3 phase rectifier

t_n_k

Joined Mar 6, 2009
5,455
Also, on reflection, I'm not sure why the load time constant indicated in the OP's question has any significant bearing on the the mean value of the rectified output. Surely the only factor would be the ripple voltage on the load resistance.
 

donpetru

Joined Nov 14, 2008
185
Joe Jester gave a working implementation in post #8.
Joe Jester simulated a three phase half-wave rectifier that does not deal with the issue of this topic.

And another point: I notice that many of yours make confusion regarding the phase voltage and line voltage.
 

JoeJester

Joined Apr 26, 2005
4,390
donpetru,

There are four answers to evaluate in that question. Which one is the "closest" or "best choice"?

Why is your load not referenced to ac return or ground?

Why are you illustrating the source with no known standard?

Why are you measuring rms when the question called for average?

The OP did not return so we may never know the source of the question or the book used in the course.

You can explore the various topologies to see which one is closest to the listed answers.

You can have a 230V source connected Wye-Wye, Wye-delta, delta-delta, and delta-Wye. Of those four averages, only one was close to any of the four answers listed.
 
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JoeJester

Joined Apr 26, 2005
4,390
Also, on reflection, I'm not sure why the load time constant indicated in the OP's question has any significant bearing on the the mean value of the rectified output.
When you think about it, the load time constant is a distractor in the scheme of things.

100 uH / 1000 ohms is 1e-7 Seconds. That certainly is less than 20 mS

In the latest diagrams, illustrating the various connections, I used 10 ohms j 0.03768 ohms as the complex impedance. No change in the results.
 

donpetru

Joined Nov 14, 2008
185
Joe Jester, I think you've missed something. Let to analyze everything from the beginning. Let's start with the first post of this topic, quote:
In three phase rectifier with resistive-inductive load (time constant of a low τ = L/R >> 20 ms), supply directly from a grid 3x230 V, mean voltage measured at the resistance of the load is approximately equal:
In the first post of this topic 3x230V means 3 x 230Vrms voltage line (that I think you've missed!). Let's go back to some theory: a three-phase circuit (star connection) dependence of line voltage and phase voltage is given by: Voltage_line = SQRT(3) * Voltage_phase. I hope that you agree with that! For example, a three-phase transformer with the secondary star connection can be a good practical example of simulation done by me a few posts ago.
So, I simulated a star connection three phase circuit where the common point of the circuit (in practice) is the grounding circuit. This point should not be the same with "-" bridge rectifier. This you do not understand and do confusion. You don't overlook the fact that I use a three-phase bridge rectifier (diode are six, not three diodes !!!).
On the other hand, I did a comparison in the picture below between star connection and delta connection for you to see, first, how a simulation is correct (if you want you can experiment practically circuits listed below and you will see that the results are very close) and which connects the reference point (ground) in the two situations below. In the case delta connections I had to use some resistors 0.01 Ohm to simulate internal resistance of V1 ... V3 power sources.
I hope the picture below it is clear now for everyone.
 

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JoeJester

Joined Apr 26, 2005
4,390
Like I said, the problem statement never stated Wye (star) or delta. The problem I have is your solution the load is not referenced to ground. I realize once you ground one of side of the load your no longer a full wave rectifier.

Neither of your diagrams use a bridge rectifier. You do have full wave rectification, but not a bridge rectification, unless you consider the combining of the three phases constitutes a bridge.

Still, we are in the dark as to what the OP was taught or the reference material he used. We will never know.

All I know is there was a poorly written question that has grey areas. I still don't know where in the world 133V is a standard. Wiki didn't have any in their list of country standard voltages and frequency.

Of course in a purely academic setting, anything is possible. Many a question has been found "irregular".
 

t_n_k

Joined Mar 6, 2009
5,455
Hi Joe,
Donpetru's schematics certainly look to be 3 phase bridge rectifiers. Why the doubt?
I agree that your schematic satisfies one of the proffered solution options.
I also agree this is a sloppy question and without further input from the OP any ongoing speculation is sonewhat akin to flogging the deceased equine.
 

JoeJester

Joined Apr 26, 2005
4,390
When i think of bridge rectifiers, i think of four diodes in a simgle phase.

I do understand that bridging something is placing something else in parallel with it .... Used in telecommunications...

When solving these best choice multiple choice questions, we are confined to the presented answers and our best interpetation of the question. There will be the stem of the question and distractos.

3x230 meant 3 phase 230 Vrms. It did not specify full wave rectification, half wave rectification, or a balanced load. It mentioned a single load.

Then we have the gem ... Approximately. The info on my side is clouded by the fact the op didnt cite their study material by title or webpage.

So we are left to fully eliminate all answers to arrive at the best choice.

All the answers could be correct given an unlimited tolerance.

We will never find out because the op is unlikely to return to this page as their immediate need was satisfied when someome gave them an answer.

Welcome to we'll do your homework for you ... If the right person answers.
 
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t_n_k

Joined Mar 6, 2009
5,455
I've always considered the single phase bridge rectifier as the generic implementation of a polyphase rectifier system. The 3 phase bridge rectifier is well established both in the literature and as off the shelf hardware. Presumably one could have a bridge rectifier with as many phases as one wished as long as the relative phase displacements between the individual phase sources are consistent.
 

JoeJester

Joined Apr 26, 2005
4,390
Cool. I hadnt run across them being called a bridge before. Im sure we referred to that topology as a full wave rectifiers in that 1950s design of a 300 kw ttansmitter power supply.
 
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