Hello, I am fairly new to the VEX robotics kit (have only been working with it for one year), and so I am very confused when my two drive motors (for the claw bot kit; seemed easy enough) suddenly burned out completely. When I took the motors off they still did not work at all except for a very minimal amount of movement, about one gear tooth moving every 3 seconds or so. I am thinking they somehow completely burnt out or broke suddenly, although it seems strange that both drive motors broke at the same time. I’m fairly sure my coding is accurate and I think the motors have bugged out in some way. I also feel I should mention that this kit was used when I bought it, so that may contribute to the issue.
My other two motors on the claw bot work perfectly fine and I have tried charging both the battery pack and the controller but to no avail. Nothing seems to be wrong with the motors when I manually took them apart and these motors are not turbo or high speed motors.
Does anyone know what happened, and if there is a fix to this issue? Thanks in advance!
Edit: If your motors start working again after a short while, it is because they have overheated from too much load and so must cool down.
You should try to determine if it is the motors that are broken by connecting the motor pins directly onto the power and ground pins of the motor ports. These are the two of the three holes closest to the edge of the cortex(shown if red and blue in the image). Alternatively, you could connect the broken motors to the motor controllers of the working motors.
Ideally, you should test the motors on a separate current limited power supply to avoid the risk of damaging the cortex, although the PTCs inside the cortex should prevent this. Do not plug the motors into ports 1 or 10 because two much power draw on these can kill the cortex.
In my experience, the brushes in old motors can fail, often causing it to not rot run at all or run very slowly in one or both directions.
The only way to fix this is to replace the motor with a working one from another 393. This is quite difficult as the circuit board traces are easily damaged when soldering new wires and is probably illegal as only repairs to external wires are permitted, although they don’t have any way to check this.
[Below: motor from a 393]
The cable can also fail where it enters the motor, causing it to short circuit which can destroy the motor controller. Sometimes when the contact between the shorted wires is small, the motor can still turn very slowly.
This can be fixed by cutting the wire off inside the motor and soldering a new wire on. This is difficult and is also likely illegal but again, they don’t have a way to check if you’ve done this.
[Below: shorted cable]
WARNING: THESE REPAIRS ARE PROBABLY ILLEGAL AND MAY BREAK STUFF/NOT WORK IF YOU DON’Y DO THEM RIGHT.
This sounds quite unsafe to me as the VEX Cortex may not have a limit to what it can output the power to the motor, as it could potentially harm the Cortex or the motor. You can output the same amount of power to two motors with a Y-Cable hooked onto the same exact port, which means that there is, indeed, a danger that can cause through directly plugging the 2 pins into the power and ground pinholes on the Cortex.
Ports 1 and 10 exist to be used and I rarely had problems with ports 1 and 10, but as many have told me they had problems, and that it has been repetitive from a multitude of people, I would suggest that nobody uses these ports and use Y-Cables instead.
Out of my 4 years in VEX the majority of electronic components (primarily motors) have been ruled legal to be repaired if they are broken. Common sense can also follow, and I doubt that there are no strategic advantages from these repairs which, in my opinion, I would think this is perfectly fine to do.
Keep mind that community responses are not official, but they can help get a relatively predicted result from the GDC.
The cortex actually has 2 4A PTCs divided between ports 1-5 and 6-10 which should protect it although its better safe than sorry so I wouldn’t leave a possibly shorted motor in for for more than the time needed to test it. The motor still has it’s own PTC(I recall reading that it trips ~2A but don’t quote me on that) in series to protect it although if the motor brushes are shorted then extra damage to it does not matter anyway.
Many of the two wire ports on my schools cortexes have died and one cortex actually caught fire, possibly through these ports.so we were told never to use them