Three-phase four-wire
In low-voltage distribution network, transmission lines generally use three-phase four-wire, of which three lines represent A, B, and C phases respectively, and are not split, and the other is the neutral line N (different from the zero line, in the single-phase transmission line entering the user, there are two lines, one of which is called the live line and the other is called the zero line. Under normal circumstances, the zero line must pass current to form a loop of current in the single-phase line, while in the three-phase system, the three phases form a loop by themselves, and under normal circumstances, the neutral line is without current), so it is called a three-phase four-wire system; in the 380V low-voltage distribution network, in order to obtain 220V line voltage from the 380V phase-to-phase voltage, the N line is set, and in some occasions it can also be used for zero-sequence current detection, so as to monitor the balance of three-phase power supply.
The armature of the three-phase AC motor has three sets of coils, and there are two types of connection: star connection and triangle connection. Generally, the star connection is used.
Star connection method
When a three-phase AC generator supplies power to the outside, the ends X, Y, and Z of the three sets of coils are connected together, and a line is drawn from the connection point. This line is called the zero line, also called the neutral line. Then a line is drawn from each of the other ends A, B, and C of the coil winding. These three lines are called phase lines or live lines. This connection method is called star connection.
This method of transmitting power to the outside of the generator constitutes a three-phase four-wire system. If the neutral line is not drawn, the power supply is supplied to the outside with three lines, which is called a three-phase three-wire system.
Because the three-phase four-wire power supply can simultaneously supply two different voltages of 220V and 380v, it is widely used. The star connection is represented by Y, also called the Y connection.
When the star connection is used. What is the relationship between line voltage and phase voltage?
When the star connection is used, the relationship between line voltage and phase voltage is: U line = U phase ≈ 1.732U phase
How does three-phase alternating current generate a rotating magnetic field?
When a sinusoidal alternating current flows through each phase stator winding of a three-phase asynchronous motor, each phase stator winding generates a pulsating magnetic field.Since the spatial positions of the three-phase windings in the core are 120° electrical angle spatial phase apart, three-phase alternating currents flow through the windings respectively, and the currents of each phase are 120° apart in time, so that the total magnetic field synthesized by the three pulsating magnetic fields generated by them at the same time in space becomes a rotating magnetic field.
The three-phase five-wire system refers to A, B, C, N and PE lines, among which the PE line is the protective ground line, also called the safety line, which is specially used to connect to equipment casings to ensure electrical safety. The PE line is connected to the N line on the power transformer side, but it cannot be used as a neutral line after entering the user side, otherwise, it will be the same as the three-phase four-wire system after confusion occurs. However, since this confusion can easily make people lose their vigilance, electric shock accidents may be more likely to occur in practice. Now, it is stipulated that the power supply of residential buildings must use a three-phase five-wire system. If yours is not, you can request rectification. For safety, you must make a firm request!
Whether it is the N line or the PE line, repeated grounding must be used on the user side to improve reliability. However, repeated grounding is just repeated grounding. It can only be connected together at the grounding point or near the grounding position, but it does not mean that it can be connected together at any position, especially indoors.
It is best to use standard/standard wire colors in the application: yellow for A line, green for B line, red for C line, light blue for N line, and yellow-green for PE line.