choosing the right motor

Discussion in 'The Projects Forum' started by devalvyas, Apr 23, 2009.

  1. devalvyas

    Thread Starter Active Member

    Nov 11, 2008
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    I am confused in choosing the right motor for my project. I am working on automating a baby cradle. i have currently used a small dc motor, 6 V, 9000 rpm produces around 320 gm-cm stall torque at around 5 amp current.

    now i am not using any gears and driving the load directly using a small shaft of around 3mm thickness. This means that actual torque produced at 3mm shaft dia is 2.1 kg at 1.5mm.

    the application involved running motor at stall as the motor will start from rest every time it has to pull the weight.

    the motor is doing fine in pulling the weigh, but gets heated up very fast. i tried replacing the motor by ac universal motor of 75 Watts capacity 12000rpm. this motor is taken out from a hand blender. i dont know the torque as yet. but that too gets heated up fast.

    how do i select the motor such that it does not get heated up fast?
     
  2. electronictech

    Active Member

    Apr 1, 2009
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    Since you are driving a shaft directly and there are no gears, I would suggest using a unipolar stepper motor that is driven with a micro-step driver. Stepper motors have ratings for the stall torque, so sizing it to your application should be easy.
    Stepper motors generate heat just as other motors do, but if the stepper motor is properly sized to the load then the heat generated should not be an issue.

    Good luck and have fun!
     
  3. devalvyas

    Thread Starter Active Member

    Nov 11, 2008
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    what should be the ideal torque sizing for a motor? should the motor stall torque be atleast twice of the actual torque its pulling? is there any formula or thumb rule for load sizing?
     
  4. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    Why not use gear or belt to boost torque ?
     
  5. devalvyas

    Thread Starter Active Member

    Nov 11, 2008
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    i want to keep the motor as silient as possible. any drive mechanism will increase the noice level.

    and even if i use a belt or gear, how shold i size the motor? how much should the torque of the motor be compared to load torque?
     
  6. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    Is it a full-sized cradle? What is it's natural period of oscillation. Once swinging, how much torque is required to sustain oscillation? Possible drives: step motor[ no experience], rotary solenoid, worth looking into, linear solenoid, a possibility. One more question, weight of cradle & length of suspencion??
     
  7. steveb

    Senior Member

    Jul 3, 2008
    2,433
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    You're scaring the crap out of me. I keep envisioning an unattended baby going up in flames. :eek: Hopefully this is just a project to demonstrate a conceptual idea, and not a way for you to watch the ball-game while your wife is shopping.
     
  8. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
    5,675
    899
    I like Bernard's suggestion of periodic pulsing. We had a baby swing that worked like that -- the rhythmic clicking would put us to sleep before the baby.

    Failing that, consider a large RC servo. The GWS SO4 has 13 Kg-cm torque at 6V ($17.50) and the S666N has 28.8 Kg-cm torque, but costs $49. Both exceed your 320 g-cm spec.

    It would be relatively easy to make a circuit to drive it back and forth continuously.

    John
     
    Last edited: Apr 24, 2009
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