# Three-phase alternating current and alternating current advantages

①Three-phase alternating current
In power transmission, the AC power output by the power station is high-voltage three-phase power. After the electricity is distributed to cities or towns, it needs to be transformed into low-voltage three-phase or single-phase alternating current. Three-phase electricity is more efficient than single-phase electricity, so it is widely used in industrial and commercial users who use high-power motors and have greater demand for electricity. Most residential communities use single-phase power distribution, because electricity demand is relatively small. Figure 1a and Figure 1b show the waveforms of single-phase electricity and three-phase electricity, respectively. Single-phase electricity returns to the zero position three times in each cycle and changes polarity twice, so its energy becomes zero three times in each cycle. Each phase of the three-phase electricity will also return to the zero position and change polarity, but the three phases will not reach the zero position at the same time, so the energy of the three-phase system will never become zero.

Because three-phase electricity is more efficient, a three-phase motor can be made smaller (smaller core) than a single-phase motor of the same horsepower. Under certain conditions, the wires in a three-phase system can be thinner.
Figure 1b shows the output waveform of a three-phase generator, with an interval of 120° for each phase. The three-phase generator contains three independent coil windings in structure, which separates the three outputs by 120°. The single-phase generator has an output winding. In practical applications, power companies use high-voltage three-phase power generation systems, and then transformers convert the power into single-phase power distribution. Single-phase generators are usually used in independent houses or small commercial occasions that cannot be connected to the grid.

The three-phase transformer is used to increase the voltage to facilitate long-distance transmission, and then lower the voltage for power distribution. The coil windings and line configurations of these transformers vary according to different applications. Figure 2 shows two common three-phase configurations, namely “delta” (△-shaped) and “Y”-shaped configurations. The windings of each phase are respectively identified as A, B and C, and the space layout shall ensure that the phases are separated by 120°. The electrician connects the output of the secondary side to a three-phase or single-phase load.

The phasor diagram (see Figure 2) can show the angular displacement and amplitude of the waveform instead of the pure waveform. The arrows in the figure represent the angles between the amplitudes of voltage and current and each phase. This method is often used as a shorthand method to express important waveform information.

Figure 3 shows a transformer with a “Y”-shaped secondary winding connected to a three-phase 240V load.