Video explains the issue, please let me know what you think.
Happening on multiple motors at random times. Motors are in good physical condition and are relatively new. Opened them up and they look normal on the inside. They are sufficiently greased and have been cleaned.
I don’t want this happening during a match at worlds! What’s going on??
Posting in this channel to allow for community feedback but I’m open to an official solution as well @VEX Support
I’ve seen that on 269s, but never a 393. I’m guessing it might have something to do with static electricity (turning the motor generates electricity, it in turn powers the motor), which is why turning them slower will allow it to pass the point. From my VERY brief experience with motors like that, powering them from the Cortex doesn’t seem to have any problems, but I wouldn’t risk it. How many motors do you have like this?
This happens to us quite often with the new motors, We have found two solutions that help. One run the motor on its own for about 5 minutes alone. Second if you also put lithium grease (found at most hardware stores, and probably just most stores in general.) on the gears inside the motors it helps as well. let me know if this helps or if you have anymore questions. See you at Worlds!
When this “stickiness” is happening, where is the plug? A few times I have been working on a system moving the motor by hand, unplugged, but the plug happens to rest against a c channel, then the motor has a lot of resistance. This is a perfectly normal motor behavior, when the circuit is completed by the c channel, the coils spinning in the magnets create an electric field, which since the wire is a coil, creates an electromagnet reversed from the permanent magnets in the motor, causing the resistance or “stickiness”. If you can make this happen with the plug isolated, I might check for an internal short , but you say they are fairly new and look good.
After re watching your video @917F, the sound the motor is making sounds very much like a back-EMF problem like I described. In the vid, I cant see the wire… was it in the c channel in your hand? that could definitely cause intermittent shorting at the plug.
We had this problem recently and we found the problem was due to the wires in the back of the motor being exposed and touching. This meant that the motor was acting as a generator when being twisted and would power itself to resist motion. @8232X explains this in better detail above.
When you say the back of the motor - is that inside the housing? We have had a few motors that did this sort of thing as well, and they all looked good in terms of wiring from the housing out to the plug. We also had one that ended up frying several motor controllers before we realized the motor must have been shorting inside (by deduction). Is there a way to check those interior wires and possibly repair them?
@TeamTX when I said back of the motor I was referring to something like this (picture below). So far we haven’t had any shorting inside the actual motor, it seems most of ours come from wires being pulled to tightly.
Let’s hope that your experience will help you deduce it more quickly next time. Though the motor controllers are a good value, they make expensive fuses.
A multimeter set to measure resistance will show zero ohms if the wires are shorted. I don’t remember the typical resistance of a good motor, but it’s easy to determine yourself. As long as you have a known good motor, that is!
If the short is intermittent, you will need to leave the meter leads attached while gently flexing the wires in order to see the zero ohms reading. This is made easier if you have an audible “continuity” setting on the meter; then you can look at the wires you’re flexing while listening for the continuity beep.
That’s iffy. It’s easy to repair them, but it’s against VEX rules to solder new wires on to the motor circuit board and then use the motor in competition. I believe it is also likely to be treated as a potential motor alteration if you have to open the motor case to solder the wires together even if you don’t have to replace the wires all the way to the circuit board. If, however the break is outside the housing you use standard wire repair techniques (solder, heat-shrink tubing) and the motor is legal for VEX competition use.
Yes, it was a very expensive and time consuming lesson overall - one that will not soon be forgotten by anyone on the team! Lol. Between that motor issue and some faulty cables that burned out several more motor controllers - weirdly all within a few days of each other (and of course the week before a competition) - we ended up having to pull and redo our entire wiring system in a couple days. We’re much better at cable and mc management now, though!
Thanks guys, I like the back EMF idea. I am familiar with the concept but didn’t think of it as an option here. It’s certainly worth looking into so I will investigate that possibility with a couple motors at our next team meeting and let you guys know what I find!
FYI for anyone else who’s experimenting, our gears were re-greased with white lithium grease and it had little effect, although it’s likely that it improved the motor’s normal operation.
The stickiness is quite random (which would be explained by shifting wires), so it’s hard for me to say under what conditions it happens. But I would like to find the culprit rather than blindly replace $15 motors left and right!