RMS or Average Current Homework - Tips & Tricks

[jaus tail]

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.

 

[cnh1995]

In rectifier/converter problems, always find the average dc output voltage/current unless it is specifically asked that the rms value be found.

Average value is of no use anyways.

Why would you think so?
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.

 

[NascentOxygen]

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.

 

[jaus tail]

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?
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.

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.

 

[NascentOxygen]

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.

 

[jaus tail]

I found this from link:
http://protorit.blogspot.in/2013/01/power-electronics-thyristor-single-phase-bridge-rectifier.html

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.

 

[NascentOxygen]

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.)