Long and firmly entered into our life terminologyelectrical engineering from the time of the first experiments with electricity. But now you somehow even become a dead end, thinking about the times when, as they say, "there was nothing." No person could have enlightened, for example, what a phase tension is, because there was not such a term. But everything is flowing, everything is changing, and now a dozen or two different options for stress are not surprising. The first application of three-phase voltage, from where everything went, belongs to the second half of the 19th century. It all began with the 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 the metal rod. Further more. If next to put another coil, turn off the current at the first and turn on the second - the rod will again be attracted, but already to the second coil. Miracles! The rod moved in space only by alternately applying current to the coils! That's so easy and, as they say, without leaving the room, in those distant times epochal discoveries were made.
If we continue the experiment, even if onlymentally, put 100 coils and alternately turn them on and off, it is clear that the rod will move from the first to the last. Change the configuration of the location of the coils - put them in a circle, and instead of the rod, fix the arrow on the axis in the center of the circle. And again, turn on and off 100 times: the arrow turned in a circle and took the starting position. What did we get? We have obtained a primitive model of a rotating magnetic field. For this purpose, each coil was in turn given an impulse voltage ("turned on / off"). Here it is very important that the next impulse was supplied with a time shift with respect to the previous one, and the current amplitude changed from zero to the maximum value - turned on, and again to zero - turned off. In principle, the process of increasing and decreasing current in each coil was the same. Such a process that is repeated in time is called cyclic, and the period of time from one maximum to the next is called a cycle.
If during the cycle the current in the coil changes andmagnitude, and direction, then it is called a variable. The 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 switched on separately from the rest (from its voltage source), and the voltage supply cycles differed only in the on-time or, as the electricians say, phase. Thus, the difference in voltage is only in the fact that they have a different moment of turning on the current in the coil, or a different phase of the voltage. Strictly speaking, the 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 supplies are connected at one end to a common point, and the second ends of each are connected to coils. It has happened historically that these feeding conductors are also called phases. In engineering, systems with a sinusoidal voltage and a phase shift of 120 ° have the greatest application, i.e. on time this is a third of the period of the operating frequency. The main technical effect of using three-phase systems is the production of a rotating magnetic field.
To compare the energy characteristicsThe voltage of various types is taken as the reference voltage of the direct current. In order to determine the equivalent voltage for a constant voltage, proceed as follows: the sources of the respective voltages are loaded by the same consumers and the adjustment of one of the voltages achieves the allocation 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 as the reference, has an amplitude of 1.42 times the constant. In practice, this is the voltage value indicated by the instruments, and it is called the operating voltage of the alternating current.
In everyday life we use industrial three-phase electrical networks, which are said to be "3 by 380 volts". In such networks, the phase voltage is 220V, and its amplitude value is 308V.