3 phase rectifier

Discussion in 'Homework Help' started by asdfgh1234, Oct 26, 2014.

  1. asdfgh1234

    Thread Starter New Member

    Oct 26, 2014
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    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:

    a). 269 V
    b). 191 V
    c). 324 V
    d). 230 V

    Can you please help me with this?
     
  2. JoeJester

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  3. donpetru

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    Nov 14, 2008
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    If it is a three-phase rectifier and 230Vrms mean voltage between grid phases, then the voltage measured at the resistance of the rectifier load is one dc voltage with a maximum value given by: 1.41 * 230Vrms = 324Vdc.
    Remember, 230Vrms it is the voltage between phases in mathematical expression above.
     
  4. WBahn

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    Mar 31, 2012
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    Can the OP contact you during their exams to get the answers, since it appears you don't think they need to even attempt to answer their homework for themselves.
     
  5. joeyd999

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    He'd also need to make himself available for consultation in the event OP gets a job.
     
  6. donpetru

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    @WBahn, @joedy999,

    If yours don't want to help the man, it's yours job. However, this forum was created for some people to understand what's up. It is too much selfishness in some of yours! It is not good !
     
  7. WBahn

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    The point is that you are NOT helping people by just working the problem for them and giving them the answer.

    Look at the terms and guidelines for this forum: "Lately we see people giving complete answers in the homework help section. This will hardly help the OP with understanding how to solve a question. The people who help in the homework help section are supposed to give hints and suggestions on how to solve the question".
     
    JoeJester likes this.
  8. JoeJester

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    Attached is the problem statement ...

    You will notice that 324.xx is NOT the mean output across the load. I removed the lower value of the AC component of the output.

    donpetru, I recommend you read the rules of the homework area. You giving out an answer like that violates those rules. Answering is not guiding.
     
  9. crutschow

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    donpetru, regardless of whether you should be answering the question directly, your answer was incorrect. An inductive-resistive load with a long time-constant does not generate the peak of the AC voltage at the load, it generates the average AC voltage.
     
  10. donpetru

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    I understand you where you want to go but considering the problem in the first post of this topic, which you assert can not be true because you don't know: the power source behavior, the rectifier bridge features and the power load. Without this information can not reach a correct conclusion.
    On the other hand, indeed, if you attach a high power inductive load on the output rectifier bridges, it will drop the voltage below 324V those calculated but you can not know by how much, if you do not know the information previously said.
     
  11. WBahn

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    You're told that the time constant of the L/R load is much, much greater than the power line period.
     
  12. t_n_k

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    So it appears that none of the four optional answers given in original question are correct for an ideal three phase rectifier configuration driving a long time constant load.
     
  13. JoeJester

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    I like the 269 volts answer for my "approximately".
     
  14. t_n_k

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    Yes, I agree that's the likely candidate. Although, this conclusion (albeit logically) presupposes a certain rectifier topology - a matter which the question (as provided) omitted to address. Donpetru also noted this point.
     
  15. crutschow

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    Below is the simulation of a 3 phase bridge rectifier fed from an assumed Wye source with 60Hz, 230Vrms to neutral and an R-L load with an L/R time-constant of 100ms. The resistor load voltage is 267V which is close to the answer of 269V if you used ideal diodes. As you can see that also is very close to the average voltage of the bridge rectified output (V+).

    Edit: But note that this answer is only for the case where the neutral of the Wye neutral is connected with the bridge output common. The answer will be significantly different if the Wye neutral is floating with respect to the bridge output common. It will also be different for a Delta connection.

    3 Phase Rectifier.GIF
     
    Last edited: Oct 30, 2014
  16. t_n_k

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    One issue crutschow - you have ground points on both the AC neutral and bridge output.
     
  17. crutschow

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    As I noted in my previous post.
     
  18. donpetru

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    @crutschow, your simulation is wrong - look, I have attached below the solution of the problem.
    If you will try to connect a capacitor in parallel with the circuit R1-L1, then the dc voltage reaches approximately 324Vdc (depends on the value capacitor and the power R1-L1 circuit).
    In the image below I simulated a three-phase circuit with 230V line voltage.

    LATER EDIT: If I try change value R1 to 100Ohm and L1 to 10H, the result is the same.
     
    Last edited: Oct 30, 2014
  19. crutschow

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    My simulation is not necessarily wrong, just different. I have the neutral connected to common and a line-to-neutral voltage of 230Vrms (I understand that may not be how you normally connect the neutral but it appears to be one connection giving a load voltage that matches one of the answer options).
     
    Last edited: Oct 30, 2014
  20. t_n_k

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    @crutschow

    Did you check the current in D1 ,D3, D5? Don't they short their related AC sources on their respective negative half cycles?
    Joe Jester gave a working implementation in post #8.
     
    Last edited: Oct 30, 2014
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