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Figuring out motor power needed

  1. Feb 15, 2012 #1
    It's been many years since I was doing physics in college, and I am beyond rusty... and I could use a little help.

    I am trying to fabricate a little hobby project involving a motorized platform (like a radio controlled car) that I can move at somewhat precise speeds and program relatively precise turns and movement into it.

    The idea is to use stepper motors to achieve this. But I really have no idea what torque I will need from the motors.

    -Weight of the unit will be 5lbs
    -I want to use rollerblade wheels that have a diameter of 80mm
    -I want the unit to be able to travel at speeds up to 20meters/sec (ideally)
    -I have read that suitable acceleration would be half top speed, so 10m/s^2
    -I would like the unit to be able to accelerate at this level on inclines up to 10 degrees

    I realize there will be mechanical and electrical losses in the drive system, which I am guesstimating would be perhaps 20%. But I really have no idea where to start figuring out the torque I need. The motors are rated in oz-in.

    I am sure this is less complicated than I am making it, but.... help! :)
  2. jcsd
  3. Feb 16, 2012 #2
    if your 5lb car is resting on an inclined plane of 10deg it would try to roll back. To stop it you would need a force of atleast F= m*g*sin(10) which for a wheel of .08 m equates to tourque of T=F/.08
    double that tourqe and you would be fine.
    Tourque output of steppet motor decreases with rpm so make sure your motor can provide the torque calculated above at required speed of 20m/s. which for .08 m wheel equates to 20/(pi*.08) rpm.
  4. Feb 16, 2012 #3


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    if you are interested in the acceleration that you will get, then you need to know more than just the torque. You need to know the Power available - that is Torque times the Speed. Doubling the min calculated torque would be a first stab at it but what speeds do you want?
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