The terminology of electrical engineering from the time of the first experiments with electricity has long and firmly entered our lives. But now you are somehow even getting into a dead end, thinking about the times when, as they say, "there was nothing." Not a single person could enlighten, for example, about what phase voltage is, because such a term has not yet existed. But everything flows, everything changes, and now a dozen or two different variants of stresses do not surprise anyone. The first application of three-phase voltage, where it all came from, refers to the second half of the 19th century. It all started with a study of the phenomenon of electromagnetic induction. The essence of the next experiment was simple - an electric current is passed through the coil and it attracts a metal rod. Further more. If you put another coil nearby, turn off the current on the first and turn on the second - the rod will again be attracted, but to the second coil. Wonders! The rod moved in space only by alternately supplying current to the coils! So easy and, as they say, without leaving the room, in those distant times, epoch-making discoveries were made.
If we continue the experiment, even if mentally, we put 100 coils and turn them on and off one by one, we can see that the rod will move from the first to the last. Change the configuration of the arrangement of coils - put them in a circle, and instead of the rod we fix the arrow on the axis in the center of the circle. And on and off again 100 times: the arrow turned in a circle and took its original position. What did we get? We got a primitive model of a rotating magnetic field. To do this, a pulse voltage (“on-off”) was applied to each coil in turn. It is very important here that the next pulse was supplied with a time shift in relation to the previous one, and the amplitude of the current changed from zero to the maximum value - on, and again to zero - off. In principle, the rise and fall of the current in each coil was the same. Such a process repeating in time is called cyclic, and the period of time from one maximum to the next is called a cycle.
If during a cycle the current in the coil changes both magnitude and direction, then it is called variable. AC voltage is characterized by the following parameters: amplitude - the maximum value of current or voltage, and frequency - the number of oscillations per second. Returning to the experiment with the coils described above, we note that each of them was turned on separately from the others (from its voltage source), and the voltage supply cycles differed only in turn-on time or, as electricians say, in phase. Thus, the difference between the voltages is only that they have a different moment of turning on the current in the coil, or a different voltage phase. Strictly speaking, phase is a synonym for the word “delay”. It should be borne in mind that the concept of "phase voltage" refers only to multi-wire systems in which the power sources at one end are connected to a common point, and the second ends of each of them are connected to the coils. It so happened historically that these supply conductors are also called phases. In technology, systems with a sinusoidal voltage shape and a phase shift of 120 °, i.e. in time it is a third of the period of the operating frequency. The main technical effect of the use of three-phase systems is the production of a rotating magnetic field.
To compare the energy characteristics of various types of voltages, DC voltage is taken as a reference . In order to determine the equivalent of alternating voltage to constant, proceed as follows: the sources of the corresponding voltages are loaded by the same consumers and by adjusting one of the voltages they achieve the release of the same amount of energy in both loads. It has been experimentally proved that the phase voltage at which the energy release in the load is the same with the standard has an amplitude of 1.42 times greater than constant. In practice, it is this voltage value that the devices show, and it is called the acting AC voltage.
In everyday life, we use industrial three-phase electric networks, which are referred to as “3 by 380 Volts”. In such networks, the phase voltage is 220V, and its amplitude value is 308V.