How to Distinguish Between Phase Voltage and Line Voltage

What is the phase voltage? What is the line voltage? What are the relations and differences between them? This paper will interpret them one by one.

Triangular connection and star connection without leading out neutral point have three terminal wires, three-phase system, three live wires, 380V, line voltage. The star connection leading out the neutral line includes three live lines, a total of four power lines, three-phase four wire system, one fire, one zero, 220V phase voltage. The line voltage is the vector sum of the phase voltages of two phases. Line voltage = 2 * phase voltage * cos30 degrees = root number 3 * phase voltage, i.e. 380 = root number 3 * 220

Many times, the phase voltage is generally understood as the voltage between phases, that is, 380V, while the line voltage is understood as 220V, which is just the opposite.


Phase voltage is what we often call 220V household voltage. The voltage between zero line and live line becomes phase voltage. In fact, all in our home are phase voltages.

Line voltage refers to the voltage of any two live lines, that is, 380V, which is actually the phase voltage. The phase voltage is 220V and the line voltage is 380V.

Relationship differences:

In the process of long-distance power transmission, the higher the voltage, the lower the loss, which is why the country has been studying ultra-high voltage power transmission (I = u / R). Generally, the high-voltage power is transmitted to the end user, and then reduced to 220V / 380V through the step-down transformer to supply different users.

For the three-phase four wire power network, the voltage between any one of the three phase lines and the zero line becomes the phase voltage; The voltage between any two of the three phase lines becomes the line voltage. The phase difference of the three-phase voltage is 120 degrees. The line voltage is the vector sum of the phase voltages of the two phases. The relationship between the line voltage and the phase voltage is: line voltage = 3 times the phase voltage of the root number, that is, 380V = 3 times the 220V of the root number (sorry, the mathematical root number 3 of the formula cannot be uploaded in the headline).

The two-phase electricity is connected to the running three-phase motor, and the motor can still work. This is what we often say that the motor runs without phase, but it is easy to burn the motor in this way. After the three-phase motor with triangular connection is out of phase, the output power of the motor will also become two-thirds of the original three-phase power supply, and the other two windings only provide one-third of the power, resulting in three-phase imbalance, current imbalance and current imbalance. Of course, the motor is easier to burn out.

After the star connected three-phase motor is out of phase, the motor output power is completely provided by two windings, and the load increases by 50% respectively. The other winding does not work, so the working current is unbalanced and the motor is easy to burn out. When two-phase electricity is connected to the stopped three-phase motor, the motor cannot be started. For single-phase motor, after connecting two 380V phase lines, the voltage becomes 380V, originally 220V. In this way, the voltage increases and the motor must be burned out.

How to Distinguish Between Phase Voltage and Line Voltage 1

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