fuse & 3-phase system

Discussion in 'General Electronics Chat' started by Manjaks, Jun 18, 2010.

  1. Manjaks

    Thread Starter New Member

    May 2, 2010
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    Hi, please help me with this question.
    In three phase systeam (star type) each phase has constant voltage, which does not change. If we make a dead short, then the fuse burns out. I can understand that, because current is getting bigger and there is relatively small resistance.
    But if we add to the circuit lots of resistance, the fuse also burns, because these elements consume much more power and current is getting bigger. But why? According to Ohm`s law if resistance is getting bigger, then current must get smaller. It really blows my mind, help please. :D
     
  2. Ghar

    Active Member

    Mar 8, 2010
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    You need to give more details because this doesn't make sense to me either.
    I'm assuming the load is a Y (star).

    What is the source; is this \Delta-Y or Y-Y?

    Does the Y load have a neutral wire?
     
  3. t_n_k

    AAC Fanatic!

    Mar 6, 2009
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    As you add more loads in parallel the effective resistance decreases - for instance if you connect twenty 1000 Watt electric heating radiators in parallel to the same source you have less total load resistance than you would for ten radiators connected in parallel. So the total load current increases as you add more parallel loads - eventually you can reach an unsafe current level and the fuse would blow.

    Only when you add loads in series does the effective load resistance increase and multiple loads connected in series is not common practice in AC power systems. Parallel operation of devices or loads is the most common situation with AC power supplies.
     
  4. AlexR

    Well-Known Member

    Jan 16, 2008
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    Get a qualified electrician to take a look at it.
    Main power can kill if you don't know what you are doing. And obviously you do not know what you are doing.
     
  5. sceadwian

    New Member

    Jun 1, 2009
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    The original question make no sense, no multiphase system has a voltage which does not change. Dead shorts of course the fuse blows, but if you add resistance it has to be enough to limit the current to the point where the fuse won't blow. You're still applying a low enough resistance that the fuse is blowing from what little information you've provided.
     
  6. AlexR

    Well-Known Member

    Jan 16, 2008
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    Which is precisely why he should seek professional help. Any advice we give must be based on information he supplies but since the information supplied is unreliable we cannot make an informed appraisal of the problem.

    Mains power is one field where if you have to ask then you probably should not be doing it unless you are under direct physical supervision.
     
  7. retched

    AAC Fanatic!

    Dec 5, 2009
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    Agreed. AC basically means constantly changing voltages.

    Now with 3 phase, between a point of time, the voltage stays the same, but individual phases still alternate themselves, it is the available power to the load that stays the same.

    With individual fuses on each phase, it is just three, single phases (120deg from each other in order), so when L1s sinusoid is swinging negative, L2 is still in the positive. A short between the two multiplies the current flowing BACK through the fuse of L1.

    It is rather confusing, and rather deadly. If you have fuses popping for a reason unknown to you, and the math is not making sense, call an electrician.

    The reason the math doesn't work is because you dont have all the variables.

    The variables are knowledge, and lack of knowledge can hurt.
     
  8. t_n_k

    AAC Fanatic!

    Mar 6, 2009
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    Interestingly if you observe the instantaneous power waveform it too varies in relation to the sinusoidal voltage and current. The power waveform frequency is twice the source frequency. If the supply (rms) voltage and load impedance values are stable then the average load power is constant.
     
  9. retched

    AAC Fanatic!

    Dec 5, 2009
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    That is ringing a bell.

    I too have to read up on polyphase circuits and power.
     
  10. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    Why are you all bragging about?

    OP is confused why the fuse is blowing when loaded, to him adding resistance means less current. He did not how ever thought of the fact that adding load means adding resistance consumes power.

    A load added needs current. which increases as the load increases, it isn't the resistance increasing, it's the load that blows the fuse, in reality more load means lower resistance, so definitely fuse should blow.
     
  11. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    star or delta connection or what ever ..does not matter.
    Load is load.

    More load ...less resistance....more current
     
  12. retched

    AAC Fanatic!

    Dec 5, 2009
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    Also if the fuses are not slow-blow, and are not rated to handle the initial LRA or locked rotor amps from the motor starting from a stall or stop, the fuses will pop.

    Because of what you said Rifaa. ;)
     
  13. t_n_k

    AAC Fanatic!

    Mar 6, 2009
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    This was the point of my post #3 which attracted no comment. I attempted to actually answer the OP's question.

    Like many threads, which take on a life of their own, the result is sometimes a case of members wandering into (albeit related) areas in which the OP would have little interest.

    It's interesting the OP hasn't returned - so perhaps they found what they needed and we are wasting our time.
     
  14. AlexR

    Well-Known Member

    Jan 16, 2008
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    Or more likely, they have successfully barbecued themselves across the 3 phase supply!!
     
  15. t_n_k

    AAC Fanatic!

    Mar 6, 2009
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    That would be a very unfortunate outcome. Not something to be deemed as successful.

    I'm of the view that the OP wasn't contemplating anything of a practical nature - they were simply trying to understand the use of electrical terminology with respect to the idea of circuit loading.

    The notion of increasing load in their mind was (not unreasonably) clearly one of increasing resistance or impedance - rather than what we 'experts' accept by convention as decreasing resistance or impedance - the latter leading to increased load current.
     
  16. AlexR

    Well-Known Member

    Jan 16, 2008
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    The OP was blowing fuses, you can't get much more practical than that!!!
     
  17. t_n_k

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    Mar 6, 2009
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    My recollection was that he was blowing his mind.
     
  18. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    fuse or mind....if it blows ....that's the way to do it.
     
  19. sceadwian

    New Member

    Jun 1, 2009
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    What is everyone talking about anyways? It's been three days since the poster has said anything at all, and that was the post that started this thread. We should probably try to tone the conjecture down until the original poster comes back and responds to what's already been said.
     
  20. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    Hey...we like to breath down on each others neck.
    So post away...just for the heck of it.

    If the OP decide to check, it will blow his mind at least :D

    Besides. to make things worst Every one is correct
     
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