Choosing Motor

  • I've been working on a 4 wheeled robot roughly the size of a laptop and less than 10 lbs. that I plan on being able to use on yards. Right now I'm using two of these motors (https://www.amazon.com/3000RPM…fRID=JE2CVZE52PGWX2Q2HJVE) connected to the front wheels to control the robot. It's powered by AA batteries being connected in series such that they have collectively 24V. I tested it out today and on pavement the robot will move but it doesn't budge when placed in the yard. I was wondering if anyone had recommendations for more powerful motors given this situation or if any adjustments should be made.

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  • yup... for given motor size you can have either speed or torque but not both. if you want both, you need to go to a larger motor.

    motor you have selected is 3000RPM. using 30RPM gear motor will be 100x slower but... also have 100x more torque (nearly, as there are always some losses)


    you can also try to find a balance, maybe 500RPM would do. this would be 6x slower bit also have 6x the torque.


    https://www.amazon.com/dp/B08F…vdExvZ0NsaWNrPXRydWU&th=1


    another possibility is to use more motors (on each wheel, not only on front wheels) or to increase wheel diameter (lower friction). for example don't try to push baby stroller on a lawn, wheels on most modern strollers are too small since today most of areas are urbanized and roads are paved. but in the old days baby strollers had wheels much larger (similar to bicycle) for obvious reasons.

    1) read pinned topic: READ FIRST...

    2) if you have an issue with robot, post question in the correct forum section... do NOT contact me directly

    3) read 1 and 2

  • Thanks for you responses. I could definitely trade some speed for more torque. I was hoping for it to go faster than 3 mph and would be willing to buy gearbox but I would prefer to just buy another motor. I checked out the one panic mode sent, but the comments indicate that it would likely not be powerful enough. Would you have any other recommendations?


    Also, I tried increasing the wheel diameter too, but to no avail. That's an interesting comment about strollers though, I never thought about that.

  • i don't know how those comments would relate what they use motor for with what you are trying to use it for....

    and how do you read those comments? are you sure that they are made by people that know what they are doing?

    this means doing MATH... so...


    can you put some numbers together?


    3mph and 10bs is known but really not much else.

    wheel diameter is 0.5"? or 12"? size DOES matter...

    motor to wheel gear ratio is....? this DOES matter...

    etc.


    once we have numbers, we can do the math... at least basic for some sanity checks...


    using just the two mentioned factors and desired speed we can do some exercises:


    if the wheel diameter is 3", then wheel circumference is about 9" (3.14*D). if motor is directly coupled then one motor revolution is one one wheel revolution.


    if motor is 3000RPM that is 3000RPM*9"=27000 inch per minute, or 1620000inch per hour (IPH=60*IPM ) or about 26mph. if the goal is to move at 3mph, this approach is unacceptable as motor power is largely wasted. this motor would do great speed if the pavement friction was sufficiently low but moving through grass is not this case and since 90% of speed (and power) is wasted, mo wonder that motor stalls.


    but lets consider different case:

    if motor is 500RPM that is 500*9"=4500 inch per minute or 270000inch per hour (IPH=60*IPM ) or about 4.25mph - clearly a much closer number.

    and since the motor speed is now matched far closer, available motor torque would be maximized, much less power is wasted.


    so....can we still make it work with 3000RPM motors? yes but ultimate goal is to match the speed (the only thing we are looking at so far).


    we can do this two ways:


    a) change wheel coupling ratio to bring the speed 5x down (using gears or pulley or whatever). For example using gears or belt.

    if using belt make sure that smaller pulley is sufficiently large to ensure proper contact area (specially if not using timing belt)



    b) REDUCE wheel diameter some 8x (26mph/3mph). this would still give you 3MPH speed which would translate in optimized torque but it would mean that wheels would have to be just 0.375" in diameter.... which is not good here. that WOULD work for paved surfaces but for an off-road vehicle you want larger wheels. therefore get the needed transmission ratio at the drive (motor) instead of wheel size. which is exactly why cars have transmissions. engines run at high RPMs and would not have enough power to move vehicle without some gearing.


    once you get the correct wheel size, motor RPM and gear ratio, the only thing to play with is the motor power (just like with car engine horse power)

    1) read pinned topic: READ FIRST...

    2) if you have an issue with robot, post question in the correct forum section... do NOT contact me directly

    3) read 1 and 2

  • Thanks for your response, it is very thorough. I went over my design and determined that 4.7. in (.12 m) would be the preferred diameter. Following your calculations, this would mean the wheel circumference is about 14.8 in. Therefore, a rpm of 300 would give about 4.2 mph. According to the website it would be reasonable to assume it operating at 60% so a 500 rpm would be enough. Does 60% sound right?


    My current concern is regarding how much motor power I'll need. My guess is that this would be caused partly by friction, slowing the rpm, and partly from inefficiencies. According to https://saferroadsconference.c…-Roadside-Grass-Types.pdf, the coefficient of friction of wet grass (the worst case) would be roughly .2. I do not have an exact weight for my model, but will set an upper bound of 10lbs (4.5 kg). If I want to go from 0 to 1 mph (.5 m/s) in a second then that will require an acceleration of .5 m/s^2. Since Tnet = Tmotor - Tfriction = Tmotor - r * Ffriction = Tmotor - r * m * g * mu = m*a => Tmotor = m * a + r * m * g * mu = 4.5 * (.5) + (.12) * (4.5) * 9.8 * .2 = 3.3 (roughly). This mean I would require at least 3.3 Nm of torque. Now, I'll probably need to calculate for things like hills and inclines too, which I can adjust my calculation for, however, my question is, assuming 2.5 Nm of torque is needed, how much inefficiency should I assume. In other words, if I theoretically need 2.5 Nm, would it be safe to assume 3 Nm of torque would be enough in the real world to get 2.5 Nm? Is there a general rule of thumb for it?


    The link you sent on amazon says that the rated torque for the 500 rpm one is .4 Kg*cm. I would think the units would be Kg * cm * (m/s^2) but assuming these are the units, the torque would be 40 Nm. Does this sound correct to you?


    My last question is how important is motor power? I didn't see any information about the motor power on the website.

  • problem is that amazon supplied info is often incomplete and merely marketing garbage.

    i would much rather see ling to a real datasheet written by someone technical, not someone trying to make a sale. i would like to see many other things like stall torque, stall current, holding current, performance curves etc.


    one thing is sure, 40Nm value is too large.... (excessive).

    this would be a torque value for a huge motor, something that can power real vehicle, not a toy car.


    according to

    https://www.onlineconversion.com/torque.htm

    kgcm is about same as Nm so 0.4kgcm is close to 0.4Nm.


    this must be torque at rated current of 0.3A

    but DC motors can do a lot more - briefly (or they will burn out)

    1) read pinned topic: READ FIRST...

    2) if you have an issue with robot, post question in the correct forum section... do NOT contact me directly

    3) read 1 and 2

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