Motor selection assistance for near-100% duty cycle

I’m doing my final year design project in engineering. We’re designing a cable-actuated robot, which uses motor-winches to provide tension to a number of cables. The cables are all attached to a central end-effector, which is then moved around. See the following link for a better idea:

What we are doing is like this, but not restricted to a plane, i.e. the cable origin points will be on top of 4 posts, and the end-effector will be able to be lowered and raised in addition to movement in a plane parallel to the ground.

I’m wondering if the Pololu 12V 131:1 or 100:1 gearmotors with encoders ( are good enough for this task. Their stall torque is about twice what we have calculated that we’ll need. However, since the cables must always be in tension, the motors will always be stalled.

What I’m wondering is:

  1. Even though the required torque output from the motors during a stall condition is only about half the stall torque, will the motors still draw stall current?
  2. Will this kind of activity be too much stress on the motor? Will they overheat? What is their duty cycle?
  3. does a torque-current or torque-speed curve exist for the motor?



If your application only needs half the rated stall torque of the motors and you are using encoder feedback to limit the voltage and current, it might work. However, even running the motors at half the rated torque constantly might be pushing the limits of what they can handle. You should be sure to avoid any stalls at full voltage. Alternatively, you might consider looking into other solutions like using a worm gear to prevent back driving or using stepper motors instead.

We do not have any graphs for our gearmotors. All of the performance information we have for them is listed on their product pages and under the “Specifications” tab of each product page. Using those specifications, you can estimate various additional motor parameters and performance curves. For example, you can approximate a linear relationship between motor current and torque, with the line passing through the free-run current at no load and the stall current at the stall torque. Similarly, there will be an approximately linear relationship between motor speed and torque, with the line crossing through the free-run speed at zero torque and zero speed at the stall torque.

- Grant

Also note that, at half the current, you will get a quarter the heat.