About the difference between three-phase power and two-phase power, the difference between three-phase power and single-phase power, the delta connection and Y-connection of three-phase power loads, the three-phase four-wire system and three-phase five-wire of three-phase power meters System, let’s get to know it together.
It is composed of three phase wires.
Mutual voltage: 380V.
Application: Three-phase motor
It consists of two-phase wires.
Mutual voltage: 380V.
Application: AC welding machine
It is composed of a phase wire and a neutral wire.
Application: Household appliances.
Three-phase Power: when the coil rotates in a magnetic field, the wire cuts the magnetic field line to generate an induced electromotive force, and its changing law can be represented by a sinusoidal curve. If we take three coils and place them at an angle of 120 degrees apart in space, the three coils still rotate at the same speed in the magnetic field, and three induced electromotive forces with the same frequency will be induced. Since the three coils are 120 degrees apart in space, the current generated is also a three-phase sinusoidal change, which is called a three-phase sinusoidal alternating current. Industrial electricity uses three-phase electricity, such as three-phase AC motors.
The voltage between any two phases is 380VAC, and any phase-to-ground voltage is 220VAC. Divided into A phase, B phase, and C phase. The lines are represented by L1, L2, and L3.
The power generated by the generator is three-phase, and each phase and its neutral point of the three-phase power supply can form a single-phase loop to provide users with electric energy.
According to regulations, the neutral point of a 380 volt (three-phase) civil power supply should not be grounded at the entrance end. A ground wire is also connected to the three-core power jack.
（Grounding at the transformer end. This grounding is to take into account that the floating point cannot cause a point higher than the power supply voltage. The grounding of the user end and the grounding of the transformer end have a certain resistance in the earth)
This is to take into account the realization of the function of the leakage protector. If the neutral point of the power supply is directly grounded, the leakage protector will lose its function and cannot protect the short circuit of the human body and electrical equipment.
The working principle of the leakage protector is: if the human body touches the line end of the power supply, namely the live wire, or the internal leakage of electrical equipment, the current flows from the live wire to the earth through the human body or the housing of the electrical equipment, and does not flow through the neutral wire, live wire and The current of the neutral line will be unequal, and the leakage protector will immediately trip to protect the safety of personal and electrical appliances after detecting this part of the current difference. Generally, the differential current is selected to be tens of milliamperes。
Divided into delta connection and Y-shaped connection.
The load leads of the delta connection are three live wires and one ground wire. The voltage between the three live wires is 380V, and the voltage of any live wire to the ground wire is 220V;
The load leads of the Y-connection method are three live wires, a neutral wire and a ground wire. The voltage between the three live wires is 380V, and the voltage of any live wire to the neutral wire or the ground wire is 220V.
The total power of a three-phase electrical appliance is equal to the voltage of each phase multiplied by the current of each phase and then multiplied by 3, that is, the total power = current × voltage (220V) × 3 (W = U × I × 3)
In the low-voltage distribution network, the transmission line generally adopts a three-phase four-wire system, of which three lines represent the three phases of A, B, and C, without splitting, and the other is the neutral line N。
In the 380V low-voltage power distribution network, the N line is set up in order to obtain the 220V line-to-line voltage from the 380V phase-to-phase voltage. In some cases, it can also be used for zero-sequence current detection to monitor the three-phase power supply balance
Repeated grounding: Regardless of N wire or PE wire, repeated grounding must be used on the user side to improve reliability. However, repeated grounding is only repeated grounding. It can only be connected together at the grounding point or close to the grounding point, but it does not mean that it can be connected together at any position, especially indoors. This must be remembered, and pay attention to whether your friend has violated it!
It is best to use standard and standardized wire colors in the application: A wire uses yellow, B wire uses blue, C wire uses red, N wire uses brown/blue, and PE wire uses yellow-green.
The three-phase five-wire system refers to the A, B, C, N and PE wires. Among them, the PE wire is a protective ground wire, also called a safety wire, which is specially used to ensure the safety of electricity use, such as the equipment casing. The PE line is connected to the N line at the power supply transformer side, but it must not be used as a neutral line after entering the user side, otherwise, it will be no different from a three-phase four-wire system after confusion.
However, since this kind of chaos makes people lose their vigilance, it may be more prone to electric shock accidents in practice. Now the power supply for residential houses has been stipulated to use a three-phase five-wire system. If yours is not, you can request rectification.
The three-phase five-wire system includes three phase wires (A, B, and C wires), a neutral wire (N wire); and a ground wire (PE wire).
The neutral line (N line) is the neutral line. When the three-phase load is symmetrical, the vector sum of the current flowing into the neutral line of the three-phase line is zero, but for a single-phase, the current is not zero. When the three-phase load is asymmetrical, the current vector sum of the neutral line is not zero, and a voltage to the ground will be generated.
The three-phase five-wire system is divided into TT grounding mode and TN grounding mode, and TN is specifically divided into TN-S, TN-C, and TN-C-S three modes.
The first letter T means the neutral point of the power supply is grounded, and the second T is the grounding of the metal casing of the equipment. This method is commonly used in high-voltage systems, but it is not suitable for low-voltage systems with large-capacity electrical appliances.
1. TN-S grounding method: The letter S means that N is separated from PE, the metal shell of the equipment is connected to PE, and the neutral point of the equipment is connected to N. The advantage is that there is no current in PE, so the potential of the metal casing of the equipment to the ground is zero. Mainly used for data processing, precision detection, and power supply system for high-rise buildings.
2. TN-C grounding method: The letter C means that N and PE merge into PEN, which is actually a four-wire power supply method. Both the neutral point of the device and the metal shell are connected to N. Since the three-phase unbalanced current and harmonic current flow when N is normal, the metal casing of the equipment normally has a certain voltage to the ground, which is usually used in general power supply places.
3. TN-C-S grounding method: part of N is separated from PE, which is a four-wire half system power supply method, which is used in places with poor environment. When N and PE are separated, they are not allowed to be combined.
ELECTRIC, WITH AN ENGE-- DAELIM BELEFIC