Three Phase Power Simplified 

By on

A single phase system is perhaps the most common type of system most people are familiar with. This is what people have in their homes and what appliances are plugged in to. For larger amounts of power, three phase systems are used.

Electricity is generated by a coil of wire moving through a magnetic field. The illustration shows three such coils in an electrical generator , spaced evenly apart. Each coil is called a phase and as there are three coils, this is called a three phase system.

ThreePhasePower

From a such a system, power can be supplied as single phase (load connected between a line and neutral) or three phase (load connected between all three lines).   In the illustration, the motor is connected as a three phase load and the socket outlets and lamp as single phase loads.

Terminology

The three winding end connected together at the centre are is called the neutral (denoted as 'N'). The other ends are called the line end (denoted as 'L1', 'L2' and 'L3').

The voltage between two lines (for example 'L1' and 'L2') is called the line to line (or phase to phase) voltage. The voltage across each winding (for example between 'L1' and 'N' is called the line to neutral (or phase voltage).

Voltage Relationship

The line to line voltage is the vector sum of the phase to phase voltage across each winding. This is not the same as the arithmetic sum and is given by the following equation:

 

myElectrical Equation  

Example:

Line to Line Voltage (VLL)

 myElectrical Equation

myElectrical Equation

myElectrical Equation

Line to Neutral Voltage (VLN)

myElectrical Equation

myElectrical Equation

 myElectrical Equation
 

 

Solving Three Phase Problems

 Tip - to solve a three phase problem, convert it to a single phase problem.

In a balanced three phase system
- each phase delivers/uses 1/3 of the total power
- convert three phase problems to single phase

Resources



Steven McFadyen's avatar Steven McFadyen

Steven has over twenty five years experience working on some of the largest construction projects. He has a deep technical understanding of electrical engineering and is keen to share this knowledge. About the author

myElectrical Engineering

comments powered by Disqus

  1. Jetstar's avatar Jetstar says:
    6/7/2012 3:59 PM

    The 4th line down from TERMINOLOGY reads.

    The voltage across each winding (for example between 'L1' and 'L2' is called the line to neutral (or phase voltage).

    Should it not be (for example between 'L1 and 'N'...)?

    Regads...

    The voltage across each winding (for example between 'L1' and 'L2' is called the line to neutral (or phase voltage).

    • Steven's avatar Steven says:
      6/8/2012 9:03 AM

      Jetstar, thanks for spotting that - your right. I've corrected the error.

  2. Matt's avatar Matt says:
    9/18/2012 5:35 AM

    What about 3 phase unbalanced loads?


Comments are closed for this post:
  • have a question or need help, please use our Questions Section
  • spotted an error or have additional info that you think should be in this post, feel free to Contact Us



Restricted Earth Fault Protection

The windings of many medium and small sized transformers are protected by restricted earth fault (REF) systems. The illustration shows the principal of...

LED Replacement Light Bulb

The inventor of the first visible light-emitting diode makes history again this year as it begins to show customers a 40-watt replacement GE Energy Smart...

Voltage Levels to IEC 60038

The standard aims to consolidate AC and traction voltages within the industry and defines the following bands: band 1 - A.C. systems 100 V to 1...

Introduction to Traction Substations

Following on from my post on railway electrification voltages, I thought an introduction to traction substations would be a good idea. Traction substations...

Periodic Electrical Installation Inspection – How Often?

How often installations are inspected is up to the owner of the installation, provided such durations do not exceed any regulatory maximums in force. ...

What is a rectifier transformer?

I've recently come across this question a couple times browsing the internet. Decided to give a quick answer here. A rectifier transformer is a transformer...

Material Properties

Everything physical in electrical engineering from insulations to conductors revolves around materials. Here we are listing common materials along with...

Electrical Engineering

Electrical engineering is a field that covers a wide variety of sub-fields, including electricity and electronics. It is a field that goes back to the...

ABB Technical Guides - Motor Operation

ABB has produced a range of technical guides that offer concise explanations of the major technologies and technical issues in low voltage AC drives. ...

Fault Calculations - Typical Equipment Parameters

A frequent problem in fault calculations is the obtaining of equipment parameters.  While it is always preferable to use the actual parameters of the equipment...

Have some knowledge to share

If you have some expert knowledge or experience, why not consider sharing this with our community.  

By writing an electrical note, you will be educating our users and at the same time promoting your expertise within the engineering community.

To get started and understand our policy, you can read our How to Write an Electrical Note