Thermal overload for Vex motors

It has been a while, but I recall reading a post about a year ago that described some of the characteristics of a Vex motor which included motor overload protection. We have been experiencing intermittent operation from a couple of motors in a multi-motor drive train. After decoupling the connnecting gears in the geartrain and testing each motor individually using the online code, we found that 2 motors in particular would run at reduced speed for several seconds then shut down. We continued testing while swapping motor ports, uncoupling the wheels, etc and finally let the motors freewheel (no load) with the same result.

We found that both of these motors were warm to the touch and suspected that they were being overloaded and we were not letting them cool down enough to avoid continually overloading them. We have taken steps to eliminate the overload by minimizing friction.

The questions are:

  1. Is there really thermal overload circuitry built into these motors and

  2. Once an overload occurs, how long should it reasonably take at room temperature before the motor cools enough to use again?

  3. Assuming no mechanical damage occured (internal motor gears), should the motor be viable for continued usage after it has sufficently cooled?

We are having the same problems with our motors.:frowning:

You could get some of those fans, like the ones that cool your computer and try adding them in…

I have never experienced this but I think a heatsink would be best because a fan will not cool the motor only the case.

Well, I definetly do not recoment putting the motor in oil as heat sink!
…fans = no effect
heatsink…How would you do that??? fill the already- crammed motor with some heatsink? I think it’s impossible.

By cooling the outside of the case that would allow more heat to tranfer to the outside and cooled part there for a fan would work. I wouldn’t bother though, just don’t put too much stress/strain on the motors.

Yes, there is overload protection built into the Vex motors. This is in the form of a PTC device, which limits motor current based on time-current-temperature constraints.

The reset time is dependant on many different factors, but the main factor is motor temperature. If you place a cold motor in a stall condition, it will almost immediately pop the limiter. It will also almost immediately come back to life, once the load is removed. If you do this same thing with a “hot” motor, the reset time will be greatly increased.

Hope this is helpful,

Thanks for the info.

It is always best to eliminate the source of the motor overload if you want to have reliable continued use of the system it is integrated in. Our issue was one where we had a combination of:

  1. too much side friction in turning
  2. modified 5" dia wheels with high COF material added
  3. long wheel base
  4. and maybe… just maybe folks that were too determined to keep the cool looking design rather than analyze it for problems and fix them.

Anyway, if this is encountered in time, the obvious solution is to design out the source of the overload. If unfortunately you find out during a competition and realistically can’t rebuild anything between matches, then either replacing the suspect motor(s) with a fresh one or using a coolant spray may be a temporary fix option ( No gurantees on how much use you will get out of the motors before they shut down though. Also… keep in mind that excessive friction reduces the performance of your system. Motors in stall or running close to stall by engaging clutches will also drain your battery much faster which further exacerbate the problem. All of this this adds up to a poor performing robot.

No, no,no. Theats not what i meant. A servo heatsink looks like this.

I dont think you’ll find one to fit a vex motor but you can make one on a cnc router or something.

well, that explains it :slight_smile: ty