November 13, 2018

During our presentations for the Florida Engineering Society's CEU program this fall, there was one question that was followed by a lively discussion.

*The question: What is meant by Line Quantityversus Phase Quantity?*

The quantity is for the voltage and current in both the supply and load sides for the 2 connections of a balanced power distribution system. These 2 types of connections are** Star** and

This seems like a simple question and distinction, but it does cause confusion. As a basic concept of electrical engineering, it is worth continuing the discussion.

To generate a 3 phase voltage power distribution system, the windings are placed at an electrical angle of 120 degrees from each other. The wave forms in each of the 3 windings (A, B,and C) are shown in *Fig. 1.*

These windings are in the stator with the poles shown in the rotor. The rotor is rotating at a synchronous speed N_{s} (rpm (revolutions per minute)). To obtain the frequency (f), the following formula is applied:

f = (p x N_{s}) / 120

where f is frequency and

where p is the number of poles (p = 2)

The 3 phase power distribution system can be connected in either * Star* or

The 3 voltages swing up and down similarly to each other, but one after another in a rhythm.

At the receiving end of the electrical load, these 3 lines can be joined together in 2 different ways: (1) a * Star* (also called a Y connection) or (2) a

The neutral point is neutral in the sense that its voltage does not fluctuate like the * Phase Voltage*. In order to make the neutral point more stable, we can safely connect this point to the ground. Once we have a 3 line terminal and a neutral terminal, we can have 2 different kinds of voltage.

A higher voltage between any 2 lines is called * Line Voltage* or

Just as there is **Line Voltage** and **Phase Voltage**, there is also * Line Current *and

In the case of the **Delta** connection, the **Line Voltage** and **Phase Voltage** are the same, but the **Line Current** is √ 3 times the **Phase Current**.

In the case of the **Star Connection**, the **Line Voltage** is √ 3 times the **Phase Voltage**, but the **Line** and **Phase Currents** are the same.

Whether a **Star **or **Delta Connection** is used, the same formula (below) is applied to a 3 phase circuit:

P = √ 3 x line voltage x line current times cos ø (power factor).

where P is Power

In our next article, we will discuss the different electrical service configurations in the United States.

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