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An anomaly in high-impedance inductors

  1. Mar 28, 2013 #1
    Hello, first post here,

    my passion is electric motors, wind turbines, etc ... yes i am very boring !

    I've been testing coils for my motor design for a couple of years now.

    I've written my own software to predict inductance, etc as well as the physical properties of the coils.

    I've recently noticed something that has me a bit puzzled, though i think i am now coming to an understanding of it.

    When i wind a coil of very high inductance (on the order of 5 Henrys plus) and attach it to a load for testing, the generator rotor increases in speed and the input current goes down (i am powering the generator with a bench PSU, in its final instance i plan it to be run from diesel).

    For a while this puzzled me, until a friend pointed out that a coil of high inductance will increase the time constant.

    The time constant of an inductor being :

    TC = L/R1+R2

    Where :

    TC = Time Constant
    L = Inductance
    R1 = DC Resistance of coil
    R2 = DC Resistance of load.

    The TC equates to about 63% of the risetime and in 5 TC's the risetime is around 98%.

    This means we can control the timing of the counter-electromotive-force by varying the coils self-inductance.

    This in turn means that we can retard or delay the equal and opposite reaction dictated by Lenz's Law.

    I think what is happening (i don't yet own an oscilloscope) is that the risetime of the inductor is delayed enough so that the CEMF is helping the rotor acceleration rather than hinder it.

    It's an extraordinary effect, and i don't think it's down to core saturation as my core is made of high quality, M6 grain-oriented steel laminations and they are experiencing about 50% saturation (they can run to 1.3 Tesla, i am running them at about half of that).

    But the effect doesn't appear at low frequencies, with these coils it only happens at around 300Hz and above.

    I can attach a small, incandescent bulb, the bulb then lights up but the rotor speeds up and the input current from my DC PSU goes down !

    I know that inductors behave more and more like capacitors as they are driven at higher frequencies, this 'parasitic capacitance' effect is designed out of items like audio amplifiers because it introduces unwanted noise, so i'm thinking that's maybe why noone i ask has heard of this weird effect before.

    Any input greatly appreciated, as it is rather odd !

    All the best,

  2. jcsd
  3. Mar 29, 2013 #2

    jim hardy

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    Gold Member

    We need a sketch.

    The picture in your mind is crystal clear to you
    but your words have only painted a jumble in mine.

    What are you describing? A motor or a generator? or just a coil?
    How does your PSU produce rotation, or does it?
    Is this 5 henry coil a part of your rotating machine?
    if so is your machine a motor or generator or alternator?
    Or your coil connectted between the machine and the load?
    What is nature of the load?

    As frequency increases your apparent inductance will decrease somewhat . That's because eddy currents in iron oppose the changing flux, so its 'effective' permeability is less.

    Re-read you post today with benefit of last night's sleep and see if you can make it more clear.

    old jim
  4. Mar 29, 2013 #3
    OK Jim.

    I think the best thing to do would be to show the anomaly.

    I'll create a youtube account and make a video.
  5. Mar 29, 2013 #4

    jim hardy

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    Science Advisor
    Gold Member

    You can also set one up at photobucket or some such place,
    draw a sketch , scan it, save it there

    and use 'insert image'..
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