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Time constant of inductors in machines

  1. Dec 11, 2014 #1
    We know that all machines or devices working on AC have high value of time constant so that there is n damage to inductor while curent reversal & high value of time constant means increase in time to reach steady state as we can see in fans , coolers , etc. but the tube-lights also have a choke coil but it give immediate response which means time constant has small value , so in that case it should be damaged..!! what is the reason exactly , I am not getting this thing..:confused:
  2. jcsd
  3. Dec 11, 2014 #2
    today my prof. taught me that if the current reversal through an inductor is done within the time constant i.e. before it attains steady state , (AC supply) inductor does'nt get damaged...buy why so ?? As we clearly know if the direction or magnitude of current is changed within a short time , inductor burns out..!!
  4. Dec 11, 2014 #3


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    Nope. Inductors don't burn out from fast transients...
  5. Dec 11, 2014 #4
    This doesn't sound right. Maybe he meant something else or you misinterpreted him. Even with relatively small current through inductor, with too quick reversal, dangerously high voltage (V=-L⋅di/dt) can be developed which can damage inductor's insulation.
  6. Dec 11, 2014 #5

    jim hardy

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    I sense a basic misunderstanding of the intent behind instructor's words (as related by OP)

    Let's get real simple here - just a DC source and a reversing switch and an inductor.

    Close the switch and current begins to rise toward Volts/(Resistance of inductor) with the time constant you mentioned.
    If the inductor has enough surface area to dissipate the heat produced by Volts2/R, then it can remain connected to the source forever. This is how DC solenoid coils work.
    If the inductor would overheat from continual application of the voltage then you must do something about it.
    You could simply disconnect
    or you could reverse the applied voltage with your reversing switch, making current start to decrease and approach negative Volts/R with same time constant.
    You could continue reversing polarity every time the current reaches what's safe for that coil, causing current to swing between a positive and a negative value that's safe.

    Isn't that simply applying AC?

    That's why AC solenoid coils have lower resistance than DC ones, maybe by a factor of 100.
    Current in the AC coil is limited by inductance(which makes no heat), but the poor DC coil has to rely solely on his resistance. So the DC coil gets very high resistance to keep power dissipation down. Setting R equal to V2 would give one watt needing only a very few square inches to dissipate..
    I don't know why teacher even mentioned time constant.

    Am i on target ? Please correct me if not.

    old jim
  7. Dec 11, 2014 #6
    according to your point , in that case all machines should be damaged when subjected to AC as mostly machines have inductive element..
    and current reversal simply is reversing of direction which an inductor oppose..!!
    what is the role of time constant then ???
  8. Dec 12, 2014 #7
    Absolutely not. In AC steady state, time constant doesn't matter.
  9. Dec 12, 2014 #8
    But the fact that inductor opposes change in magnitude & direction of current is true for all , we have to keep time constant in concern !!
    Please make it clear .
  10. Dec 12, 2014 #9
    In AC steady state we don't have. Relation between magnitudes of current through inductor and voltage across it is just I = V/ω⋅L.
    Time constant comes in play during switching on and off. Fast switching off is more dangerous as explained before.
  11. Dec 12, 2014 #10
    that's the thing I am asking.. See , if we consider a sinusoidal wave , the curent changes its direction from positive half cycle to negative & the time is very short ..so that's the same case..!! Then , why the inductor will not damage here??
  12. Dec 12, 2014 #11
    Calculate and see. Every electrotechnical component is designed to withstand certain voltages and currents.
  13. Dec 12, 2014 #12


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    "Very short" is a relative statement.
  14. Dec 12, 2014 #13
    so ultimately there's no use of time constant here.. you means the machines & devices are designed such that they are not affected by current reversals..
  15. Dec 12, 2014 #14
    They are affected, but we design them having on mind circumstances they are expected to work under.
  16. Dec 12, 2014 #15

    jim hardy

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    getting back to original question:

    Time constant is L/R for the whole circuit.
    Clearly there's something besides the inductor in a tube-light circuit that limits current.
  17. Dec 12, 2014 #16
    Do you have any idea , what could be there in the particular design of the machine which does'not let the machine get damaged during ac supply??
  18. Dec 12, 2014 #17
    So is this right that tube-lights have a low value of time constant.?? And again the same point is arising here that they are designed such that current reversal is not affecting it.>!! So ultimately we don't have any practical example in which induxtor is getting damaged due to current reversal??? :confused:
  19. Dec 13, 2014 #18
    In a first place, it is capability to withstand max voltages and currents in a steady state, and in occasional transient states. For example insulation of a new 230V rated machine must be capable to withstand something like 1500 V in 1 min tests, and few kVs in standard lightning impulse tests. It should be emphasized that some machines (especially rotating machines) are subjected to great electromechanical stress in extreme modes of their operation and are designed to withstand it.
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