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dc motor resistor in parallel to slow motor down

Discussion in 'Sensors and Actuators' started by supak111, Jan 10, 2013.

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  1. supak111

    supak111 ★ƃuᴉɯǝɥɔs sʎɐʍlɐ★

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    Apr 29, 2012
    I'm using a 12v GEARED DC motor to move something and need the motor to not move or move as little as possible after the power is OFF to the motor.

    Would adding a large resistor to the 2 wires on the motor add resistance and there fore the motor would move much harder if someone tries to move it by hand when there is no power to the motor?

    How do I figure out OHM/Watt of the resistor if a resistor would indeed help?

    Also would the resistor be a problem when there is no 12v power to the motor?

    PS. motor is never used for more then 1 second then a long pause so I don't think the heat on the resistor should be much of a problem.

    Thanks everyone
     
    Last edited: Jan 10, 2013
  2. davenn

    davenn Moderator

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    Sep 5, 2009
    you could just switch in a short cct across the motor terminals when the power is switched off
    that may counter the EMF generated if the motor is manually turned .... never tried it, some one else may comment :)
    Any gearing on the motor will also add to make it difficult to turn

    Why is all this important ?? what is the motor being used for ?

    Dave
     
  3. supak111

    supak111 ★ƃuᴉɯǝɥɔs sʎɐʍlɐ★

    327
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    Apr 29, 2012
    Gearing does make it pretty hard to move the motor but wind still movies it.

    I made a custom setup so I can rotate my tv antenna. So when the motor is OFF I don't want the wind to easily move the antenna out of the position I left it in. But I also don't want to complicate the setup. I need to know if this resistor is gonna do anything and if so will it mess with the power going to the motor when I do turn it ON once in a while.

    Thinking about using the same set-up for solar panels later
     
  4. supak111

    supak111 ★ƃuᴉɯǝɥɔs sʎɐʍlɐ★

    327
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    Apr 29, 2012
    Yup your right, short circuiting the DC motor terminals def makes it 4 5 times harder to turn, so that works.

    I still wish the resistor would work because it would be super easy to do. Think I'm just gonna find a random resistor tomorrow and try it
     
  5. davenn

    davenn Moderator

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    Sep 5, 2009
    putting a resistor permanently in circuit is going to affect the power supply to the motor and not in a good way

    thats why I suggested switching in a short across the motor when the power is switched out ...

    Dave
     
  6. Merlin3189

    Merlin3189

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    Aug 4, 2011
    Dave's short cct is the way to go - the 'shorter' the better!
    If you think about it, the highest resistor you can put across the motor, is an open cct, which will do nothing. A smaller resistor will allow some current to flow and provide some breaking. The smallest resistor you can use is a short cct, which allows max current and most breaking.

    But in your context this may not work. It only provides a force when the motor moves and the force is proportional to the speed. The wind may move the antenna very slowly and cause very little breaking. It's a bit like monting the ant in treacle: you wouldn't be able to turn it quickly, but a steady small pressure would easily turn it very slowly.
     
  7. supak111

    supak111 ★ƃuᴉɯǝɥɔs sʎɐʍlɐ★

    327
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    Apr 29, 2012
    Yea that is true, wind is small but constant pressure which will probably move it slowly no matter what.
     
  8. Merlin3189

    Merlin3189

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    Aug 4, 2011
    A bit late I know, but I just had a thought about this.
    Forget electronics. Use a worm gear. That gives a good stepdown gear ratio from motor to load and is almost impossible to run in reverse. So when the motor is not powered, the antenna is locked.
    I think it would theoretically be possible to turn a worm gear the wrong way, if you got friction and sticktion low enough, but in practice a typical high ratio worm is a one way device.
     
  9. supak111

    supak111 ★ƃuᴉɯǝɥɔs sʎɐʍlɐ★

    327
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    Apr 29, 2012
    That indeed is a good idea. I can make the pole the main gear and then just put the motor with the worm gear on the side to spin the main gear/pole.

    PS not only would the wind not be able to move it but I can still move the pole in both directions if I use a DC motor.
     
  10. Lord_grezington

    Lord_grezington

    115
    2
    May 3, 2013
    If you directly short the motor terminals, the motor will create a back emf which will brake the motor abruptly. if you connect a low value resistor across the motor terminals, this will use the back emf created from the motor to brake the motor less abruptly. if you used a higher value resistor the back EMF will be reduced and the braking of the motor will be much less.

    Keep in mind that the smaller value resistor you connect between terminals the higher your power consumption. Use Ohms law to calculate this.

    a worm and wheel gearbox will work well because it will eliminate back driving, using a risitor will only use the back EMF from the motor to electronically brake the motor. A worm and wheel gearbox can not be back driven, however inertia will still make a difference. (this will depend on the size of the motor).

    Graham
     
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