meaning of henry

Discussion in 'General Electronics Chat' started by PG1995, Oct 27, 2011.

  1. PG1995

    Thread Starter Active Member

    Apr 15, 2011
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    Hi

    It is easy to conceptualize meaning of one farad for a capacitor. If a capacitor is rated at 2 farad then that means it would take 2 coulombs of charge to for raise of every 1 volt. But I don't get the meaning of one henry for an inductor. One henry stands 1 volt per (ampere per second). What does it really mean? Please help me with it.
     
    Last edited: Nov 26, 2011
  2. joeyd999

    AAC Fanatic!

    Jun 6, 2011
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    The current increases at a rate of 1A per second when 1V is applied across 1H.

    The formula is:

    V*t = L*I

    which is the dual of:

    I*t = C*V
     
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  3. Adjuster

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  4. steveb

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    Since you feel that visualizing charge is good for the capacitor, then you should visualize flux (F, which is the dual of charge) for the inductor.

    F=LI is analogous to Q=CV

    dF/dt=V is analogous to dQ/dt=I

    Hence, if an inductor is rated at 2 Henry, then that means it would be 2 Webers of flux for every 1 Amp of current.

    If you still find that confusing, then don't feel bad because you are not alone. Even after years of study, I still have a better intuitive feel for charge, than for flux.
     
    Last edited: Oct 27, 2011
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  5. VoodooMojo

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    that's because it is easier to compute the quantity of dung in a bucket than it is to count the number of flies buzzing around it!
     
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  6. steveb

    Senior Member

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    Ha, true. But a key difference is that i hate flies and dung, but i love flux and charge ! :p
     
  7. PG1995

    Thread Starter Active Member

    Apr 15, 2011
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    Thank you, everyone. Sorry for the belated thanks.

    Someone might find this definition useful: A coil is said to have an inductance of 1 henry if an e.m.f. of 1 volt is induced in the coil when the current in the coil changes at the rate of 1 ampere per second.

    So, when 1 Amp of current (let's say DC) is flowing through an inductor, the magnetic flux around the inductor would be 2 Webers and it acts as a storage of 1 Joule of energy (energy in an inductor is: 1/2*L*I^2). Am I correct? Please let me know. Thanks.

    Regards
    PG
     
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  8. steveb

    Senior Member

    Jul 3, 2008
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    You are correct. An alternate form of the magnetic field energy equation is E=0.5*F*I, where F=LI.

    Again, drawing an analogy with capacitors, capacitor electric field energy is E=0.5*C*V^2=0.5*Q*V.
     
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  9. PG1995

    Thread Starter Active Member

    Apr 15, 2011
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    Hi

    Isn't there a subtle important difference between the two definition? The second one work with constant value of current but first one requires current to be non-constant.

    Definition #1:
    A coil is said to have an inductance of 1 henry if an e.m.f. of 1 volt is induced in the coil when the current in the coil changes at the rate of 1 ampere per second.
    v = L*dI/dt => L = v/(dI/dt)
    [according to this definition current cannot be constant; it should be changing]

    Definition #2:
    A coil is said to have an inductance of 1 henry if 1 Amp of current flowing through it develops magnetic flux of 1 Weber around it.
    F=LI => L = F/I
    [according to this definition current can be constant]

    Thank you.

    Regards
    PG
     
  10. Yako

    New Member

    Nov 24, 2011
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    It think that it is some people's first refuge man.
     
  11. steveb

    Senior Member

    Jul 3, 2008
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    That seems correct to me, but they are really the same thing. The fundamental definition of inductance is F=LI (analogous to the capacitance definition Q=CV). To obtain the other form of the equations we simply take the time derivative of the equations to get V=L dI/dt and I=C dV/dt.

    By the way, which equations are easier to use to measure L and C? Personally, I don't know a good way to directly measure total flux F or total charge Q for devices sitting on the table. However, it is very easy for me to measure currents and voltages, and their rates of change, using an oscilloscope.
     
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  12. PG1995

    Thread Starter Active Member

    Apr 15, 2011
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    Hi Steve

    Could you please help me with the query included in the attachment? Thanks for the help.

    Regards
    PG
     
  13. crutschow

    Expert

    Mar 14, 2008
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    In a transformer the AC flux is proportional to the voltage and thus the magnetizing current due to the primary inductance only. There is no net contribution from the load current. This is because the load current in the primary is countered by the opposite direction load current flowing in the secondary.

    To see that this is so, look a the polarities of the primary and secondary of a transformer and think about the current flow. The primary load current flows into the plus terminal but the secondary load current flows out of the plus terminal. Thus the load amp-turns of the primary are countered by the load amp-turns of the secondary and no net flux is generated by this current. In other words the net LI flux is zero for the load current.
     
  14. steveb

    Senior Member

    Jul 3, 2008
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    The above answer seems good to me. I can't think of another way to clear up your confusion.

    In general, voltage is proportional to rate of change of flux, but for AC signals the text says voltage is proportional to flux. The implication here is that frequency is constant and the signals are sinusoidal, hence voltage is proportional to ωλ, where λ is flux and ω is frequency. In a constant frequency application (like 50/60 Hz power), the text is correct.

    The flux is also proportional to LI, as you say, but here you have to be careful what you mean by L, as crutschow pointed out.
     
  15. PG1995

    Thread Starter Active Member

    Apr 15, 2011
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    Thank you, Carl, Steve.

    But I'm still missing the main point. I understand that the primary current has two components: one component is used to generate flux and is only 2-3 percent of the full-load current, and the other component supplies hysterisis, eddy current losses etc.

    Please have a look on the attachment to see where I'm going wrong. Follow the link for high-resolution copy of the attachment: http://img194.imageshack.us/img194/4652/boylestademfequationsin.jpg

    Thanks a lot for the help.

    Regards
    PG
     
  16. crutschow

    Expert

    Mar 14, 2008
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    So what's the "main point" you are missing?

    I believe the comment by Steve should be "be careful what you mean by I", not L.

    The primary load current does not contribute to the flux because it is always balanced by current in the opposite direction from the secondary load. You can't have primary load current without the corresponding secondary current. That's the crux of it.
     
  17. PG1995

    Thread Starter Active Member

    Apr 15, 2011
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    Thank you for the reply.

    As I mentioned in my last post that the magnetizing component is only 2-3 percent of the full-load primary current (full-load current: the current which flows in the primary when the secondary is fully loaded), so my confusion is this: In short circuit full-load primary current is flowing therefore magnetizing current can have its full 2-3 percent share and generate the maximum flux. Now, could you please help me? Thanks.

    Regards
    PG
     
  18. steveb

    Senior Member

    Jul 3, 2008
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    PG,

    I'm having a hard time coming up with an explanation that gives a good intuitive feel (I'll keep thinking). I tend to think of this situation in terms of formal field theory, which may not be helpful to you yet. I did notice that our Ebook here has a good explanation, so you might want to look at the following.

    http://www.allaboutcircuits.com/vol_2/chpt_9/1.html

    crutschow,

    That's probably a better way to say it. I kind of meant both really. The way I see it, inductance L is really defined through the relation L=λ/I, and is determined by geometry. However, we have many things we might mean by L, it could be self inductance, mutual inductance or leakage inductance. So, once we change the flux or current we are relating, we are changing the definition of inductance. I can't help but wonder if this is causing confusion for the OP. He says L is constant, but it is not constant if one changes the definition midstream, by considering the wrong I.
     
  19. PG1995

    Thread Starter Active Member

    Apr 15, 2011
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    Thank you, Steve.

    I hope you will come up with some explanation soon. I'm still struggling with the concept and don't know how to state my confusion any better thank this. I'm also waiting for Carl's reply.

    Best wishes
    PG
     
  20. crutschow

    Expert

    Mar 14, 2008
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    The same rules apply whether the output is open, carrying a load, or shorted. In each case it's the primary voltage that determines the flux level and the magnetizing current. When the transformer is shorted, the primary voltage is due mainly to the winding resistance, so consequently the flux level and magnetizing current are very low.

    That 2-3% rule-of-thumb applies to the normal operating voltage. It would be much less under shorted conditions.

    Make sense?
     
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