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Need help with circuit design

Discussion in 'General Electronics Discussion' started by Itgetsthehose, May 8, 2016.

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  1. Itgetsthehose


    May 8, 2016
    I have an electric motor, 12V...When turned on, the motor instantly goes full speed. When the motor is turned off, it instantly stops. This "full speed or nothing" causes a lot of torque and noise in the apparatus the motor is mounted to.

    Basically, I'm wondering if I can have a circuit built that, when turned on, would slowly increase the current (over several seconds) until full speed is reached...AND then when turned off slowly decrease the current (over several seconds) until stopping. Automatic variable resistors? Caps? Magic fairy dust?

    The motor I'm using cannot be swapped out for any other type of motor. I can get more info on the motor if that is needed to answer the question.

    Is doing something like this possible? I'm obviously a complete nube, take it easy on me :)
  2. Itgetsthehose


    May 8, 2016
    It's a 12 volt brushed motor, draws about 30 amps
  3. davenn

    davenn Moderator

    Sep 5, 2009
    is that current running when under load or no load ?
    and what is the load ?

    the initial start up current is likely to be at least double that

    soft start is the main thing you are looking for .... and for stopping, not sure

  4. Itgetsthehose


    May 8, 2016
    I'll have to test the load, not sure.

    Hmm, so what happens if you use a thermistor and a capacitor? A capacitor will store some electricity and allow for a soft stop?
  5. HellasTechn


    Apr 14, 2013
    so you need dc motor drive ?

    I came up with this after google search.

    stopping should not cause trouble.
  6. hevans1944

    hevans1944 Hop - AC8NS

    Jun 21, 2012
    A PWM controller with a slow ramp up to 100% duty cycle when turned on, and a slow ramp down to 0% duty cycle when turned off should do it.
  7. Itgetsthehose


    May 8, 2016
    Hevans, I saw some pwm controllers on the internet, but they all look like they're manually controlled? Maybe I'm not fully understanding how they work.

    What would happen if I just wired in a large capacitor between the on/off switch and the motor? Would a capacitor steal some amperage on startup, and then release it on shut down, hence giving soft starts and stops?
  8. duke37


    Jan 9, 2011
    A capacitor would do it but at 30A it would be enormous. You would need to bypass the charging resistor after a suitable time. If you did not use a charging resistor, then you would probably melt the switch.
    The same principle is used for the PWM controllers to raise the current but the charging resistor and capacitor are more realistic.
  9. Alec_t


    Jul 7, 2015
    A cap in parallel with the motor would make hardly any difference on start-up and not much on stop, unless it had a humongous value.
  10. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    The best option is what Hop suggests, plus some magic fairy dust to control it.
  11. ramussons


    Jun 10, 2014
    I go with Hop. PWM has the ability to maintain the required torque (at least to a large extent) al low speeds.
  12. hevans1944

    hevans1944 Hop - AC8NS

    Jun 21, 2012
    You can visit this page to see what your options are. Buy or build, but I would avoid any of those five dollar imports you can find on the web. You are switching some serious current here and proper heat sinks and back emf protection are required, especially to dissipate the energy when the motor slows down. Motors can store a LOT of mechanical energy as angular momentum. Depending on gearing, some of this energy will be converted to electrical energy as the motor slows down. There has to be a way to safely dissipate that energy.
    Last edited: May 10, 2016
  13. Itgetsthehose


    May 8, 2016
    Thanks...I'm checking into it and doing some research. If I get stumped I'll probably check back in
    hevans1944 likes this.
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