In each branch of technology you can always find a peculiar echo of ancient times, namely names that reflect a kind of history of the development of this direction. And few people know that this or that technical concept has a long way of becoming, getting used to, and at the very beginning of its birth it marked the next, often very significant, step of technological progress. So, for example, among electrical terms, one can often hear the expressions “three-phase voltage”, “line voltage”, “constant” or “alternating voltage” and many other names with the word “voltage”.
Initially, as a physical quantity, the voltage is defined as the potential difference of the electric field, capable of performing the work of moving the electric charge from one point of the field to another. The energy of the field is spent on moving the charge, so its value, more precisely, the potential difference, decreases to zero. In a real closed circuit, the work of moving electric charges is treated as an electric current - the result of the movement of electrons from one point of the circuit to another. So that it does not change, it is necessary to maintain the potential difference unchanged. As you know, maintaining the current in the circuit "knows" the power source. It depends on it whether the current in the circuit is constant, i.e. not changing its size and direction, or variables changing according to some law. The term "line voltage" only makes sense for AC networks .
The most widely used in electrical engineering are ac sinusoidal networks. The maximum value of the voltage during its oscillation is called the amplitude Ua. For such a voltage, additional units of measure are used - the frequency F and the phase ψ. The frequency is determined by the number of oscillations per unit time, and the phase is the time shift of the same oscillation points. It so happened historically that the term "phase" was also called the AC power line, if it is part of a system of many phases - usually three. Three-phase networks were another achievement of electrical engineering and have so many advantages that it is simply impossible to pass by. And the most important of them is the ability to extremely easily, practically without any effort, to receive a rotating magnetic field - the basic principle of operation of any electric motor. In a three-phase circuit , phase and linear voltage are distinguished, and its feature is that each of the phases has a shift with respect to the other two +/- 120 degrees. The three-phase voltage generator has output windings in which the phase shift is structurally set. Each of the windings has an end and a beginning: 1-1, 2-2, 3-3. In a three-phase system, two options for connecting the phases are possible - “star” and “triangle”.
When connecting a “star”, all ends are connected to one point - “pin 0”, and the beginnings serve as output ends for the generator and input for the device it feeds. In such a system, the line voltage is the value measured between any pair of output ends H1, H2, H3, and it is denoted by Ulin. There is another characteristic of a three-phase network - phase voltage. It is denoted by Uf and measured between the points "output 0" and any of the output ends K1, K2 and K3. Omitting the details, it should be noted that, based on the vector diagram for a three-phase network, the relationship between these voltages is Ulin = Ѵ3 * Uf. When connecting a "triangle" the ends of the windings are connected in a ring: K1-H1-K2-H2-K3-H3-K1. Each end-to-start connection is a terminal, and the linear voltage does not differ from the phase voltage, i.e. Ulin = Uf. It is interesting to compare the constant voltage Udir and the amplitude of the alternating voltage Ua, for example, based on the same energy released in the load. For this case, Udir = Ѵ2 * Ua.
So, for decades, knowledge about the nature and nature of electricity has been accumulating, and an imperceptibly simple concept of “voltage” has been overgrown with related terms, expanding our capabilities in using natural phenomena for human needs.