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measurement of inductive reactance

Discussion in 'Electronic Basics' started by nag, May 16, 2006.

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  1. nag

    nag Guest

    Hi ,
    It is a general question regarding the measurement of
    Inductive reactance. If we apply the sine wave means the reactance will
    be 2*pi* Frequency* Inductance. But if the applied wavefrom is a non
    sinusoidal one means how to determine the inductive reactance.

    Also will the inductace change depends on the applied shape
    and frequnecy of the input signal? if so can some one explain the
    details behind that?

    I have tried checking these details in the Books but o
    could not get that. So help me to solve this basics doubts.

    Regards,
    M.Nagarajan.
     
  2. Chris

    Chris Guest

    Hi, Nag. Technically, a 20Hz square wave can be seen as the sum of odd
    integer harmonics (i.e. 20Hz, 3 * 20Hz = 60Hz, 5*20Hz=100Hz, &c).

    http://en.wikipedia.org/wiki/Square_wave

    I would guess you're thinking about that solenoid you're trying to
    drive at 20Hz. You're overthinking this again. You've got a solenoid
    you're trying to drive at a high frequency. Your goal is to get the
    current flowing in the solenoid coil as quickly as possible to turn it
    on (magnetic force is proportional to current flow, and inductance
    impedes that), and then get the inductive kick to dissipate as quickly
    as possible when you turn it off, to stop the magnetic force.
    Inductance isn't your friend here, but there are things you can do to
    speed things up. A higer voltage power supply would be the first.

    Cheers
    Chris
     
  3. Chris

    Chris Guest

    Hi, Nag. One more thing. You had a similar conversation on s.e.b. and
    s.e.d. last March. Your focus solely on the inductance of the solenoid
    is only seeing half the picture.

    A solenoid is an electro-*mechanical* device. That means F=m*a is
    every bit as important as the L/R time constant. Given a DC current in
    the coil, you will generate a magnetic force. However, the speed of
    the solenoid, and the speed of its return, has a top limit dependent on
    the physical factors involved, too (not only magnetic force, but also
    stroke length, work required to compress the return spring, and other
    factors).

    By switching, say, a 60V supply and a 200 ohm resistor in series with
    your solenoid, you'll get the current going very quickly -- an order of
    magnitude faster than if you were using a 6V supply and switching the
    solenoid on with a transistor or relay. But it will still take a
    finite amount of time for the solenoid to pull in. F=ma. And if your
    solenoid is spring-released (probably your limiting factor), you *will*
    have to wait for the spring to do its work, even if you use Transzorbs
    to dissipate the inductive kick quickly. A spring return on a solenoid
    could easily take 50 or 75ms, which would make 20Hz unrealistic.

    There are several ways to get the current going through the inductor
    more quickly, just as there are ways to dissipate the power of the
    inductive kick on turn-off more quickly. But you might just want to
    make sure the solenoid is capable of 20Hz operation before you spend a
    lot of time on this. I'd assume you're not rolling your own solenoid
    here, so you might just want to contact the manufacturer of the device
    and ask.

    Also, remember that a solenoid is a wear part. They are usually rated
    for number of cycles. 20Hz is going to burn through those cycles
    pretty quickly. If this is an automation application, you might want
    to consider another method of getting the thing done. Solenoids
    typically don't work very well for high speed cycling on machines.
    While you're on the phone with the manufacturer, be sure to ask about
    rated life.

    Feel free to post again. I'd be interested in knowing what you're
    using your solenoid for. Possibly someone might be able to recommend
    something better.

    Good luck
    Chris
     
  4. nag

    nag Guest

    Hi Chris. The solenoid which i was talking about is similar to Facet
    solidstate pump. You would have heard about the Facet pump it is a
    universal one. The operating frequency is 16Hz only. After seeing your
    reply i wonder how that pump is operating with 16Hz.

    But the question i asked regarding the reactance is general one not
    related with this solenoid.

    Also i much intrested in applying the mathematics in the design of
    electronics like integration, differentiation etc. i could not find any
    site or books which explians mathematics along with the engineering
    applications. i expecting this kind of things from a long back since i
    have not practiced the integration & differentiation and all that much.
    so only i asked. Also could you help me in this regards?

    Regards,
    M.Nagarajan.
     
  5. Chris

    Chris Guest

    Hi, Nag. Since you have a step function input, you can bypass the
    business of determining impedance of complex waveforms, and just use
    some basic math. Possibly that will help explain where I'm coming from
    here, too, saying you might be overthinking this.

    This is the first page I found on Google -- you can use this as a frame
    of reference:

    http://www.ac.wwu.edu/~vawter/PhysicsNet/Topics/Inductance/LRCircuits.html

    Just as something to ponder, look at this (view in fixed font or M$
    Notepad):

    |
    | _/ _/ ___
    | .--o/ o------. .---o/ o-----|___|-------.
    | | SW1 | | SW1 60 ohm |
    | | | | (external) |
    | | .-. | .-.
    | | 20| | | 20| |
    | | | | | | |
    | | '-' | '-'
    |6V +| | | |
    | --- o +| o
    | - | 24V --- |
    | | 60mH C| - 60mH C|
    | | C| | C|
    | | C| | C|
    | | | | |
    | '-------------' '-------------------------'
    |
    (created by AACircuit v1.28.5 beta 02/06/05 www.tech-chat.de)

    See what happens to the time constant and the current when you use a
    higher battery voltage and a series resistor to take up the slack.

    This effect may help you in getting the solenoid to move in as quickly
    as possible -- and if you're looking at trying to get maximum speed
    from a solenoid, you need all the help you can get.

    Good luck
    Chris
     
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