Odd Motor Behavior?

Today I started experiencing some odd behaviors in one of the 393 motors in my lift. Upon taking apart the motors, I found one to do some really odd things that doesn’t really make any sense to me. The motor goes forwards fine, but when going backwards, it appears to have some problems moving. Also, sometimes it looks as if a PTC trips and completely shuts off the motors for about ~10 seconds, when running backwards for a few seconds.

This motor is very new, probably has ~10 hours of testing done on it, so I would find it extremely surprising if it’s already dieing. All the wiring is done right (393 -> Motor controller -> Cortex), and the other lift motors are all working correctly so it’s not a code problem.

Any ideas?

Switch the motor controller
Switch the cortex port
Switch the cortex
Run clean code
See if a known good motor works with the same port/motorcontroller/cortex
Unplug and handturn, same issue?
Flip the connection between motor and motor controller (red to black and black to red) does direction of resistanc change?
Check joystick calibration
Try motor without motor controller in ports 1/10
Take apart the motor and check components

Not necessarily in this order, but there’s a lot of options to try out.

Already did most of those.

Tried multiple motor controller, no difference.
Tried different cortex port, no difference
Tried ports 1/10, no difference
Tried clean code, no difference
Other motors work in same port
Flip motor controller direction, no difference
When turning by hand, forwards is smooth, backwards is pretty tough to move
I only have one cortex :stuck_out_tongue:
I checked all the internal gears, they’re all intact and as expected.

Just to make sure, did you handturn while it was unplugged?

Yup, just double checked it too, there’s definitely some resistance going backwards.

Try opening the motor, removing the actual motor module and hand turning the internal gears?

The internal gears spin freely both directions, so it appears to be the motor itself that’s dead. Oh well, looks like I’ll need to get a new one. Thanks for your help.

No problem, unlucky with the motor I guess :confused:

I think that you will find that the brushes are damaged within the motor, I posted a photo of this some time ago as we have had a similar problem.


Yeah, do you know why the brushes get damaged? Is it just having it stressed for long periods at a time? Or does it just happen naturally to every motor over time?

From what I have observed, the brushes on one side of the commutator may (there are two sets) get temporarily spot welded to the commutator during a motor stall event. Once the motor breaks free the brushes bend on one side only, or the brushes simply get caught in one of the three gaps in the commutator causing them to bend.

yes its the brushes
weve had way too many of those :stuck_out_tongue:
but keep the motor housing and gears etc for spare parts :slight_smile:
its just a thing you have to deal with with brushed motors :confused:

Your post piqued my curiosity – I had to back track to here to visualize what was happening, but your explanation makes sense – thanks!

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Has anyone ever opened up a dead motor to confirm that momentary spot-welding, or bent brushes in general, is the reason for our dying motors?

It makes sense, but still…
I’d love to see it if anyone’s willing to go through the hassle!

yes i have pulled appart some of these motors and here is a link to the photos.(I hope)

Thanks! Interesting how that works. I suppose we just push these motors too hard.

It’s too bad Vex doesn’t add connectors to the motors and sell motor replacements.

Our team was having similar issues and we found out something very interesting. The Vex Cortex was designed with 2 internal circuit breakers (4 amps on each breaker). The 1st breaker is for motor ports 1-5 the 2nd circuit breaker is fro mtor ports 6-10. if you run your robot in the defualt EasyC operator control pattern (left side 2 and 4/right side 3 and 5) all these go to the 1st circuit breaker, and will at some point give you issue.

If you split the config of your robot to both sides so that each circuit breaker has an equal load (drive train say leftside 2 and 4/ right side 6 and 8) this will kep the amps and heat (friction low) in the motors and prevent the motors internal thermal breakers from stopping your motors.
It worked for my team!:slight_smile: