how to gradually start up a DC motor?

Discussion in 'General Electronics Chat' started by ann0yed, Oct 18, 2010.

  1. ann0yed

    Thread Starter New Member

    Oct 18, 2010
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    I have a tiny DC motor that I'm powering with a few watch batteries. I'd like to build some tiny component that will gradually spin up the motor (preferably analog); right now it just goes full throttle when the circuit is closed. I've read about soft-starts and thyristors but its all foreign to me... Thanks!

    -jonathan
     
  2. Kermit2

    AAC Fanatic!

    Feb 5, 2010
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    The component you seek is called a Rheostat.

    The BIG ones back in 'the day' were sometimes made with a wooden barrel, two copper plates and some salt water.

    :)
     
  3. Audioguru

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    Dec 20, 2007
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    A rheostat in series with a motor reduces the torque so much that nothing happens as you slowly turn it then suddenly the motor starts running too fast.

    You need pulse-width-modulation that gives high frequency but full power pulses so that the width of the pulses controls the motor's speed and the torque is high enough to strart the motor at a low speed.

    A wheelchair uses pulse-width-modulation for adjusting its speed. Can you imagine a huge and hot rheostat used instead that suddenly starts the motor to smoke the tires?
     
  4. ann0yed

    Thread Starter New Member

    Oct 18, 2010
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    Hey, thanks for the replies but I want this spin-up to happen automatically, on its own. I just want to flick a switch and the motor gradually spins up to full rpm over the course of several seconds...
     
  5. Kermit2

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  6. Audioguru

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    Flick your switch that slowly charges a capacitor through a resistor. The slowly rising voltage on the capacitor controls the width of the pulses that slowly start the motor to run. The voltage keeps rising then the speed of the motor keeps increasing until it is max.
     
    Last edited: Oct 18, 2010
  7. ann0yed

    Thread Starter New Member

    Oct 18, 2010
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    Yeah, that's the concept I desire, but I need this thing to be tiny, like the size of a peanut! Is anything like this possible?
     
  8. ann0yed

    Thread Starter New Member

    Oct 18, 2010
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    Hey Audioguru, that sounds promising! Ugh, see, this is where my infinitesimal knowledge of electronics leaves me with an empty head. I see capacitors and diodes in the RadioShack drawers all the time but how can one go about determining which ones will best suit a project? I'm using a tiny cellphone vibrator motor and a 3v coin cell...

    -jonathan
     
  9. Audioguru

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    You need to understand electronics and find out the spec's of your motor and battery cells. Then you can calculate the value of the resistors and capacitors needed.
    RadioShack used to sell many parts. Now their parts are old, huge and over-priced.
     
  10. ann0yed

    Thread Starter New Member

    Oct 18, 2010
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    "You need to understand electronics" - Ha, yes, I agree with you there. I probably will increase my knowledge a bit after this exercise.Ok, well that gives me a starting point, thanks man, I was hoping this would be doable in simplicity and size and it sounds like it is.

    -jonathan
     
  11. Audioguru

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    A motor speed controller with PWM adjusted with voltage is not simple. A few of us might attempt to design such a circuit and maybe somebody will design the circuit for you. You can make it with tiny parts that are placed and connected by a robot.
     
  12. ann0yed

    Thread Starter New Member

    Oct 18, 2010
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    Ok, not simple. <snip>. So are you saying people on here do freelance/contract work?

    -jonathan
     
    Last edited by a moderator: Oct 20, 2010
  13. tom66

    Senior Member

    May 9, 2009
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    I have wondered how this is done. I don't think it is done using a complex motor driver. I have seen it in bargain basement devices (a vacuum cleaner and a blender.) I believe it is done using a NTC device. These devices decrease their resistance over a few seconds to almost zero allowing a motor to get to full speed.
     
  14. Audioguru

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    A cold PTC resistor applies full power to the degaussing coil around the picture tube of an old colour TV. It warms and increases its value so then it reduces the current in the coil.

    You cannot use a resistance to slowly start a motor since the resistance reduces the torque of the motor then it cannot start slowly.
    A cheap light dimmer uses PWM and also cheap RC toys so it is not complex nor expensive.
     
  15. tom66

    Senior Member

    May 9, 2009
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    I suspect you can do this but you have to get the time constant right and it has to be designed for the motor, so it gets the motor spinning slowly then allows it to reach full speed. i.e. it's not a drop in solution.
     
  16. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
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    Can you share your secret with us? How will it soft start the DC motor?

    John
     
  17. beenthere

    Retired Moderator

    Apr 20, 2004
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    Sorry - Jessica was a spammer & got banned. Her secret is lost to us forever.
     
  18. retched

    AAC Fanatic!

    Dec 5, 2009
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    NNNNNOOOOOO!!

    She had the cure to cancer too!

    Where is the anti-beenthere list? ;)
     
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