When I first encountered the terms "phase voltage," "line voltage," "phase current," and "line current," the phrase "é‘é…éé‰" (chÄ« mèi wÇŽng liÇŽng) suddenly popped into my mind. What do these four words mean?
I remember first coming across "é‘é…éé‰" during a high school Chinese exam. At that time, I was confused: what exactly are these four ghosts? After graduating, I gradually forgot about them… until I came into contact with phase and line voltages and currents again. Suddenly, those old memories resurfaced, bringing back the 10,000 points of pain from my past exams. If you don’t believe me, take a look at the language test question in Figure 1.
To heal the emotional wounds from that test, I decided to clarify what phase voltage, line voltage, phase current, and line current really mean. This is my way of resolving long-standing doubts and sharing the knowledge with you.
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 diagram, the stator core is fixed, and three identical coils—A, B, and C—are embedded within it. The magnetic poles inside rotate due to the prime mover (like a steam or water turbine), and as they turn, they cut through the magnetic field lines, inducing electricity in the A, B, and C 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 (also known as a hot wire). Then, we connect the ends of the A, B, and C coils together to form a common point. This line is called the neutral wire (denoted by N), and it's usually connected to ground. The grounded point is referred to as the zero point, and the wire itself is called the zero line. The voltage between the neutral line and ground is essentially zero.
When connecting a three-phase load to the power system, there are two common connection methods: star (Y) connection and delta (Δ) connection, as shown in Figures 3 and 4.
**Phase voltage** refers to the voltage between a live wire and the neutral wire. In a star connection, this is the phase voltage. In a delta connection, there is no neutral, so there is no phase voltage.
**Line voltage** is the voltage between two live wires. It applies to both star and delta connections.
**Phase current** is the current flowing through each load. In both star and delta configurations, the current represented by the red arrows is the phase current.
**Line current** is the current flowing through the power supply’s output lines. In both configurations, the blue arrows represent the line current.
After this explanation, I hope you now have a clearer understanding of phase voltage, line voltage, phase current, and line current. But here's another question: what do these terms actually mean for us in real-life applications?
In systems without a neutral line, phase voltage is typically not considered meaningful because we can't determine the true neutral point. Most testing instruments calculate total three-phase power based on line voltage and line current. As shown in the image below, only the total power value is meaningful, while other values are intermediate variables.
This means that single-phase power in a three-phase system has no practical significance. In the example above, only “17.2914K†represents actual total power. The other values like “8.73475K,†“8.55673K,†and “0.30716K†are just intermediate calculations and don’t reflect the power of any individual phase.
**In summary**, phase voltage, line voltage, phase current, and line current may sound similar, but their physical meanings are distinct. Understanding them clearly helps avoid confusion. When conducting tests, it's important to know which system you're working with and which parameters are relevant. Finally, using a power analyzer ensures accurate measurements.
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