Electric current

What is an alternating current? Definition and advantages

What is an alternating current? Definition and advantages

Alternating current (AC) is the flow of an electric charge that periodically reverses its direction. The variation can be in current intensity or direction at regular intervals.

The AC voltage varies between the maximum and minimum values cyclically. The voltage is positive half the time and negative the other half. It means that half the time, the current flows in one direction and the other half in the opposite direction.

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

In contrast, direct current (DC) maintains its voltage value constant and without polarity change.

Any current-carrying conductor can produce a magnetic field around it. Therefore, both AC and DC are helpful to create electromagnets.

AC peak voltage is the maximum or peak voltage the source can or will achieve. Peak voltage, which we designate as VP, is measured from the horizontal axis (at 0 reference height) to the top of the waveform.

What are the advantages of alternating current?

The use of this type of electricity has the following advantages if we compare it with direct current:

  1. It is possible to increase or decrease the power supplies voltage using transformers.

  2. It facilitates the transport of electrical electricity with a minor loss of energy.

  3. It is possible to convert it into direct current with ease.

  4. By increasing its frequency electronically, voice, image, sound, and control commands can be transmitted over long distances wirelessly.

  5. Electric motors and generators using AC power are more straightforward to maintain than those using DC power.

Frequencies of an alternating current

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

The sinusoidal curve represents the variation of the AC voltage. The frequency of the same current is the number of turns or cycles that the radius of the trigonometric circle gives in a unit of time.

What are the usual frequencies in alternating current?

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

Most electrical power is generated at 50 or 60 Hertz. Moreover, some countries have a mix of 50 Hz and 60 Hz supplies.

A low frequency facilitates the design of electric motors, especially those that require a high speed of rotation. 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. Using lower frequencies also reduces impedance losses.

Effects of high frequencies

A direct current flows uniformly through the cross-section of a uniform cable. However, the electrical charge is forced away from the cable’s center towards its outer surface in an alternating current of any frequency. It happens 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 line within a shallow thickness of the crust.

The depth of the crust is the thickness to which the current density is reduced by 63%. Thus, 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 to save in mass and cost.

Since current tends to flow at the periphery of the conductors, the effective cross-section of the conductor is reduced. It 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 more significant loss of energy due to ohmic heating.

When an alternating current is passing through a coil, it generates a magnetic field. The AC current is a time-varying current, and it is often a sine wave. Thus, the magnetic is also time-varying. There are several techniques for generating a high-frequency magnetic field.

Electricity distribution

In Europe, the power distribution is made in 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 through the power lines is very efficient if it is carried out at high voltages. High voltages are achieved quite easily with the use of transformers.

  • Alternators are more straightforward: 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 through 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 into appropriate frequency, waveform, and voltage parameters.

How to convert alternating current to direct current

Alternating current can be easily switched to direct current using a rectifier. On the other way round, it is much more challenging. It is precisely the reason for the widespread use of this type of electric current.

The electrical energy is given by the product of the tension, the intensity, and the time. Since the section of the conductors of the electric lines depends on the amperage, we can raise the voltage at high values. As the voltage increases, the current intensity decreases.

The great advantage of distributing current at a higher voltage is that we can distribute power over long distances at low currents through transmission lines from power plants. Thus, it reduces energy losses.

Publication Date: August 9, 2016
Last Revision: July 15, 2021