Dc motor start, reverse automatically and stop

Discussion in 'The Projects Forum' started by Jabs, Nov 11, 2014.

  1. Jabs

    Thread Starter New Member

    Nov 11, 2014
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    Hello folks, im new here. I need help with ideas or methods in creating a circuit for a project at school.
    Im using a high torque low speed dc motor, 5rpm, 12v, 5amps. It supposed to work like this:
    I push a knob, the motor turns on, rotates the shaft about 145degrees, hits a limit switch(of some kind) automatically reverses the polarity to reverse the motor direction.
    The motor turns, until it reaches back at its initial position and shuts off when it hits another switch(of some kind,a limit switch?) note that the motor is not making a complete rotation.
    I am only pushing the button to turn on. The system must automatically reverse and stop by itself.

    Any help and ideas, circuit diagrams will be helpful. Im not knowledgable about circuits however. Im a student pursuing mechanical eng bsc.
     
  2. djsfantasi

    AAC Fanatic!

    Apr 11, 2010
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    Note that practically speaking, you need a pause before reversing direction. Motors don't like immediate changes in polarity. That being stated, what you want is very do-able.

    What is this for? A homework assignment? A device for your own use ? Or for someone else? How much will it be used?
     
  3. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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  4. Jabs

    Thread Starter New Member

    Nov 11, 2014
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    It is a coconut dehusker. A blade will be welded onto a shaft. The shaft turns moving the blade that is perpendicular to it. Its already there, just to automate it using the electrical scenario I stated. Its for domestic use for home owners. Its a final year project. I will be building after xmas this year or early January. I live in the Caribbean so its a feasible project :p
     
  5. Jabs

    Thread Starter New Member

    Nov 11, 2014
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    Initially we considered using mechanical motion/cams but its too much linkages and to get a motor to suit it is difficult as high torque small motors are hard to find and expensive.
     
  6. djsfantasi

    AAC Fanatic!

    Apr 11, 2010
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    There are many circuits on this site which you may find useful. Search for "relay H-bridge" or H-bridge... Here is a link to such a circuit developed by SgtWookie. The schematic is below:

    [​IMG]
     
  7. williamj

    Active Member

    Sep 3, 2009
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    I suggest you take mcgyvr's recommendation, especially if there has to be any amount of speed involved.
     
  8. MaxHeadRoom

    Expert

    Jul 18, 2013
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    As others have mentioned, if you constantly reverse the motor, especially from high rpm or high inertia for many times over a short period without allowing stopping between reverse, the motor will heat up and could cause failure of the motor itself and/or burning of relay contacts.
    Max.
     
  9. MrChips

    Moderator

    Oct 2, 2009
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    TNT to de bone, eh? Think of windshield wiper motors. They go back and forth and stop at the starting position when the switch is turned off.
     
  10. Denesius

    Member

    Feb 5, 2014
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    I think if you look at one, you'd realize the windshield washer motor most certainly does not go back & forth. It runs in one direction and drives a cam on a small wheel, attached to a larger lever arm, and as a result the larger arm (the wiper drive) goes back & forth!
     
  11. djsfantasi

    AAC Fanatic!

    Apr 11, 2010
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    Which is using the mechanical principal Mcgyvr mentioned earlier. Although I've supplied information about the H-bridge, I agree a mechanical solution is the best.
     
  12. MrChips

    Moderator

    Oct 2, 2009
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    Sorry for my wording. You are absolutely correct.
    What I wanted to point out is what everyone is saying. Use a motor that turns in one direction and use mechanical means to reverse the action.
     
  13. djsfantasi

    AAC Fanatic!

    Apr 11, 2010
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    @MrChips,
    Didn't mean to appear to be argumentative with you. I've use windshield motors for animated Halloween props, and there is a lot of confusion as to how they work. Just trying to clear that up.
     
  14. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Also think of piston, con rod and flywheel as a mechanical answer.
    I imagine you do not need really high speed operation so ~1:5 reduction by belt pulley will increase the motor torque x5.
    Max.
     
  15. Jabs

    Thread Starter New Member

    Nov 11, 2014
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    Yup trini. We considered using windshield wiper motors but they are not powerful enough. The motor im using is 91Nm of torque at 5rpm. The lab tech in controls lab told me we can use limit switches but the procedure will work when u push the botton, the motor turns hit the limit switch cuts off. Then u push button again and it reverses and hits other limit switch then stops at initial. I wanted it to reverse n stop automatically. I was reading up on timer relays where u can set the time the turn on or off the relay. Was thinkin if the motor stops with limit switch, timer relay turns on after set time and sends signal to reverse motor? Then it stops at the initial position with the other limit switch.
    Is this posible by the use of timer n limit switches? I understand the motor must stop for it to be reversed, so it can stop for below 1second.
     
  16. Jabs

    Thread Starter New Member

    Nov 11, 2014
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    Screenshot_2014-11-13-01-04-12.png

    I also obtained this circuit by emailing this Institute. I told them my situation and this is what they came up with. Can this also work?
     
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