Motors stops operating with high load

  1. 8 months ago
    Edited 8 months ago by mr.d

    My students noticed that if they’re pushing an INZ mobile goal around with a “super clawbot” claw held tightly around the goal and both joysticks at full power, that after a few seconds all the motors will stop operating and will merely shudder until the joysticks are released for about 5 seconds. Then it will operate again as normal with no other changes. The vexnet lights stay on the whole time.

    We did some troubleshooting and noticed that the super clawbot can push around an INZ mobile goal without the claw, just with the drive base, without having the same effect.

    Is it possible that when too much amperage is drawn, the cortex stops sending power temporarily? How can we resolve this?

  2. Aponthis

    7 Oct 2017 Gilbert, Arizona Formerly 127A, 127C, 127X

    @mr.d Is it possible that when too much amperage is drawn, the cortex stops sending power temporarily?

    You are exactly right (I think that is basically the technical explanation, anyway). Each motor has a PTC which will melt and temporarily prevent current from flowing if it gets too hot. Ports 1-5 and 6-10 also each have their own PTCs. So, if you draw too much current for too long on either of these sets of ports, you will trip the PTCs. I would suggest two solutions. First, split your motor allocation among ports 1-5 (the first cortex PTC) and ports 6-10 (the second cortex PTC). This will spread the load more evenly. Second, avoid having any motors run at full speed while not being able to make any progress. This will draw lots of current, and burn out that individual motor's PTC for sure.

  3. Avoid stalling a motor at any motor power above 15 to 20 ish out of 127, any longer than that and you'll "burn out" (trip the ptc).

  4. Rick TYler

    9 Oct 2017 Teachers/Coaches, Event Partner, V5 Beta Moderator Redmond, Washington Founder of Exothermic Robotics
    Edited 8 months ago by Rick TYler

    To clarify, the PTC does not actually melt, but as it heats up, it increases resistance and reduces power flowing (in this case) to the motors. Once the PTC cools off, it resumes its normal operation, although resistance may remain higher than normal for a few hours. (You should look this up, these are fascinating devices, where heat changes the crystalline structure to an amorphous structure, which has much higher electrical resistance.)

    Triggering a PTC does not burn it out. They are designed for <some large number> of thermal shutoff events. They aren't fuses.

    If you repeatedly overheat DC motors, they will fail. The PTC protects them, but not perfectly, and repeatedly overheating the motors will eventually cause them to die. In the early days of the Tetrix parts kit in FTC, the motors had no thermal protection and they would fail pretty much instantly when stalled. After using VEX for a couple of years, we were insufficiently cautious with the Tetrix motors, and managed to burn out an entire paper grocery bag-worth of motors in 3 months (with four teams). They behaved like $30 fuses. At the time, VEX was still using the 3-wire motors, and I think we burned out one motor in three years (we did replace lots of the plastic internal gears, at a cost of 50 cents per motor). I think I'm the only one in VEX who misses the 3-wire motors.

  5. Splitting the motors between ports 1-5 and 6-10 really helped! Thanks, Aponthis!

    theone1728, I guess I don't really understand what you're saying: Are you saying we need to keep motors at less than 20/127 to avoid ever tripping the PTC? That seems like a pretty low power to maintain!

    Rick, that's exactly the question I was wondering: how could a melted fuse reform in just a few seconds?! You answered the question and now I do want to do more research on this fascinating tech. Thanks!

  6. 536Mentor

    10 Oct 2017 Event Partner, V5 Beta Tester Appleton, WI 536

    @mr.d theone1728, I guess I don't really understand what you're saying: Are you saying we need to keep motors at less than 20/127 to avoid ever tripping the PTC? That seems like a pretty low power to maintain!

    I believe that what is meant is that while the motor is being stalled (motionless yet receiving power), you don't want to apply more than 20/127 power. There are times where you will intentionally stall motors, such as holding a claw closed or holding an arm in position, in those cases try not to exceed 20/127 and not for long periods either. We try to run our motors at approximately 80% or around 100/127 as that gives us some head room and the motors are near peak efficiency at that level (or so I was led to believe).

  7. jpearman

    10 Oct 2017 Moderator, ROBOTC Tech Support, V5 Beta Moderator Los Angeles 8888
    Edited 8 months ago by jpearman

    @536Mentor We try to run our motors at approximately 80% or around 100/127 as that gives us some head room and the motors are near peak efficiency at that level (or so I was led to believe).

    See these
    (from this thread https://www.vexforum.com/index.php/8089-motor-torque-speed-curves-rev2/0)

    -image-
    -image-

    For peak efficiency (that is power out/power in), when sending 127 (or any value greater than about 95 as they don't respond after that) to the motor, try and make sure the motor is running at about 85% of free speed.

    For max power, the motor would be running at 50% of free speed, but at that point current draw will trip the PTC after a few seconds.

  8. Rick TYler

    10 Oct 2017 Teachers/Coaches, Event Partner, V5 Beta Moderator Redmond, Washington Founder of Exothermic Robotics

    A good rough-and-ready rule for VEX drivetrains is that the robot should appear to jump from a stand-still to full speed nearly instantly. If you can see the robot accelerating, your gearing is too aggressive. Similarly, an arm should not visibly struggle to lift its load, it should just lift. Any time you see a VEX system running slowly or accelerating slowly your motors are probably overheating.

  9. _7682

    10 Oct 2017 V5 Beta Tester

    @Rick TYler In the early days of the Tetrix parts kit in FTC, the motors had no thermal protection and they would fail pretty much instantly when stalled. After using VEX for a couple of years, we were insufficiently cautious with the Tetrix motors, and managed to burn out an entire paper grocery bag-worth of motors in 3 months (with four teams). They behaved like $30 fuses. At the time, VEX was still using the 3-wire motors, and I think we burned out one motor in three years (we did replace lots of the plastic internal gears, at a cost of 50 cents per motor). I think I'm the only one in VEX who misses the 3-wire motors.

    Funny thing was the smoke leaked from an under-rated inductor which was intended to suppress commutation noise. Sort of like a non-resettable fuse with visual indication. Much smoke during 30sec auton routines unless the students included stall checks. Biggest problem was knowing when to bin a motor since often they'd still be working after leaking smoke.

    Yes you probably are the only one who misses 3 wire motors with plastic gears!

  10. This is all really great info. I'm going to have to study that chart to really understand it, or find a youTube video explaining power and efficiency on the torque-speed spectrum.

    One of my concerns now is not knowing when to replace a motor and when to change out its internal gears. Do the gears need replacing if and when they get stripped? In which case they'd just spin freely? Does the motor need replacing when it stops running altogether?

  11. _7682

    10 Oct 2017 V5 Beta Tester

    @mr.d This is all really great info. I'm going to have to study that chart to really understand it, or find a youTube video explaining power and efficiency on the torque-speed spectrum.

    One of my concerns now is not knowing when to replace a motor and when to change out its internal gears. Do the gears need replacing if and when they get stripped? In which case they'd just spin freely? Does the motor need replacing when it stops running altogether?

    Here's an educational video entry from 2014 our team made explaining motor PTC's and the motor speed/torque curve. More focused on the drive motors but you'll no doubt encounter this at some stage too. Open up your unhappy motor(s) and inspect the gears. You can buy a refurb kit from VEX.

  12. Great video! I love it!

  13. Aponthis

    11 Oct 2017 Gilbert, Arizona Formerly 127A, 127C, 127X

    @mr.d Do the gears need replacing if and when they get stripped?

    Correct, you will hear a clicking sound. Make sure to isolate this to the motor; a clicking could be skipping external gears or skipping chains on sprockets. After a while, you will be able to tell these sounds apart (and even front/back gears on the motor). I've been doing this for a while....

    @mr.d In which case they'd just spin freely?

    Not necessarily, often just one tooth will be broken, so you will get a rhythmic skipping. This will occur before it becomes completely free-spinning.

    @mr.d Does the motor need replacing when it stops running altogether?

    I wouldn't make that generalization. Sometimes wires can short or split or something like that, and after (legally) repairing them you can get plenty of use out of them. Gears are not the only issue, but replacing gears can save you a lot of money compared to replacing every problematic motor.

  14. 7 months ago

    biglesliep

    17 Oct 2017 Menlo Park, CA 1666

    I've written several in-depth posts about these very topics, and all are geared toward the new mentor or coach who is learning these things for the first time. I hope you find some use in them:

    Motor gears (the ones inside the motors): https://renegaderobotics.org/motor-gears/
    Gears & Torque: https://renegaderobotics.org/gears-torque-crash-course/
    VEX Motors: https://renegaderobotics.org/vex-motors/
    Motor overload: https://renegaderobotics.org/motor-overload/
    Motor ports: Spread out the load: https://renegaderobotics.org/motor-ports-spread-out-the-load/

  15. 97934U

    22 Oct 2017 Nashville, TN 97934U

    Slew Rate controllers are the way to go

 

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