Three-phase AC power supply is a power supply composed of three AC potentials with the same frequency, equal amplitude, and phase difference of 120°. Three-phase AC power supply is widely used in daily life and industry.

In daily life, the single-phase AC power we use is actually provided by one phase of three-phase AC power. Three-phase AC power supply has many advantages, such as low manufacturing cost, material saving, simple structure, and excellent performance. In addition, three-phase AC power has obvious advantages in power generation, power transmission and distribution, and conversion of electrical energy into mechanical energy. For example, the capacity of a three-phase motor made of the same material is 50% larger than that of a single-phase motor. Under the condition of transmitting the same power, a three-phase transmission line can save 25% of non-ferrous metals compared with a single-phase transmission line, and the power loss is less than that of a single-phase transmission line.

There are two common connection methods for three-phase AC power supply: star connection (Y-connection) and triangle connection (△-connection).

• Star connection (Y-connection): In a star connection, each end of the three power sources or loads are connected together to form a common point, usually called a neutral point. Then, a line is drawn from the other end of each power supply, and these three lines are the three phase lines of the three-phase power supply. Star connection is often used for low-power equipment and household electricity. In star connection, the relationship between line voltage and phase voltage is the square root of 3 times, that is, U = √3 * U phase. The relationship between line current and phase current is equal.
• Delta connection (△ connection): In a triangle connection, the head end of each power supply is connected to the tail end of the next power supply to form a closed triangle. Delta connection is usually used in situations where high voltage is required, such as motors. In a triangle connection, the line voltage is equal to the phase voltage, that is, U = U phase. The line current is the square root of 3 times the phase current, that is, I = √3 * I phase.

These two connection methods have their own advantages and disadvantages, and which method to use needs to be selected according to actual conditions and needs. When choosing a connection method, factors such as the voltage, current, and power of the power supply, as well as factors such as the needs and safety of the equipment, need to be considered.