Alternating current (AC) is a type of electrical current that is characterized by changing over time, either in current intensity or direction, at regular intervals.
The voltage varies between the maximum and minimum values cyclically, the value of the voltage is positive half the time (positive half cycle or half positive period) and negative the other half. This means that half the time the current flows in one direction, the other half in the other direction. The most common form of undulation follows a sine-type trigonometric function, since it is the most efficient and practical way to produce electrical energy by means of alternators. However, there are certain applications in which other waveforms are used, such as the square wave or the triangular wave.
Frequencies of alternating current
The frequency of the electrical system varies according to the country and, sometimes, within a country; Most electric power is generated at 50 or 60 Hertz. Some countries have a mix of 50 Hz and 60 Hz supplies, especially the transmission of electric power in Japan. A low frequency facilitates the design of electric motors, especially for lifting, crushing and rolling applications, and commutator type traction motors for applications such as railroads.
However, the low frequency also causes a noticeable flicker in the 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 on alternating currents
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 that cancel the propagation of electricity to the center of materials with high conductivity. This phenomenon is called the film effect.
At very high frequencies, the current no longer flows in the cable, but actually flows on the surface of the cable, within a thickness with little depth of the crust. The depth of the crust is the thickness at which the current density is reduced by 63%. Even at relatively low frequencies used for power transmission (50 Hz - 60 Hz), non-uniform distribution of current still occurs in sufficiently thick conductors.
For example, the film depth of a copper conductor is approximately 8.57 mm at 60 Hz, so high current conductors are generally hollow to reduce their mass and cost. Since the 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 area of the cross section. Alternating current resistance is often many times greater than direct current resistance, causing a much greater loss of energy due to ohmic heating.
Electricity distribution by alternating current
In Europe, electricity is distributed in the form of sinusoidal alternating current at a constant frequency of 50 Hz.
The use of alternating current is consecuéncia of the following reasons:
- The transport (especially at long distances) of high electrical powers is very efficient if carried out at high voltages, the latter is quite easily achieved with the use of transformers.
- Alternators are constructively simpler and have greater efficiency than dynamos.
- In direct current it is not possible to exploit the advantages of a three-phase system. Almost all consumer electronic devices operate in direct current, but this can be achieved by alternating current by means of a simple rectifier.
On the other hand, it is possible to obtain alternating electric current from the direct current, generated in appropriate parameters of frequency, waveform and voltage by means of devices called inverters.
History of alternating current
In the industrial origins of the use of electricity in the nineteenth century was used direct current, which offers the advantage of being able to be stored in batteries, but with the arrival of alternating current by the scientist Nikola Tesla the world was revolutionized new. The efficiency of this new type of current that is allowed to drastically reduce energy losses over long distances thanks to the increase in electrical voltage. This feature allowed the high-voltage electric power transmission and the current dissipation losses drastically reducing low in the line and then the thickness of the conductor used for transport, compared to Edison's direct current.
The alternating current was extended thanks to the use of the transformer, which allows to take the difference of potential (voltage) to very high levels (high voltage) and correspondingly the current to very low values, thus maintaining the power unaltered and then transmitting it to a large distances and small losses, achieving considerable economies of scale.
In addition, alternating current electric motors are more reliable and efficient than direct current electric motors.
Currently in the world, alternating electric power is distributed in two frequencies, 50 Hz (Europe, Asia, Africa) and 60 Hz (America, part of Japan) and different voltages (see electrical standards in the world).
Last review: May 17, 2019