RMS or Average Current Homework - Tips & Tricks

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Discussion Overview

The discussion revolves around the distinction between RMS (Root Mean Square) and average current in the context of rectifier and converter problems. Participants explore when to use each value and the implications of their usage in power electronics, particularly in relation to DC output voltage and current.

Discussion Character

  • Homework-related
  • Debate/contested
  • Technical explanation

Main Points Raised

  • Some participants question how to determine whether a problem requires RMS or average current, suggesting that output is typically considered RMS.
  • Others argue that in rectifier/converter applications, the average DC output voltage or current should be calculated unless specifically asked for the RMS value.
  • A participant mentions that specifications often describe the output as a constant and ripple-free DC current, implying that RMS and average values may be equivalent in such cases.
  • It is noted that if the current is ripple-free and constant, then the RMS value equals the average value, leading to the same result when using the RMS formula.
  • Concerns are raised about the assumptions made regarding the waveform of the current, with some participants pointing out that the original problem may have incorrectly assumed a sine wave for the current waveform.
  • One participant emphasizes the importance of determining the waveform of the current before applying formulas, as misapplication can lead to incorrect answers.
  • Another participant suggests that the averaging function of a large series inductor plays a critical role in determining the steady current through the load.

Areas of Agreement / Disagreement

Participants express differing views on the relevance of RMS versus average current, with no clear consensus reached on which is more appropriate in various contexts. The discussion remains unresolved regarding the assumptions made about current waveforms and the implications for calculations.

Contextual Notes

Participants highlight potential limitations in the assumptions made about waveforms and the definitions of RMS and average values, but these remain unresolved within the discussion.

jaus tail
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Homework Statement


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Homework Equations


I calculated for RMS current.

The Attempt at a Solution


How to know whether they've asked RMS or Average current? Isn't Output always RMS? Average value is of no use anyways.
 

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In rectifier/converter problems, always find the average dc output voltage/current unless it is specifically asked that the rms value be found.
jaus tail said:
Average value is of no use anyways.
Why would you think so?:wink:
Have you studied speed control of dc motor using rectifier, converter and chopper? You never bother about rms there, it's all about average values.
 
jaus tail said:
How to know whether they've asked RMS or Average current? Isn't Output always RMS? Average value is of no use anyways.
The specifications tell you that this arrangement is outputting a load current they describe as constant and ripple-free DC current as a result of inductive smoothing.

Purists may insist on inclusion of the word "approximately", but in power engineering that goes without saying. Everything in power electronics is geared towards an approximation of convenience.
 
cnh1995 said:
In rectifier/converter problems, always find the average dc output voltage/current unless it is specifically asked that the rms value be found.

Why would you think so?:wink:
Have you studied speed control of dc motor using rectifier, converter and chopper? You never bother about rms there, it's all about average values.
I've left that out for exam. There is too much to study so I've not studied some parts in each subject.
NascentOxygen said:
The specifications tell you that this arrangement is outputting a load current they describe as constant and ripple-free DC current as a result of inductive smoothing.

Purists may insist on inclusion of the word "approximately", but in power engineering that goes without saying. Everything in power electronics is geared towards an approximation of convenience.
If it's ripple free constant DC current then RMS = Average value. So I should get same answer by rms formula.
 
jaus tail said:
If it's ripple free constant DC current then RMS = Average value. So I should get same answer by rms formula.
First determine the waveform of the current, then use a formula appropriate to this waveform. If you misapply a formula you will get a wrong answer.

Yes, if you determine the RMS value of a steady current you must arrive at its average (or DC) value.
 
I found this from link:
http://protorit.blogspot.in/2013/01/power-electronics-thyristor-single-phase-bridge-rectifier.html

EEEbold99_thumb2.jpg

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I know that since current is said to be constant, there'll be one horizontal line for current. But in the answer in post#1, they've assumed current waveform to be sine wave. That's how they got the equation for average value. The equation of 1 + cos firing angle is for average value.
I still don't understand why average value is taken whereas rms value should be considered. RMS value means DC value means the value that will give same power. Like for heating coils rms value is taken. DC value is never used. Even in network equations rms value is taken.
Transformer also measures RMS Value in rating. Power = V rms times I rms times power factor.
 

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jaus tail said:
But in the answer in post#1, they've assumed current waveform to be sine wave. That's how they got the equation for average value. The equation of 1 + cos firing angle is for average value.
No, they assumed the voltage to be sinusoidal with a portion missing and used a formula to find the average value of that voltage rectified. The large series inductor performs an averaging function, and the steady current through the load has its value determined by the average of the voltage across L + R. (The average of a periodic voltage across an inductor is zero.)

It would be instructive for you to sketch the expected current waveform in one thyristor, and also the current in the diode. When the thyristor is conducting, it is supplying all the load current. When the thyristor is not conducting, the backswing diode is supplying all the load current (a fixed value).

(I think they have glossed over something in determining that average voltage, those negative-going portions, but I won't mention it either.)
 
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