When I first encountered the terms "phase voltage," "line voltage," "phase current," and "line current," the phrase "é‘é…éé‰" (which translates to "ghosts and monsters") suddenly echoed in my mind. What were these four mysterious terms? I had come across them during a high school Chinese language test, but at that time, I didn’t really understand their meaning. After graduation, I forgot about them—until I met those electrical terms again. The memory came back with a vengeance, bringing back the 10,000 points of pain from that old test. If you don’t believe me, just take a look at the language question shown in Figure 1.
To heal the emotional damage from that painful exam, I decided to dig deeper into what phase voltage, line voltage, phase current, and line current actually mean. This is my attempt to explain it clearly so that others can understand as well.
Electricity starts from the generator. Most power plants use three-phase generators. The following figure shows a cutaway view of a synchronous three-phase generator:
In this setup, the stator core is fixed, and three identical coils (A, B, and C) are embedded inside. When the magnetic poles rotate (driven by a prime mover like a steam or water turbine), they cut through the magnetic field lines, inducing electricity in the coils.
Most low-voltage power systems use a three-phase, four-wire supply. From each of the A, B, and C coils, we draw a live wire. These are the "hot wires" we commonly refer to. Then, the ends of the coils are connected together, forming a common point known as the neutral line (denoted as N). This line is usually grounded, and the point where it connects to the earth is called the "zero point." The neutral line is also referred to as the "zero line," and its voltage relative to ground is zero.
When connecting a three-phase load to the power system, there are two main connection methods: star (Y) connection and delta (Δ) connection, as shown in Figures 3 and 4.
**Phase Voltage**: The voltage between a live wire and the neutral line is called the phase voltage. For example, in a star connection, the voltage between any one coil and the neutral is the phase voltage. In a delta connection, there is no phase voltage because there is no neutral point.
**Line Voltage**: The voltage between two live wires is called the line voltage. It exists in both star and delta connections.
**Phase Current**: The current flowing through each individual load is called the phase current. For instance, the red arrows in the diagrams represent the phase current in both star and delta configurations.
**Line Current**: The current flowing through the output lines of the power supply is called the line current. The blue arrows in the diagrams indicate the line current.
After understanding these definitions, it becomes clear how these terms relate to real-world electrical systems. But then comes the big question: How do these concepts affect us in practical situations?
In systems without a neutral line, the concept of phase voltage doesn't apply, since we can't determine the true "neutral point." Most testing instruments calculate total power based on line voltage and line current, not phase values. As shown in the diagram below, only the total power (17.2914K) is meaningful, while the other values (8.73475K, 8.55673K, and 0.30716K) are intermediate variables with no real-world significance.
In summary, phase voltage, line voltage, phase current, and line current may seem confusing at first, but they become much clearer once you understand their physical meanings. In real-world testing, it's important to know which system you're working with and which parameters are relevant. Using a power analyzer can help ensure accurate measurements.
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