long and firmly established in our lives terminology of electrical engineering since the first experiments with electricity.But now, somehow even become a dead end, thinking of the time when, as the saying goes, "Nothing."No one would be able to educate, for example, that is the phase voltage, because that term did not exist.But everything flows, everything changes, and now a dozen or two different variants of the stress does not surprise anyone.The first application of the three-phase voltage, where it all went, belongs to the second half of the 19th century.It all began with a study of the phenomenon of electromagnetic induction.The essence of the next experiment was simple - a coil of an electric current, and it attracts a metal rod.Further more.If put next to another coil, turn off the current to the first and second turn - the rod is attracted again, but the second coil.Miracles!The rod moved in the space only by alternately supplying current to the coil!This is so easy and that is called, without leaving the room, in those days were made epoch-making discovery.
If you continue the experiment, even if only mentally, put 100 coils and turns their turn, turned off, it can be seen that the rod is moved from the first to the last.Change the location of the coil configuration - put them in a circle, instead of the rod will fix the arrow on the axis at the center of the circle.And again, on or off 100 times: the arrow turned around and took the starting position.What have we got?We got a primitive model of the rotating magnetic field.For this purpose, each coil in turn was applied pulse voltage ("on-off").It is very important that the next pulse is fed to the time shift with respect to the previous and current amplitude varies from zero to a maximum value - inclusive, and again down to zero - off.In principle, the process of rise and fall of the current in each coil was the same.This process is repeated over time is called cyclic, and the period of time from one peak to the next - cycle.
If during the cycle of the current in the coil changes and the magnitude and the direction it is called a variable.AC voltage is characterized by the following parameters: amplitude - the maximum current or voltage and frequency - the number of oscillations per second.Returning to the experiment described above, with the coils, note that each of them separately from the rest (from its source voltage), and the cycles of the supply voltage differed only on-time or, as electricians, phase.Thus, only the voltage difference that they have different moment of switching the coil current or different voltage phase.Strictly speaking, the phase - a synonym for the word "delay".It will be appreciated that the term "phase voltage" refers only to the multiwire system in which one end of the power supplies connected to the common point, and the second ends of each of them are connected to the coils.It so happened historically that the supply conductors are also called phases.The technique most widely used system received with a sinusoidal voltage and a phase shift of 120 °, i.e.the time period of one-third of the working frequency.The main technical effect of the three-phase systems - to provide a rotating magnetic field.
comparison of energy characteristics of various types of voltage as a reference take a DC voltage.In order to determine the equivalent AC voltage DC, proceed as follows: appropriate sources of stress loaded the same consumers and regulation of one of the strains sought allocation of the same amount of energy in both loads.Empirically proved that phase the voltage at which the energy release of the load the same with the standard, has a magnitude of 1.42 times the DC.In practice, it shows that the voltage devices, and it is called the current AC voltage.
In everyday life, we use three-phase industrial power networks, which they say "3 to 380 volts."In such networks, phase voltage is 220 V and its peak value - 308B.