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Drive Motor Sizing

Drive Motor Sizing

The Drive Motor Sizing Tool is intended to give an idea of the type of drive motor required for your specific robot by taking known values and calculating values required when searching for a motor. DC motors are generally used for continuous rotation drive systems, though can be used for partial (angle to angle) rotation as well. They come in an almost infinite variety of speeds and torques to suite any need. Without a geardown, DC motors turn very fast (thousands of revolutions per minute (rpm)), but have little torque. To get feedback of the angle or the speed of the motor, consider a motor with an encoder option.

Gear motors are essentially DC motors with an added geardown. Adding a geardown both reduces the speed and increases the torque. For example, an unloaded DC motor might spin at 12000 rpm and provide 0.1 kg-cm of torque. A 225:1 geardown is added to proportionally reduce the speed and increase the torque: 12000 rpm / 225 = 53.3 rpm and 0.1 x 225 = 22.5 kg-cm. The motor will now be able to move significantly more weight at a more reasonable speed.

If you are not certain about what value to enter, try to make a good “educated” guess.

Click each link for more explanation about the effect of each input value. You are also encouraged to look at the Drive Motor Sizing Tutorial, where you will find all the equations used in this tool complete with explanations.


Output (for each drive motor)

* Note: Although kg-cm is used throughout the RobotShop site, it is actually kgf-cm. Similarly, oz-in should actually read ozf-in throughout the site.


20 Responses to “Drive Motor Sizing Tool”

  1. larry seibold

    thank you for the drive motor tool. It would be nice to save or print the results in some easy manner. I would like to know how you handle friction at the various locations, generally promotional to weight. I would also recommend a %slope time variable to separate peak power from watt-hrs.

  2. Samantha

    It would be great if it showed you the motor options after.

  3. Frank

    Thank you for this tool! Do the output(for each motor) refer to the free-run and stall torque? Or, are these values for a specific operating point?

    • Coleman Benson

      The torque value would what is required in that specific situation, and can be considered the maximum continuous torque. Based on observations and what motor manufacturers provide, the stall torque would be ~4x or 5x this value.

  4. Mitch Berkson

    Can you tell me why my result is different than yours?

    Mass: 25 kg
    Motors: 1
    Wheel radius: 0.2 m
    Velocity: 2 m/s
    Incline: 0
    Acceleration: 1 m/s2

    You get:
    Torque = 78 kgf-cm

    I get:
    Torque = (25 kg) x (1 m/s2) x (0.2 m) x (1/9.8 m/s2) x (100 cm/m) = 51 kgf-cm


  5. Mitch Berkson

    Or in this simpler example:

    Mass: 1 kg
    Motors: 1
    Wheel radius: 1 m
    Velocity: 0 m/s
    Incline: 90
    Acceleration: 0 m/s2

    I would expect a torque of 9.8 N-m, but the calculator says 15 N-m.

  6. Mitch Berkson

    Thanks. The calculator and explanation are so well done that if you’re going to throw in an arbitrary number, it’s a shame not to mention it up front.

  7. Mitch Berkson

    Whoa. Now I see that you had efficiency as a parameter. Sorry about that. You can delete any superfluous comments I made.

  8. Amit

    HI! I was building an Electric Wheelchair. And i calculated the output values required for the motor using this Tool. This tool is very useful. Can you please provide the formulae used in this tool to calculate torque, speed and power? Thanks

  9. Karol

    @Coleman Benson I’ve read the tutorial provided, but I have 2 questions regarding the content.

    #1 Can you explain in details what does the efficiency value stand for, and where should I look for it.
    #2 Why didn’t you account for rolling resistance ?

    • Coleman Benson

      #1 There are general losses in efficiency: DC brushless motors are not 100% efficienct, nor are the gearing to which they are connected. Batteries are also not 100% efficient, and there are also losses in cabling. The value provided would be a total of these losses. Since none of the parts are known until after choosing the products, we can only estimate a total efficiency.
      #2 Honestly for simplicity, but if you can derive a simple equation using the input above, we’d be happy to add it.

  10. Rabea

    Hi. i need information about EMA actuators Application in Mobile Robotics

  11. Rabea

    Dear Coleman Benson
    could you send to me your contect

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