Understanding Angles: Vab, Vbc & Vca

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Discussion Overview

The discussion revolves around understanding the angles Vab, Vbc, and Vca in the context of three-phase power systems. Participants are exploring how to differentiate these angles and their reference points, particularly focusing on the assignment of angles in phasor diagrams and practical applications.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • Some participants inquire about how to determine the angles Vab, Vbc, and Vca from a given diagram, noting that the reference angle is always 0 degrees.
  • One participant suggests looking up "Three phase power" to understand the angular relationships between supply leads.
  • Another participant asserts that while the angles (0, 120, -120 degrees) are fixed, their assignment to specific leads is arbitrary, depending on the context provided by the phasor diagram.
  • It is mentioned that practical systems have standards that define wiring colors associated with specific angles.
  • Participants note that in worked examples, phase angles are provided, but subsequent questions may not specify these angles, leading to confusion about their assignment.
  • One participant expresses concern about the lack of clarity on how angles are chosen, emphasizing its importance for accurate answers.
  • A suggestion is made to post specific questions to clarify the issues being faced, as general suggestions may not address particular concerns.

Areas of Agreement / Disagreement

Participants express uncertainty regarding the assignment of angles and how to approach problems without clear definitions. There is no consensus on a definitive method for determining the angles Vab, Vbc, and Vca, indicating multiple competing views on the topic.

Contextual Notes

Participants highlight that the assignment of angles can affect overall answers, and there is a reliance on the context provided by specific examples or diagrams. The discussion reflects a need for clarity in the definitions and assignments of angles in three-phase systems.

DunceKirchhoff
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http://i60.tinypic.com/nmntpj.jpg

How do you differ what angle for example Vab, Vbc, and Vca are?

I notice the reference angle is always 0 degrees but how can you tell for the other two?

Thanks in advance.
 
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DunceKirchhoff said:
http://i60.tinypic.com/nmntpj.jpg

How do you differ what angle for example Vab, Vbc, and Vca are?

I notice the reference angle is always 0 degrees but how can you tell for the other two?

Thanks in advance.

Look up "Three phase power". The power supply is designed to deliver voltages with a specific angular relationship between the supply leads.
 
Yes but just by looking at the diagram you must be able to tell which is which?

Using Vab as a reference, the reference is always 0 degrees.

So if Vxy is used as a reference in another question that will also be 0 degrees.

How do I find out the other two?
 
The angles used (0, 120, -120 degrees) are fixed, but their assignment to particular leads is arbitrary in theory. It looks as though they've made the assignment for you by providing the phasor diagram and formulas. You take them as given.

In practical systems, such as the power grids of countries or industrial generators, there are standards in place that define the wiring colors associated with the particular angles.
 
The phase angles are given here because it is a worked example. Questions there after do not give you these angles, instead they say for example use Vab for reference.
 
DunceKirchhoff said:
The phase angles are given here because it is a worked example. Questions there after do not give you these angles, instead they say for example use Vab for reference.

Then feel free to assign the angles as you please to the otherwise undefined leads. Or take the worked example as a motivation to assign them in the same way. Usually the only important criteria is that the each of the required angles is represented.
 
I can't see any indication on how they have chosen which are angles which. Its important to know as it obviously effects the overall answers.
 
DunceKirchhoff said:
I can't see any indication on how they have chosen which are angles which. Its important to know as it obviously effects the overall answers.

Why don't you post one of the questions that's giving you problems so we can see the issue? We can only give general suggestions to general concerns... a specific example might shed some light.
 

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