs in thick enough conductors.

For example, the film depth of a copper conductor is approximately 8.57 mm at 60 Hz. For this reason, high current conductors are generally hollow. This saves mass and cost.

Since current tends to flow at the periphery of the conductors, the effective cross section of the conductor is reduced. This increases the effective alternating current resistance of the conductor, since the resistance is inversely proportional to the cross-sectional area.

The alternating current resistance is often many times greater than the direct current resistance. this difference causes a much greater loss of energy due to ohmic heating.

Conversion from alternating to continuous

Alternating current can be easily switched to direct current. The continuum does not have this facility. This is precisely the reason for the widespread use of this type of current.

To convert direct current into alternating current, the voltage rise is achieved by connecting dynamos in series. This technique is very impractical compared to the alternative that has a transformer, which allows the voltage to be raised efficiently.

Electric energy is given by the product of voltage, intensity and time. Since the section of the conductors of the electrical lines depends on the current, by means of a transformer the voltage can be raised to high values. High voltage electric current. Increasing the voltage decreases the current intensity.

The great advantage of distributing current at high voltage is that electricity can be distributed over long distances at low intensities. This reduces the energy losses caused:

  • The Joule effect.

  • Hysteresis.

  • Foucault currents.

It is common for photovoltaic solar energy installations to use a direct current to alternating current converter so that the energy generated can be supplied to the grid.

Electricity distribution

In Europe, electricity is distributed in the form of sinusoidal alternating current at a constant frequency of 50 Hz.

The use of this type of current is due to:

  • The transport of high electrical powers is very efficient if it is carried out at high voltages. High voltages are achieved quite easily with the use of transformers.

  • Alternators are simpler. They are also more efficient than dynamos.

  • In direct current it is not possible to exploit the advantages of a three-phase system. Almost all consumer electronic devices operate on direct current. Despite this, this conversion can be easily achieved by means of a simple rectifier.

On the other hand, it is possible to obtain alternating electric current from direct current. Power inverters can generate this conversion and supply the current in appropriate parameters of frequency, waveform, and voltage.

History of alternating current

In the industrial origins of electricity use in the 19th century, direct current was used. Direct current offers the advantage of being able to be stored in batteries.

However, with the arrival of alternating current by scientist Nikola Tesla the world was revolutionized again.

Alternating current expansion

The efficiency of this new type of current is due to the fact that it can drastically reduce energy losses over long distances thanks to the increase in electrical voltage.

This characteristic allowed to overcome the direct current of Thomas Edison.

The alternating current was extended thanks to the use of the transformer. The transformer allows to bring the potential difference (voltage) to very high levels (high voltage) and the current to very low values.

In this way, the power remains unchanged when transmitting it over long distances.

Also, AC electric motors are more reliable and efficient than DC electric motors.

Currently in the world, alternating electrical energy is distributed in two frequencies, 50 Hz (Europe, Asia, Africa) and 60 Hz (America, part of Japan) and different voltages.

War of the currents

The war of the currents was a series of events that surrounded the struggle motivated by the introduction of electric power transmission systems in the United States. It happened between the end of the 1880s and the beginning of the 1890s.

The struggle was motivated by the expectation of the enormous benefits that large companies hoped to derive from the rapid growth of the electricity supply business.

The DC direct current systems of the Edison Company and AC alternating current systems of Westinghouse Electric, (whose owner was George Westinghouse) with their respective advantages and disadvantages, became the protagonists of the confrontation between companies.

Thomas Edison was on the side of the DC advocates. On the other hand, the engineer Nikola Tesla was in favor of alternating current.

Despite Edison's popularity and his discoveries and inventions, it was Tesla's alternating current that won the battle.


Published: August 9, 2016
Last review: April 18, 2020