What is three-phase alternating current? Presumably most of the friends engaged in the electrical industry are not strange, but for a beginner it may be half understood. In fact, three-phase alternating current is a form of power transmission, referred to as: three-phase electricity. General household electricity is single-phase AC, but the large-scale production and distribution of current and most industrial electricity are in the form of three-phase AC circuits. For example, electric motors use three-phase alternating current, which is often referred to as the three-phase four-wire system. In daily life, more single-phase power supply is used, also known as lighting. When the lighting power supply is used, one of the three phases is used to supply power to electrical equipment, such as household appliances, and the other wire is the fourth wire of the three phases and four wires, that is, the neutral line, which leads from the neutral point of the three phases. So what is the difference between line voltage, phase voltage, phase current and line current? Let’s take a look.

## (1) What is three-phase alternating current?

Three-phase AC is a power system composed of three AC circuits with the same frequency, equal potential amplitude and 120° Angle of phase difference. At present, China’s production and distribution are three-phase alternating current.

Three-phase AC has many advantages over single-phase AC. In terms of electricity consumption, three-phase motor has simple structure, cheap price and good performance than single-phase motor. In terms of power transmission, the three-phase system is adopted to save the amount of copper used in transmission lines compared with single-phase transmission under the same conditions. In fact, the single-phase power supply is to take a phase of the three-phase power supply, therefore, three-phase AC power has been widely used.

To turn a coil in a magnetic field, only an alternating electromotive force is generated in the circuit, and the alternating current emitted at this time is called single-phase alternating current. If there are three mutually angled coils rotating at the same time in the magnetic field, three alternating electromotive forces occur in the circuit, and the alternating current emitted is called three-phase alternating current.

In the AC motor, three identical coils AX, BY, CZ are fixed on the iron core, the beginning end is A, B, C, and the end is X, Y, and Z. The planes of the three coils are at a 120 degree Angle to each other. By turning the core at a constant speed, the three coils rotate at a constant speed in the magnetic field. The three coils are identical, and they emit the same three electromotive forces, maximum values, and frequencies.

Although the maximum value and frequency of these three electromotive forces are the same, their phase is not the same, because the three coil planes are 120 degrees Angle to each other, so the phase difference of the three electromotive forces is 120 degrees.

## (2) What is line voltage? What is phase voltage?

The voltage between any phase line and neutral line in the three live lines is called phase voltage Ua, Ub, Uc. In the low voltage power supply system of our country, the voltage between the three phase lines and neutral line is 220V.

The voltage between the three phase lines is called the line voltage. In a symmetrical three-phase system, the size of the line voltage is 1.73 times that of the phase voltage, and in China’s low-voltage power supply system, the line voltage is 380V.

## (3) What is the difference between line voltage and phase voltage?

A, B, C three phases commonly used in power systems.

The phase voltage is a single voltage, that is, a single voltage to ground, which is generally 220V for civil use.

The line voltage is often said to be the phase voltage, that is, the voltage between every 2 phases, and the power electricity is generally 380V.

In the Y-type connection, the line voltage is equal to 3 times the square root of the phase voltage, and the phase current is equal to the line current.

In the triangular connection, the line voltage is equal to the phase voltage, the phase current is equal to the square root of the line current 3 times, and the power P= square root 3*UI.

## (4) What is the difference between phase flow and line current?

Three-phase four-wire distribution, the difference between phase current and line current, mainly depends on the connection method of the load, if it is a star connection, the phase current and line current are the same, and the line voltage is 3 times the square root of the phase voltage. If the load is triangular, then the line current is 3 times the square root of the phase current, and the phase voltage and line voltage are the same.

In three-phase AC, the relationship between line current and phase current is determined according to the load connection method. In star connection, line current = phase current; In the triangle connection, line current = square root 3 times phase current.Uppercase U represents the line voltage and lowercase u represents the phase voltage

Capital A represents the line current, and lowercase a represents the phase current

The difference:

1) Phase voltage: the voltage of any phase line and the neutral line in the three-phase line. Line voltage: The voltage between lines in a three-phase line. Relation: U phase =U wire /1.732.

2) Phase current: current of phase line and neutral line load. Line current: The current between lines of a three-phase load. Relationship: l phase =P/U phase/power factor, l line =P/1.732/U phase/power factor.

In the three-phase generator star connection, the ends of the three windings are connected together to form a common end – neutral line – neutral line. The power line connected with the three winding ends forms a phase line, also known as the fire line. The voltage between (live line) and neutral line is called phase voltage U1. The current flowing through each phase load in a three-phase power supply is phase current.

3) Line Voltage is the voltage between the two lines of the polyphase power supply system, taking the three phases as an example, the voltage between the A, B, and C three phase lead lines, also known as the phase voltage. The magnitude of the star-connected line voltage is 3 times the square root of the phase voltage. The phase voltage of a triangular power supply is equal to the line voltage.