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Low RPM, high torque DC motor help

  1. Dec 12, 2010 #1
    Hey folks,

    I have been thinking about doing a little hobby project for while, but didn't have the right knowledge to do it until now. Basically, I need a relatively small Low RPM, high torque DC motor that can be controlled by a microcontroller. I'm using the arduino duemilanove and have become pretty familiar with it. Depending on the rating of the motor, i'll probably use an h-bridge to actuate the motor, instead of powering it straight from the arduino. I'll need the motor to spin both ways. It needs to spin at relatively low RPM, probably like 0-5RPM, maybe just a tid bit higher. The rpm needs to be able to be varied. I think that all depend on the power you supply it, right? I'll just use PWM to control the RPM. The motor will be pulling a wire string tight. It also needs to stay locked in place once it has pulled it to a certain tension (i'm not sure if a motor can do this. would i be thinking of a gear in this case?). I think the max tension in the wire will be something like 60N, which is like 13lbs, I believe. I know torque (in physics) is related to the length of the arm crossed with the tension force. is this true in torque of a motor? Or is it more just have to do with force?

    Does anyone know where I can get a motor with these characteristics?

    -low rpm
    -relatively small
    -high torque
    -idealy low power, but can work with higher power
    -hopefully cheap
    -motor able to rotate both ways
    -motor able to lock in place

    Thanks in advanced for any help!
    Last edited: Dec 12, 2010
  2. jcsd
  3. Dec 12, 2010 #2
    Yes, You need a stepper motor. Read up on them, you will need a pair of H-Bridges for bi-polar, or four mosfets for unipolar. Stepper motors are all about stall/holding torque, not about RPM. Research it, use it.

  4. Dec 12, 2010 #3
    Great! Thanks! That is exactly what I need.

    I have another question for you. Okay, so back to the issue of pulling and holding a wire tight at about 60N. How do I go about looking at a datasheet for these motors to see if they can achieve this force? For instance,


    This motor says it has a holding torque of 0.23 Nm. Again, I know torque = force*(length of arm perpendicular to force). So, if I look at the radius of the rotor (which says is 2.5mm), do I just devide this holding torque by that radius, since that would be the arm of the torque? So, that would be 0.23Nm / 0.0025 m = ~ 100N, which is more than I'd need. So this one would work for me? Or no? Am I thinking about this wrong?

    I apologize if I sound ignorant. I had a really bad elec. machinery prof in college.

  5. Dec 13, 2010 #4
    I think it might be best if you give a little better description of what you are attempting to achieve. For instance, if your goal is simply to tension a wire, why not attach a 6kg weight and let gravity do the work. Obviously there are many systems where this approach would be impractical, but w/o knowing what you are attempting to do, it is hard to help you. Again, for instance, would a spring and a tension screw meet your needs? Does your system require active load adjustment, and how important is the accuracy of the force? What type of response time is required? How frequently does the tension need to be adjusted? Engineering is almost always about finding the simplest/cheapest/most reliable solution; it sounds to me like you are attempting to solve a fairly simple problem with a fairly complex/expensive approach.

    To answer your question, almost any size motor could be configured to achieve your 60N, we are simply haggling over complexity, accuracy, response time, cost. I am not convinced a motor is requisite at all. If the load is dynamic then you have not provided nearly enough information.

  6. Dec 22, 2010 #5

    I am thinking about building a tennis raquet stringing machine and i have the same problem. Let's say that it would look like this one:

    What torque should the motor have to achieve a 24 Kg tension in the string? And how can I achieve that torque? By applying a certain Current Intensity to the motor ?

    Sorry I'm a newbie

    Last edited by a moderator: Sep 25, 2014
  7. Dec 22, 2010 #6
    Ahh, fish, I apologize. With the information you gave me, I felt confident that a stepper motor was what I needed.

    To be more specific, I'm doing a little project on my own for fun. I wanted to try to build a prototype to autotune a guitar string. My system will have a mic pick up the note of the guitar, use some fourier transforms to get the fundamental frequency, and then see how far off my frequency is from the actual frequency of the note (low E, for instance, is about 80hz). I'll do some calculations in a microcontroller (arduino) to output a nice PWM signal to my motor that is proportional to how far off I am from the actual note to correct it. The motor also has to go both ways, whether the note is sharp or flat, so a bidirection stepper motor seemed my best bet, thanks to your input!

    This all seems doable. I found a cheap bi-directional stepper motor online, but I'm still unsure if the torque numbers are enough.

    That leads back to my initial question. If i'm given a spec for a motor that has a max torque of X mN-mm (milinewton, millitimeters) [not sure if this is the standard unit for torque for small motors, might be mN-cm]. Do I just divide this number by the radius of the rotor to get the max tension it can hold? Since N (torque) = F (force) * r (radius of arm).

    If that's so, then I think this motor will work.

    Unfortunately I'm still in phase one with the mic. I'm getting a signal, but it's rather noisy. When I try to pick up notes, it's very faint, so I need to amplify it some more. I have a feeling the motor part won't be as difficult as this part. I've worked on past projects with h-bridges and PWM, so I'm not too worried about that part.

    Thanks again!
  8. Dec 22, 2010 #7
    There are several types of motors that can supply high torque at low RPM.

    1) A stepping motor can supply high holding torque at zero RPM, but stepping motors are designed to hold a specific preset angular position at a range of torques (Not a specific preset torque at a range of angular positions).

    2) A true torque motor can provide a fixed preset torque at a range of RPMs including both forward and reverse RPM, and zero RPM ("locked rotor").

    The difference between these two motors is similar to the difference between a constant-voltage regulator and a constant-current regulator in electrical engineering. Both can supply a specific current at a specific voltage, but under very different constraints.

    If the application is to maintain a constant tension in a wire, cable or string, even if the wire, cable, or string is constantly changing length (e.g., spooling on/off a reel, or thermal expansion or stretching of a wire between two fixed points), then the only choice is the torque motor. The torque is set by regulating the input current.

    A dc gear motor operated at constant current could provide some features of a torque motor, but any brush motor operated under these conditions will damage the commutator.

    Most/all the true torque motors will have a permanent magnet rotor and a Hall Effect (or other) sensor to determine rotor position. DC gear motors have a permanent-magnet stator and a brush/commutator wound-coil rotor.

    Bob S
  9. Feb 7, 2012 #8
    hi friends, i need dc motor with high rpm nearly equal to 10000 rpm with high torque which will be controlled by PWM signal & its shaft will drive the supercharger,So please suggest me where can i get it?????
    Last edited: Feb 7, 2012
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