Questions about three-phase four-wire system
Thread Content
Three-phase four-wire systems are generally used for industrial power supply; the three wires among the four constitute the three phases, with a voltage of 380V between any two of these wires, while the fourth wire serves as the ground wire. Clearly, two live wires plus one neutral wire are sufficient to provide 380V and 220V voltages; then why add another live wire unnecessarily? ? Is it for insurance purposes? ?
In power systems, the electricity coming from the power source is three-phase. The loads, on the other hand, can be either single-phase or three-phase. However, regardless of the type of load, in order for the system to operate properly, the loads on the three phases should be as balanced as possible. The three-phase four-wire system is a standard configuration, and it isn’t designed for any specific exception cases.
Firstly, the secondary side of 0.4/10kV transformers is usually three-phase five-wire, namely phases A/B/C, neutral N, and ground PE. The phases A/B/C are 120 degrees apart from each other; the voltage between the lines is 380V, while the voltage of each phase relative to the neutral is 220V. It is the 120-degree separation between the phases A/B/C that enables the formation of a rotating magnetic field. If there are only two phases, the phase difference is only 180 degrees, which is not sufficient to create a rotating magnetic field. Similarly, with four phases, the phase difference is only 90 degrees, again insufficient to generate a rotating magnetic field.
Let’s take an example: Question: Why do we have 5 fingers on our hands? When using chopsticks, only 3 fingers are needed; sometimes 4 fingers are used as well. Is the little finger then meant only for wearing divorce rings or for picking out earwax? Answer: People are born with 5 fingers……
In industrial plants and factories, most equipment such as motors requires three-phase alternating current as a power source
What a profound question; it’s so philosophical