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Lesson 1-4 Phase Angle

Phase of a Sinusoidal Waveform
Before starting this module, you should be able to: When you complete this module, you should be able to: 
  • Use the formula for instantaneous sine voltage and current to sketch an accurate sinusoidal waveform.
  • Describe the meaning of phase angle.
  • Expand the formula for instantaneous sine voltage and current to include a phase angle, then apply the formula to sketch accurate sinusoidal waveforms.
  • Define the terms leading and lagging as they apply to sinusoidal waveforms.
  • Determine whether a given waveform is leading or lagging a reference waveform.

 

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Phase Angle 

The phase angle of a waveform is angular difference between two waveforms of the same frequency. 

  • Math symbol: q  (theta) 
  • Unit of measure: degrees or radians
Two waveforms are said to be in phase when they have the same frequency and there is no phase difference between them. 

Two waveforms are said to be out of phase when they have the same frequency and there is some amount of phase shift between them.

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Leading and Lagging Phase Angles 
 
A leading waveform is one that is ahead of a reference waveform of the same frequency. 
 
In this example, the blue waveform is taken as the reference because it begins at 0 degrees on the horizontal axis.

The red waveform is said to be leading because it is already at about 90 degrees when the reference waveform begins at 0 degrees.


A lagging waveform is one that is behind a reference waveform of the same frequency. 
 

In this example, the blue waveform is taken as the reference because it begins at 0 degrees on the horizontal axis.

The red waveform is said to be lagging because it has not yet completed its cycle while the reference waveform is beginning a new one at 0 degrees.

 
Keeping straight whether one waveform is leading or lagging another is commonly a confusing point for students of AC electricity (and no small number of practicing technicians as well). So it pays to keep in mind whatever pictures or gimmicks that are required for helping you specify which of two out-of-phase waveforms is leading and which is lagging.
 

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Author and Content Provider: David L. Heiserman
Publisher: SweetHaven Publishing Services

Copyright 1997, 2003, 2005 David L. Heiserman
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