current absorber

Discussion in 'The Projects Forum' started by cpleng7, Sep 9, 2010.

  1. cpleng7

    Thread Starter Active Member

    Dec 18, 2008
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    currently I am work out with this circuit. can some one teach me how to create a circuit that can detect the over current in the AC system?
     
  2. Dyslexicbloke

    Active Member

    Sep 4, 2010
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    That depends on what you want it for .... I suspect that if its purpose is anything other than indication there would be serious safty issues.
    You will also need to specify exactly wht sort of AC system you are talking about.
    Unless you just want the theory .....
    Is this another homework question?
     
  3. marshallf3

    Well-Known Member

    Jul 26, 2010
    2,358
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    Perhaps if all this homewoerk type stuff were posted in the appropriate thread:
    http://forum.allaboutcircuits.com/forumdisplay.php?f=7
    there would be a better chance of others working on the same problem working together as chances are if it's the subject of the week in one class it probably is in another.
     
  4. cpleng7

    Thread Starter Active Member

    Dec 18, 2008
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    actually what am I doing is not a home work type stuff.
    what I am doing now is reduce inrush current for the induction motor.

    actually before this that have a lot solution for this like:
    1)star delta starter
    2)autotransformer
    3)resistance or reactant starter
    4)soft start
    5)and the lastest starter is inverter start

    But all this starter just can lower down the inrush current and not going to eliminate the inrush current.

    What am I try to work out is do a circuit that can be absorb current with some reference current. If the current is larger than the reference current that have been set, the circuit will absorb the exceed current.

    So can any some suggest me any refrences that I can do research. or any circuit diagram can as a references. Thank you
     
  5. PackratKing

    Well-Known Member

    Jul 13, 2008
    850
    215
    Inrush current to an induction motor, is a fact of life.........They draw what they want to start, up to 8 times the rated nameplate current as a pulse until operating speed is achieved, and any attempt to "cushion the blow" will only serve to shorten the life of the motor.

    Just be sure your wiring can take the load, and the connections are clean and tight. Soft start, is employed in a number of current consumer power tools, but bear in mind, the unused portion of that current has to go somewhere, like incandescent dimmers.......likely the single largest waste of energy on the planet.
     
    Last edited: Sep 12, 2010
  6. eblc1388

    Senior Member

    Nov 28, 2008
    1,542
    102
    How would soft starting induction motor using series inductor or Star-delta method shorten the life of motor?
     
  7. PackratKing

    Well-Known Member

    Jul 13, 2008
    850
    215
    I'll concede that soft start, is about the only option for control, tho' what's the point ? Given a normal load, say a table saw with a cap-start motor, that blade needs to be up to speed asap .

    if you dog a cap-start [ any motor for that matter ] in any manner that prevents achieving operating speed, not allowing the centrifuge to open the start switch / windings, that motor will commence to smoke in short order, and hopefully any thermal protector onboard will open up in an attempt to save it, and give the operator long enough to reconsider what they are doing before the motor pukes completely.

    I work in a motor [ HVAC ] shop......for 8 years now............ I've seen, and repaired it all :p
     
    Last edited: Sep 12, 2010
  8. marshallf3

    Well-Known Member

    Jul 26, 2010
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    This is what your original post appears as. There is no circuit, no mention of what it's for, what it's made of etc.
     
  9. marshallf3

    Well-Known Member

    Jul 26, 2010
    2,358
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    I *think* i have a very vague idea of what you're trying to do but can you explain why you'd want to do that?

    If you want to "soft start" a motor a professional programmable speed control inverter is the only logical solution.
     
  10. cpleng7

    Thread Starter Active Member

    Dec 18, 2008
    120
    0

    actually I didn't want to create a soft start for my induction motor. What am I want to do is to create "current absorber starter" for the induction motor. the purpose of the "current absorber starter" is for the eliminate the high inrush current of the induction motor.

    put it as a easy understand way is the current will flow to the induction motor in the nominal current which is enough for the induction motor start up.

    In here I would like to understand the method the how a current absorber work out. I have go though to the google , I can't find any relevant circuit that can let me to study, so did anyone have done this current absorber before? Can you share some idea to me. Thank you.
     
  11. PackratKing

    Well-Known Member

    Jul 13, 2008
    850
    215
    The device that will detect the "overcurrent" you are chasing, is the common Ammeter....:p

    Unless the [ analog ] ammeter you connect to the motor circuit twice exceeds the expected start current pulse, it will momentarily slam the needle fullscale, until the motor is up to approximately 3/4 of its normal operating rpm's

    second quote:

    " actually I didn't want to create a soft start for my induction motor. What am I want to do is to create "current absorber starter" for the induction motor. the purpose of the "current absorber starter" is for the eliminate the high inrush current of the induction motor. "

    Once again, High inrush current is unavoidable, short of a soft-start circuit --- and again, what's the point ??
     
  12. marshallf3

    Well-Known Member

    Jul 26, 2010
    2,358
    201
    Very, very large NTC thermistors will limit the inrush current.
     
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