Drawing 3-Phase Voltage Waveforms

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Discussion Overview

The discussion revolves around the task of drawing and labeling 3-phase voltage waveforms based on given parameters, including specific voltages and frequency. The scope includes homework-related challenges and technical explanations regarding the conversion of voltage readings from the frequency domain to the time domain.

Discussion Character

  • Homework-related
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • One participant expresses uncertainty about how to start the assignment, noting a lack of coverage in lectures.
  • Another participant suggests converting from the frequency domain to the time domain as a potential first step.
  • A third participant outlines the characteristics of classical 3-phase systems, emphasizing the need for sinusoidal voltages that are equal in peak value, frequency, and displaced by 120 degrees.
  • This participant provides example voltages Va, Vb, and Vc to illustrate the phase differences and suggests plotting these waveforms based on their relationships.
  • Another participant points out that the provided information only describes two phases and questions whether the third phase is implied or assumed, highlighting the differences in magnitudes of the voltages.

Areas of Agreement / Disagreement

Participants do not reach a consensus on how to interpret the given voltages and whether the third phase is explicitly defined. There are multiple viewpoints on how to approach the plotting of the waveforms, indicating ongoing uncertainty and exploration of the topic.

Contextual Notes

Participants mention the need to understand phase differences and the relationship between frequency and wavelength, but do not resolve the implications of these factors for the assignment. The discussion reflects varying interpretations of the problem's requirements.

JGrecs
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Homework Statement



Draw and label the 3-phase voltage waveforms where VAN= 347∠0° Vrms, VAB= 600∠30° Vrms, Frequency: 50Hz, Volts/Division 200, Time/Division: 2 ms

2. The attempt at a solution

Im really not sure how to tackle this one. My prof hardly went over this in lecture, but gave this out as an assignment (wtf). Does anyone how to go about starting this?
 
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So for starters do you know how to convert from the frequency domain to the time domain? It might help get you in the right direction if you convert your readings.
 
3 phase systems in the most classical sense are provided by 3 sinusoidal voltage sources with:

(1) the same peak values
(2) which alternate at the same frequency
(3) which are displaced in time by 1/3rd of a cycle or 120 degrees

I would expect that you have three voltages which are out of phase (if you want to do the polar plot with the rotating vectors).

So if you had my three classical voltage waves - let's call them Va Vb Vc
and let's say that they are all oscillating at the same frequency, with a magnitude of 5V, but are out of phase.

then Va = 5<+120
Vb = 5<0
Vc = 5<-120

To convert from frequency to time domains in a really proper way is to move from a Fourier transform to a laplace transform. However I think this may not be needed in your case.

Were you to plot out the three waveforms that I have described you would see the following:
http://commons.wikimedia.org/wiki/File:3-phase-voltage.svg

So what you (probably) need to do is:
1) work out the phase difference (displacement along the time axis of your waves.. which ones lead and lag which other ones)
2) assume they are all operating at the same frequency (which they most likely are)... and understand the relationship between frequency and wavelength.. which can help you draw waveforms on the squares
3) use the voltage and time/division to space out your plot squares.
 
I think the information you have put up at present only describes 2 phases of your 3 phase plot.

VAN= 347∠0° Vrms, VAB= 600∠30

Unless the third phase is somehow "implied" or assumed to be at 0. Ie that the VAN phase happens at exactly the same time as this phase. Those two phases definitely have different magnitudes.. ie different heights on your plot if it is voltage against time... or different vector lengths on your rotational plot..

What exactly has he asked you to plot voltage against? :)
 

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