Q: What is the difference between the general three-phase four-wire system and the three-phase three-wire system?

A: Generally, there are only three-phase four-wire system in practical application (not counting ground wire, which is called three-phase five-wire system), and three-phase three-wire system will not be used. In the three-phase four-wire system, in addition to the three live wires, the fourth wire is called the neutral wire (note: the single-phase only has the neutral wire, but the three-phase wire does not). If the loads connected to each of the three-phase circuits are the same, and the sum of the currents flowing through the neutral line at each moment is zero, it is possible to remove the neutral line and use a three-phase three-wire system for power supply. But this is an ideal situation. In fact, the three-phase load formed by multiple single-phase loads connected to a three-phase circuit cannot be completely symmetrical. In this case, the midline is particularly important, not optional. If there is no neutral line in the case of asymmetrical load, a three-phase three-wire power supply with asymmetrical load is formed. Due to the asymmetry of the load impedance, the phase current is also asymmetric, and the load phase voltage is naturally not symmetrical. Some phase voltages may exceed the rated voltage of the load, and the load may be damaged (the bulb of the phase is too bright and burnt out); some phase voltages may be lower, and the load cannot work normally (the bulb is dim). The load impedance of each phase changes with the lights on and off. The phase current and phase voltage will change accordingly, the lights will flicker, and other electrical appliances will not work properly or even be damaged. Therefore, there must be a neutral line, that is, a three-phase four-wire system.