What do three-phase three wire system and three-phase four wire system mean respectively? What's the difference between them?
       Whether the three-phase three wire system or three-phase four wire system is used depends on the incoming line of the user and the nature of electricity consumption. If the user is a pure three-phase generator, such as three-phase transformer, three-phase motor, etc. Three phase three wire circuit can be used. The three-phase three wire system has only three wires. If there is no zero wire, you can only use a three-phase three wire meter. If the user has single-phase load and three-phase load, it is three-phase four wire system or three-phase five wire system (multi neutral line grounding wire) line. A three-phase four wire meter will be used. The three-phase three wire system line has no adjustment capacity, and the three-phase load is required to be basically balanced.

      What is the difference between three-phase three wire system and three-phase four wire system? What are the differences in application sites?

      Three phase four wire has one more power supply neutral wire than three phase three wire. The three-phase three wire power supply can only supply 380 volts. Three phase four wire power supply can provide both 380 V and 220 V voltage.

      Three phase and three wire, obviously saving money. However, when the load is unbalanced, it is impossible to feed back the current through the zero phase, which is easy to burn things. However, three-phase four wire can solve this problem.

      What are the advantages of three-phase three wire system, three-phase four wire system and three-phase five wire system?

      Three phase three wire system is to use only three phase wires.

      Three phase four wire system means three phase lines plus one zero line.

      Three phase five wire system means three phase lines, one neutral line and one protective grounding wire.

      What is the difference between three-phase three wire motor and three-phase four wire motor (power motor)?

      The three-phase motor is a balanced load, and the phase current is equal to the line current. The vector sum is zero, so there is no need for a zero line. The motor with angle connection has no zero line to connect. The star connection can connect a zero line at the neutral point. But it doesn't make sense. It's the same whether to connect or not.

       In low-voltage power supply system, what is the scope of application of three-phase four wire system compared with three-phase three wire system? What are the advantages?

       The three-phase three wire power supply system is only applicable to three-phase symmetrical load (such as three-phase power transformer, three-phase motor, etc.). If three-phase load is asymmetrical, voltage will appear at neutral point. The three-phase four wire power supply system can obtain line voltage and phase voltage, which is convenient for users. In addition, when the three-phase load is asymmetric, the voltage displacement of the neutral point can also be eliminated due to the small impedance of the neutral line.