DC motor rough speed control

Discussion in 'General Electronics Chat' started by bug13, Jul 20, 2013.

  1. bug13

    Thread Starter Well-Known Member

    Feb 13, 2012
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    Hi guys

    A friend of mine asked me, he wants a QUICK and DIRTY way to control a 12V DC motor between 90RPM to 100RPM (+/- 10%)at a constant load, what is the easier way to do it?

    Thanks.
     
  2. panic mode

    Senior Member

    Oct 10, 2011
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    Pwm .
     
  3. strantor

    AAC Fanatic!

    Oct 3, 2010
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    how many amps is the motor rated? how many RPM is it rated? give all the info you have on the motor.
     
  4. bug13

    Thread Starter Well-Known Member

    Feb 13, 2012
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    Good question, my friend didn't give me tho info, but he said something about a DC motor uses in a car's wiper. (I assume that's the one he has)
     
  5. bug13

    Thread Starter Well-Known Member

    Feb 13, 2012
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    Is there a dirty and easy way to get to 90RPM to 100RPM +/- 10%? like some sort of estimate of a good thumb of rule or something?
     
  6. tubeguy

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    Nov 3, 2012
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    bug13 likes this.
  7. strantor

    AAC Fanatic!

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    The quick and dirty answer could be as simple as putting a rheostat in series with the motor, but not knowing anything about the motor, this could be a terrible suggestion.
     
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  8. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    A regulated and adjustable voltage supply will run a DC motor at a more-or-less constant speed. So something like a bench power supply or a LM317 adjustable voltage regulator will work.

    The big problem is your speed range. To reliably get 90 RPM you need a geared DC motor so the motor can be going many times faster RPM but the output shaft is going 90 RPM. Otherwise if you try to run the motor too slow it will have no effective torque and be very unstable in RPM.
     
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  9. bug13

    Thread Starter Well-Known Member

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  10. MaxHeadRoom

    Expert

    Jul 18, 2013
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    You can set or provide the voltage at a fixed value sufficient to provide the torque need for the particular load, this is only good for constant torque loads however.
    Anything else and some kind of feedback, current sense resistor etc is needed.
    Max.
     
  11. THE_RB

    AAC Fanatic!

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    That's true with higher RPMs. At very low RPM a DC motor will be near or below it's no-load stall speed, so will stop and start or behave erratically, and even the smallest change in load will stall the motor. You also have detent load-torque pulses caused by the motor poles and commutation.

    90 RPM is well below the stable open-loop running speed of most small DC motors.

    If Bug13 could provide some motor specs that would help a lot, particularly the RPM per volt value (sometimes called k/V). A photo of the motor would help a lot too. :)
     
  12. MaxHeadRoom

    Expert

    Jul 18, 2013
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    That really depends a lot on the technology of the motor, how many poles, skewed rotor etc.
    Most of the T.M. motors, which are not generally Hi-Tech, can perform below 90rpm with simple SCR type control.
    Max.
     
  13. bug13

    Thread Starter Well-Known Member

    Feb 13, 2012
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    I will get more info of my friend when I see him next time, I don't have any more info at the moment:(
     
  14. THE_RB

    AAC Fanatic!

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    Sorry but I've never seen a typical "12v DC motor" that was good at 90 RPM. Most have a top speed of a few thousand RPM and 90 RPM performance is either very poor or simply below the stall speed.

    I'm not sure what a T.M. motor is.
     
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