Everyone is told about the types of electric current in physics lessons at school. For some, this knowledge remains only theoretical, expanding the understanding of the world order, while others choose a specialty that is directly related to energy. Regardless of this, everyone knows that there is direct and alternating current. In electrical engineering, its second variety is widely used, since it is easier to transform, and electric motors based on it are more simple and reliable.
Alternating current is the movement of charged particles along the conductor, which at certain time intervals (periods) changes in direction and magnitude. The famous sinusoid just illustrates its behavior. To understand where the alternating current comes from, we will consider the principle of operation of the simplest generating device. It is based on the phenomenon of electromagnetic induction, which consists in the appearance of current in a closed conductive circuit that crosses magnetic lines of force.
On the axis, between the two poles of the magnet (north and south), is a frame of conductive material (copper wire). Its ends by means of the sliding contacts of the brush mechanism are connected to the circuit with a load or a voltage measuring device. The frame can rotate around the axis on which it is placed. Between the poles of the magnet there are invisible magnetic lines of field strength. When the frame is rotated, its two sides intersect these lines, resulting in an alternating current. The reason for its occurrence is caused by the “knocking out” of the electrons by the magnetic field from orbits distant from the nucleus. Although the directions of EMF (electromotive force) in two intersecting sections of the frame are always opposite, if you look at the circuit as a whole, it becomes clear that they are unidirectional. EMF reaches its maximum at the moment of vertical intersection of the lines of force and passes through zero when the parts of the frame are horizontal (remember the sine wave, it also periodically crosses the zero value). So simple.
Of course, real alternating current generators look more complicated than a wire frame, but the principle of their operation is the same. An armature with windings (in fact, many frames from the example) rotates an external force in the stator magnetic field: it can be the energy of falling water; the movement of steam created by the heat of a nuclear reactor; wind pressure, etc. As a result, voltage appears on the terminals of the windings. It remains to connect the load and the alternating current will not take long. Usually, not one, but three phases are generated at once.
As already indicated, in addition to alternating current, there is direct current. Its name speaks for itself: no changes in the direction of movement occur. Such a current is always directed from plus to minus. Secondary energy sources, for example batteries, accumulate just such a current through chemical reactions, hence the notation “+” and “-”. Compared to a variable, it has a number of features. I would like to say “advantages and disadvantages”, but this is not so - namely “features”. For example, a direct current electric motor makes it possible to smoothly control the speed of rotation of the armature, rather than stepwise and, moreover, without the need to use complex frequency converters. Moreover, almost all electronic circuits are designed to work with this particular type of current, since it is easier to control. Obtaining direct current from an alternating current is very simple - it must be “rectified” with special semiconductor elements (diodes and diode bridges). There is a cut of the peaks of the sinusoid in two half-periods at once. The remaining pulsating nature of the resulting current can also be smoothed out.