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# How to Apply Single-phase Power Supply to Three-phase Power System?

New in January 2020

The form of AC power can be categorized as single-phase, two-phase and three-phase. Compared to single-phase AC power, three-phase power is relatively unfamiliar to the public since it is mainly used in industrial applications, while the single-phase AC power is more prevalent due to its wide application in our daily lives.

As shown in Figure 1, the waveform of a single-phase AC power source is a sine wave. The frequency is mostly 50Hz or 60Hz. Single-phase AC power is generally used in low-voltage (voltage below 600V) power systems, such as household application and lighting. According to the connection method with the load, there are two types of single-phase connection, single-phase two-wire (1Φ2W) and single-phase three-wire (1Φ3W).

The three-phase AC power supply system is composed of three alternating currents of the same frequency, equal voltage amplitude, but with a 120° phase difference in between, as shown in Figure 2. Generated by a three-phase generator with three windings, the three-phase AC power supply system is commonly used in industrial applications. To ensure the stability of the generator operation, the generator requires at least three windings. Theoretically, the number of phases can be higher, but three phases are the most economical solution. Therefore, the three-phase power system is generally applied in most countries.

There are two ways to connect the windings of the three-phase system, star (Y) and delta (∆). The star connection is to connect one end of the three windings to the common point N, and the other end to the load, as shown in Figure 3. The common point N is called the neutral point. The lead from the neutral point is connected to the load, and there is a total of four wires connected to the load end, called a three-phase four-wire system (3Φ4W). As shown in Figure 4, there is no lead at the neutral point, called a three-phase three-wire system (3Φ3W). Generally, the neutral point is grounded. The ground is used to replace the neutral line.

The advantage lies in that the protective equipment can be added to increase safety. The voltage and current relationship between the phase and the line of the star connection is as follow.

a Line current IL = phase current IP
b Line voltage VL = √3phase voltage VP

The delta connection is to connect the two ends of the three windings to the ends of the opposite ends of the load, forming a closed-loop, as shown in Figure 5. As there is no neutral wiring, the wiring method for delta connection is only three-phase three-wire (3Φ3W). The relationship between phase and line voltage and current is:

a Line voltage VL = phase voltage VP
b line current IL = √3 phase current IP 