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How to make a device which is "Reverse solenoid"

  1. Sep 18, 2015 #1
    Hi Everyone,

    This is Sentil from India. I'm a professional in the field of commerce. But i love science!! then the obvious question "Why /How did you end up in Commerce". Thats really a long story.

    But today im here to get help on one of the side projects i am into to develop something like "solenoid in reverse". That is, i want to generate some power from the solenoid by mechanically moving the plunger of the solenoid.

    I did some basic web research ( i still do ) and together with my lil elementary science knowledge, i understood how solenoid works and what is the principle and all the theory part. but i cant find anything on this " Solenoid in reverse" or "reverse solenoid" ( not sure if im confusing you guys. this device is suppose to work just like "linear alternators") like no of turns of copper wire, how powerful should be the magnet, what should be the speed at which this magnet should move to and fro to cut the magnetic flux and generate output power.

    It would be very help if somebody could help me with these doubts and help me build one.

    Looking forward to hear from you all..!!


    P.S:- Do i need to post this somewhere else?
    Last edited: Sep 18, 2015
  2. jcsd
  3. Sep 18, 2015 #2


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  4. Sep 18, 2015 #3
  5. Sep 18, 2015 #4
    To generate some power from the solenoid by mechanically moving the plunger of the solenoid the solenoid has to be a magnet..
  6. Sep 18, 2015 #5


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    From the pictures, the "antiques" are probably conventional rotary type dynamos with "slipping" linear actuators. I couldn't pass up Wiley Coyote and the Roadrunner, though.
    More relevant to your inquiry, https://www.google.com/?gws_rd=ssl#q="faraday+flashlight" .
  7. Sep 18, 2015 #6
    Yup...the configuration what i'm thinking of is like a copper coil and the plunger will be magnet!!

    This magnet im intending to move it "to and fro "mechanically. Thats where the doubt listed below:-

    1. No of turns of copper wire to get a specific power in Watts
    2. How powerful the magnet should be?
    3. What should be the speed at which this magnet should move "to and fro" to cut the magnetic flux and generate output power.?

    Any equation available to get atleast some ball mark figure??
  8. Sep 18, 2015 #7


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    What you are doing is designing another of these shaker torches.
    I do not recommend this product but show it as an example of the “see through technology”.

    The magnet is on the RHS in the picture and slides along an internal guide through the solenoid coil. A rubber block at each end helps cushion the change in direction. You might think it is silly moving a heavy magnet backwards and forwards, but it is actually the torch that moves when you shake it, the battery remains in the same place. The best technique is wrist action that does not move the mass of your arm.
  9. Sep 19, 2015 #8
    But how is "linear alternators" different from shaky flashlights? Do they work on the same principle or some other laws of physics?
  10. Sep 19, 2015 #9
    Thank you so much for that link. I did some research on farady's law and figure out the relation between current and number of turns of copper coil. Now, that i understood farady's law and the current is depended on the "rate of change of magnetic flux", how do i increase this variable in a practical situation? Is it like i just have to increase the magnet's strength? if yes, could you suggest some ways to do it? or is there any other way?
  11. Sep 19, 2015 #10
    An increase in the rate of change of the flux, thus producing an increase in the solenoid voltage, can be achieved simply by moving the magnet (or "plunger mechanism") more rapidly, since a faster moving magnet will cause the flux through the coil to change more rapidly.
  12. Sep 19, 2015 #11
    Yup...Just as i tot..!! but in my project the speed of the moving magnet has a upper limit, beyond which i may not be able to increase the rapidity. In that case what should i do?? increase the number of turns of the coil?
  13. Sep 19, 2015 #12
    Also increasing the number of turns obviiously increaese the rate at which the flux changes, since if you double the flux you will also double the rate at which the flux changes through the coil. Same with increasing the strength of the magnet.
  14. Sep 19, 2015 #13
    Remember that the total effective flux through the coil is the flux through one turn times the number of turns.

    Let's illustrate this with an example. Say the magnetic field is uniform across the cross section of the solenoid but it changes as B*sin(wt), where f = w/(2*pi). Therefore the flux is B*N*A*sin(wt), where N is the number of turns of the soilenoid and A is the area of the solenoid turn. If you take the derivative of the flux to get the "rate" at which it is changing you get B*N*A*w*cos(wt). So B (which is related to the "strength" of the magnet), N (turns), A (diameter of the coil), and w (how fast the magnet moves) arep retty much the parameters you can control. Lowering the resistance of the wire by using a low resistivity material and heavy gauge wire will also perhaps be beneficial
  15. Sep 19, 2015 #14
    Thanks leright.

    Hey tell me something, the copper coil through im moving the magnet, does it has to be of particular area ,that is, the pie*r^2 of the coil should be larger to get a high rate of change in magnetic flux?
  16. Sep 19, 2015 #15
    If you have to compromise between gauge of wire and number of turns I would choose mrore turns. I don't know what types of constraints you are dealing with.
  17. Sep 19, 2015 #16
    Thanks leright. With that para, you just answered almost all of my doubts except one. And that is about of the strength of permanent magnets!! Earlier i was searching the net for powerful magnet and one of the site said N35 is the strongest and can operate in high temperature like 150 degree celius. Could you help me understand thats true?
  18. Sep 19, 2015 #17
    Yes, I believe N35 is the strongest room temperature permanent magnet.
  19. Sep 19, 2015 #18
    Thanks again..!! cheers
  20. Sep 19, 2015 #19
    Good luck with your project. Let us know how it turns out.
  21. Sep 19, 2015 #20
    Definitely!! Will post a picture as well..!!

    But just to understand, how much power do you think i can generate if the turns are like 1600 with N35?
  22. Sep 19, 2015 #21
    One tip, instead of moving the magnet into a stationary coil it might be best to use a configuration like a generator, which rotates a coil with a crank in a magnetic field produced by a stationary magnet. Of course, the amount of power is limited to the amount of mechanical power you put in, but there will be of course some losses.

    You might want to read up more on Faraday's Law and electric generators to get some ideas.

    You'd be surprised how much work it takes to light up even a very small light bulb with a hand crank generator. :)
  23. Sep 19, 2015 #22
    So if you want to light a 20 Watt light bulb you will need to give at least 20 Watts of mechanical power. You burn about 300 kcalories per hour walking briskly (I think), or about 1.2*10^6 Joules/hour, or 333 Watts of power.
  24. Sep 19, 2015 #23


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    They are the same.
  25. Sep 19, 2015 #24


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    Do some experimental engineering . Design and make something starting from best guess at configuration and with available materials .
  26. Sep 19, 2015 #25


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    One way of improving electrical machines, that rarely seems to get mentioned (except by Eric Laithwaite), is improving the magnetic circuit.
    Air is thousands of times less permeable than iron (I leave that vague, because there is a lot of variation between different alloys), so the length of air gap the flux has to pass through is often the limiting factor.

    For rotary machines this idea is well understood and applied (except by inventors of free or cheap energy generators!) but I'm not sure how it will work here.
    Perhaps instead of one large sliding magnet in one long coil, it would be better to have a series of short magnets sliding between a toothed stator and a series of coils?
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