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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.


34 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

  12. Mustafa Mirza

    Dear Coleman Benson.

    I’m looking to make a hobby robot of max mass 10kg incline 30degree incline using 2 drive motors and all other parameters included the max torque your application gives me is about 300 ozf-in. i was looking to buy the pololu 100:1 or 50:1 motor set for better speeds however for 100:1 the specification lists the stall torque as 220 ozf-in. My question is how high is the stall torque supposed to be. ive read above that you said its basically 4x-5x the max torque listed by the application and in your view point which motor would be more suitable for me? the 50:1 or 100:1? please reply as soon as possible as i have a tight window to order this motor.

    • Coleman Benson

      @Mustafa Mirza If the sizing tool gives 300oz-in, the motor needs to have a stall torque of around 1200 to 1500oz-in stall torque. Neither motor would be suitable.

  13. Lars Poster

    The incline value…if I enter a value of 20 degrees and a desired Runtime of 20 minutes, is the provided value for battery capacity calculated on the basis of the 20 minutes having been at 20 degrees for the whole period? ie.the Extreme case of 20 minutes running uphill at 20 degrees?


    • Coleman Benson

      @Lars Poster Correct. Since it’s rather difficult to estimate the percentage of time the robot will be going up an incline, the incline value is largely there for the max torque required. The battery estimate comes from the estimated current draw. Feel free to recalculate based on 0 degrees (to get an estimate of the battery pack for flat terrain) and then estimate the time it will be on flat terrain vs. incline to get a better approximation of the actual batteyr pack you’ll need.

  14. M.Ismail

    Thank you so much for this helpful tool & tutorial , but u have 1 question :
    *** If the torque is for example 1 N.m. , now i want to use 2 wheels connected together through a shaft (like simple cars) and will connect one motor only to one of them from outside the car using coupling. does this mean i need a motor with 2 N.m. so 1 n.m. per wheel OR do i need a motor of 1 n.m. and that will be divided on the 2 wheels to be 0.5 n.m. for each ??

    i.e : the torque output from the tool is it The Total Required torqur OR is it per wheel ??

    thank you in advance

    • Coleman Benson

      @M.Ismail If you still only have one drive motor per side (regardless of if you use multiple wheels or tracks), you still need to put that you have two drive motors. If you have four wheels and four motors, you put four drive motors. If you have six wheels but each set if three is connected to one side, you still need to put two drive motors.

  15. M.Ismail

    Sorry for typing mistakes in my first reply , i still have one more question:

    If i have my motor with brakes should i still worry about stall torque ?

    • Coleman Benson

      @M.Ismail Yes, and ideally you would not choose your motor based on stall but rather the maximum continuous torque the motor can provide, which is ~1/5 to 1/4 stall. The brake will simply prevent it from rotating but won’t help it move.

  16. M.Ismail

    for the motor_wheel issue : thank you a lot i understand better now .

    well the datasheet i have has 2 terms only : one is called “rated torque” which i use in selecting motors and the other is called “Peak torque” and i do nothing with it ..i simply choose the motor that has rated torque=my required torque at rated speed=my required speed ,so is this enough ?

    • Coleman Benson

      @M.Ismail “Rated torque” should be the torque at which the motor can be operated continuously for the longest period of time, so you can base yourself off that. Peak torque should only be for a few seconds.

  17. Vivek

    Really nice tool… quite practicle too…but didn’t really understand role of acceleration in the calculation. I had calculated power using rolling friction coefficient, which was overestimated. Would be helpful if you share your thoughts..thanks!

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