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B, H, MMF, which one is the fundamental "driving" force?

  1. Jun 1, 2014 #1
    I was looking at a problem regarding solenoid with an iron core inside. the iron core has an air gap . once the MMF was turned on, a constant flux will flow thru the core and air gap (ignore fringing), so B's are the same for air gap and core but H's are different.

    I am guessing some similar situations would apply for E and D as well. D will remain the same thru out different materials but E actually varies.

    My conclusion is that MMF or EMF are the "fundamental driving forces". whenever new materials/objects are added for MMF to drive thru, B and H will all change everywhere. just like voltage as the fundamental driving force in a circuit, with Rs being the different materials and I and J as flux and B. E as H.
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  3. Jun 1, 2014 #2

    jim hardy

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    what's in a name ?

    MMF and EMF are abbreviations for
    Magneto Motive Force
    Electro Motive Force ...

    your reasoning sounds fine to me.
  4. Jun 1, 2014 #3

    Simon Bridge

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    None of them are fundamental.
    They are all emergent features of underlying quantum-level events ... changes in one may be said to cause changes in the other depending on how the situation is rigged up.

    But on the scale of events you are considering - it is reasonable to think of the MMF and EMF together as driving a circuit.
  5. Jun 2, 2014 #4
    Very good Simon, I was going to reply along those lines. This forum has had numerous threads re B & H, as to which is more "fundamental". Which is the driver and which is the result can vary.
    Re mmf, H and mmf are related via Ampere's law. To say which is most "fundamental" requires that you define the term "fundamental".

    In an electric circuit I cannot exist w/o V and vice-versa. Neither I nor V is in general the "driver". One of the most common prejudices encountered in electrical science is the myth that voltage is the driver of current. At times it can appear that way, but not true in general. The power company spins their turbines at fixed speed for good reasons. Fixed frequency provides sync motors ability to lock onto fixed speed, and allows multiple generators on the grid to sync.

    The power company could produce fixed torque on their turbines and output a constant current source. The voltage would vary with load. It's not done because transmitting at full current all the time incurs higher losses. Constant voltage is chosen for those reasons. Mother Nature does not decree that voltage determines current. It is a man made condition.

    Likewise B & H are mutually inclusive, neither is the "driver". I can elaborate if needed. Regards.

  6. Jun 2, 2014 #5

    jim hardy

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    I yield to the higher level thinking in above two posts.

    Electric circuits are taught simplistically, E = IR without going into fields. It works because the moving charge is generally constrained to the conductors.
    Magnetic circuits are taught similarly though flux is less well behaved - op's reference to 'fringing' .

    We're also taught Force = Mass X Acceleration
    either force or acceleration can be considered the 'driver'

    but a student has to start somewhere.
  7. Jun 2, 2014 #6
    Hi, so many brilliant minds in one post, thank you very much

    The reason why I was asking:


    That's the plot of a coil with a Sin voltage source exciting a core with hysteresis loss.
    I was utterly lost when voltage and ø are Sinusoidal but the current's shape is screwed up. shouldn't voltage drives a sinusoidal current and that current will result in a screwed up shape of ø?

    or at least: I know the L of the coil will be changing as well, so current shouldn't be really sinusoidal. But NI is the MMF, and to me that's a "fundamental driver". There is absolute no reason for a voltage on a coil to induce a perfectly shaped sinusoidal flux.
  8. Jun 3, 2014 #7

    jim hardy

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    [STRIKE]oops - have you edited your post ? I quoted but it looks a little different now..[/STRIKE]

    oops2 my mistake, old jim

    There's a mathematical reason.... and it's shown on your graph.

    remember e= n d[itex]\Phi[/itex]/dt ? Voltage(per turn) is rate of change of flux?

    Now remember your trig identities
    d sin(ωt) = ωcos(ωt)

    Now - do a sine and cosine wave resemble one another? Sure, they're just 90 degrees out of phase.
    Next - do your voltage and flux waves resemble one another? Sure, they're just 90 degrees out of phase.


    Try exciting your inductor with a small square wave voltage. You'll see a triangle wave flux, because during each half cycle d[itex]\Phi[/itex]/dt is constant.

    If you force a triangle wave current through an inductor your voltage will resemble a square wave. That is a great way to observe the imperfections of an iron core. Eddy currents that limit frequency response show up as rounded edges.

    Here's some 'scope traces from a 12 foot tall inductor with a non-laminated stainless steel core.
    In each photo upper trace is triangle wave current through the inductor(our triangle generator wasn't perfect)
    and lower trace is counter emf d[itex]\Phi[/itex]/dt as measured by a second coil on same core . We used the second coil as a flux detector - with no current it has no IR drop so just reports the counter emf.


    As you see it resembles an inductor on top trace ~ 3 hertz, but square wave's edges are rounded.
    middle is trace ten hz , resemblance to an inductor is bit of an imagination stretch
    bottom trace is 60 hz and it's just not at all well behaved.

    I guess that's why transformer cores are laminated.

    Our current was very low so we didn't approach saturation,

    What made us try triangle waves is we wanted a function that didn't look so much like its own derivative. Our sinewaves just showed us confusing phase shifts.
    Remember - Mother Nature loves to make sinewaves and that's why they're so common. But they are a mathematical oddity in that differentiating them doesn't change their appearance.
    But the derivative of a triangle wave is a square wave. Seeing is believing.

    Sorry to be so simple minded about this - but i learned a lot playing with those big coil stacks.

    If you have a lab in your school, give this a try. It'll help you understand inductance. A doorbell transformer will do, and surely you have a computer with D/A . A triangle wave into a 4-20 milliamp converter should work well as an inductor driver. Ours was home-made and ran on lantern batteries.

    old jim
    Last edited: Jun 3, 2014
  9. Jun 3, 2014 #8

    jim hardy

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    Now - since current is mmf, as you said,
    and it takes more mmf to increase flux on those flat parts at extremes of your B-H (phi-I) curve,
    that's why current deviates from sine.

    It's counterintuitive because flux is greatest at voltage sinewave zero crossings.
    Think derivatives, think slopes.

    old jim
    Last edited: Jun 3, 2014
  10. Jun 3, 2014 #9

    jim hardy

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    Your example imposed a sine voltage on the coil

    If instead you forced cosine current through your iron-cored coil
    your flux wave would be flattened near its peaks, again because of iron's nonlinearity.
    So the voltage 'sine' wave would be deformed around its zero crossings.

    Air core inductors are better behaved. Those twelve foot coils behaved just like textbook inductors until we inserted the core..
    Last edited: Jun 3, 2014
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