Solenoid Valve Operation

  1. Briefly, a solenoid valve utilizes a solenoid to move a soft iron core in such a way as to open/close an orifice to allow/inhibit flow of a fluid. I am a bit rusty on my E&M and I have a couple questions about this operation.

    1) When a magnetic field is created using the solenoid, the soft iron core becomes magnetized, producing a north and south pole on the core, and essentially becoming a bar magnet. How does this magnetized core behave in the presence of the magnetic field? Since the solenoid will, in general, be short, the magnetic field is non-uniform inside the solenoid, so does that mean the magnetic field causes the core to move? Doesn't this violate the principle that magnetic fields can do no work? If the magnetic field does not cause the core to move, why does the core move?

    2) I looked around a bit, but I have not seen any literature showing an analytical model for the behavior of the valve as the coil (insulation and wire) degrades. I've seen equivalent circuit models, but not a more specific model. If anyone has leads on something of this nature, I would greatly appreciate it.


    Thanks in advance for your help.
     
  2. jcsd
  3. You must be aware that every electric engine uses magnetic fields to do work, aren't you?
     
  4. Sure, I can see that magnetic fields are employed to perform work in electric motors (as well as magnets being used to pick up cars in scrap yards). But there is an absolute principle that magnetic fields can do no work. What is the best way to view the movement of the solenoid core? Is the magnetic field changing the velocity of the core? If so, isn't this work?
     
  5. Vanadium 50

    Vanadium 50 17,581
    Staff Emeritus
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    The statement "magnetic fields do no work" is more confusing than enlightening, and I don't understand it's sudden recent popularity. It is true that magnetic fields do no work on electric charges, and ultimately the work is done by "hidden" electric charges, such as those that hold a piece of iron together, but this is not helpful in understanding how magnetic systems behave - for example, why iron is attracted to regions of increasing magnetic field density.

    My advice is to stop treating "magnetic fields do no work" as something deep and fundamental, and more as a footnote.
     
  6. Thanks, but honestly, I wasn't trying to be deep; I am only trying to understand how the solenoid valve operates on a fundamental level. Does it work because "iron is attracted to regions of increasing magnetic field density"? If so, does that mean the valve is designed such that the highest magnetic field density is at the core's final position? In other words, if the valve is designed to close when energized, is the highest field density located where the center of the plunger should be (or one of the core's poles) when the valve is closed? I imagine this location of highest field density is probably at the center of the solenoid (central in terms of its diameter and length). So, the solenoid would need to be positioned such that its center (or one of its poles) is at the same location (or further along the trajectory) as the center of the core when energized.

    For a while I think I had the misconception that the core was magnetically attracted to one end or the other of the valve due to the mutual magnetization of the core and the solenoid coil housing. Thanks again for any help. (If you know of any literature on this subject, I would really appreciate a reference.)
     
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