Timed 12v reversed polarity

Discussion in 'The Projects Forum' started by billybob12, Jan 18, 2010.

  1. billybob12

    Thread Starter New Member

    Jan 18, 2010
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    I am trying to to rev. the polarity on a 12v dc motor that turns 400 rpms. My problem is that I need to do it very fast. One rpm one way and then reverse it to one rpm the other. Is this possible. Thanks Anthony
     
  2. SgtWookie

    Expert

    Jul 17, 2007
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    It is not advisable to attempt to suddenly reverse the direction of a DC (universal) motor, as this will result in very high current and resulting power dissipation as heat.

    Just as an example, your motor might be rated for 12v @ 1A when running at 400 RPM, however when the motor is first started from a stop (or at stall; rotor locked), it may draw many times that current. If the motor is rotating at full speed and the polarity is suddenly reversed, it will have nearly double the current flow as it would from stall.

    If you wish to reverse the motor as quickly as possible without damaging it, you should first remove power, and short the + and - terminals of the motor until it is stopped, then remove the short, and apply power to the motor in reverse polarity.
     
  3. billybob12

    Thread Starter New Member

    Jan 18, 2010
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    Do they make a switch that could do this. Motors are cheap and so are batteries.
     
  4. SgtWookie

    Expert

    Jul 17, 2007
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    Then just use a DPDT switch or relay.

    Don't expect either the motor or switch/relay to last a long time, though.
     
  5. Chris15

    Active Member

    Apr 15, 2009
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    SgtWookie is right

    You can wire a DPDT to change the direction yes but like SgtWookie it is not reccomended and will shorten the life of all the components used. the motors would have to be at a complete stop before you reverse the polarity. A motor (dynamo) can generate electricity as it spins for example if you connect just an LED to the leads of a small motor and you manually crank the motor shaft (the right way as an LED is bias) the LED will flicker, so if you have a large motor spinning at 400 RPM and you switch the polarity that is very bad circuitry and is not reliable.

    Chris

    What is it that you are trying to achieve?, there may be a way around it,
     
  6. billybob12

    Thread Starter New Member

    Jan 18, 2010
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    I am trying to give the illiusion of a spinng blade without it makeing a full 360 revelution. It needs to spin at 400 rpms. If it was to make 1 complete revalutation it would be against the law
     
  7. cumesoftware

    Senior Member

    Apr 27, 2007
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    I think you might need to use a stepper motor for that. Stepper motors allow you to control the rotation in a precise manner. But I think you might need a gearbox as well to speed up the blade, ans the stepper motor might not be enough.
     
  8. KMoffett

    AAC Fanatic!

    Dec 19, 2007
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    It's not going to happen. Imagine your motor spinning CW at 400RPM for 0.15 seconds (that's one 360° rotation at 400 RPM), stopping "instantly", "instantly" spinning up to 400RPM CCW for another 0.15 seconds, back to the starting position....and on and on. It's basically limited by the laws of physics governing mass and inertia.

    Can you elaborate on that law?

    Ken
     
    Last edited: Feb 17, 2010
  9. billybob12

    Thread Starter New Member

    Jan 18, 2010
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    The unit is to attract animials during hunting season. After Dec. 1st it is leagal to let the device spin at 360 deg. But for the first 6 weeks of the season It cannot spin. It can flap back and fourth but it cannot make a complete rev. I have made numerious devices that move back and fourth but it doed not have the same effect. The best I can get it to flap is 90 deg. Thanks Anthony
     
  10. SgtWookie

    Expert

    Jul 17, 2007
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    Ahh, I see. So, you really don't want the "bait" to spin at 400 RPM; it's much slower than that, right?

    Rather than reversing the motor (which will kill it and the power source quickly), consider using a dual V-belt drive.
    Two small-diameter pulleys on the motor, two larger-diameter pulleys on the "bait wheel". One belt goes directly from a motor pulley to a "bait wheel" pulley. The other is twisted 180° so that it goes in reverse.
    Make a belt tensioner that's actuated by a lever that gets struck by the bait wheel, that alternately disengages/engages the belts.
     
  11. retched

    AAC Fanatic!

    Dec 5, 2009
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    Is it for scent dispersement? If so, the reversing prop will just switch from suck to blow, not really helping in the dispersion.
     
  12. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    Thats aout 7 flip-flops / sec. 'do not think our coyotes can shake a rabbit that fast. Several weeks ago a post about like this came up; solutions included non reversing motor.
     
  13. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    Loook at cv935 , with rack and pinion, think you can get almost 360 deg.Flexable steel cable will work as well as gear.
     
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