Hi, my team was competiting at the Northern Lights regional and we were playoff match 10 (it was a tie-breaker). About 3 seconds after auto our robot started doing circles. Our driver stated that one of our CIM’s was extremely hot. Could we have melted the ceramic resin that lines the motor casing? Any answers are accepted.
Thanks,
Taggerun
P.S. We use a single CIM per Toughbox Mini. (We have two Toughboxes). Am I right about the ceramic resin?
You are correct in that it could be issue. When the resin melts the coils become one big wire (essentially), thus being a real big short.
I presume the hot motor was on the side where the motors stopped rotating, no?
Do you have two toughboxes total or per side? If your robot has only 2 drive motors, I would highly recommend adding another. Even if it is only a mini cim, it will still relieve some of the strain on the single CIM. I am not sure if the motor is damaged but am just trying to help figure out how to prevent an issue if it is.
We have one toughbox per side and one CIM per toughbox. Yes, the side that was hot was the side that stopped working. One more question: What would’ve caused this??
Thanks,
Taggerun
2 CIMs to drive your robot??? Probably overheated the CIM, and it did not have enough time to cool down before the next match.
At a minimum, we have always used 2 CIMs per side (4 total) to drive our robot.
Can you post some pictures of your chassis and drivetrain?
You either have a mechanical issue or a faulty motor.
Our team was working on their practice robot and none of the 3 wheels on each side could turn. They were using the kit chassis and had over-tightened four the bolts that serve as the axles for the front-most and rear-most wheels. Once we backed off those bolts, we could turn the wheels by hand.
My impression is that many teams have used a single CIM per side with no issues, especially rookie teams. If the motors are stalling, they self-limit the current so they will all draw pretty much the same stall current, i.e. a heavier robot won’t cause the motors to have a higher stall current than a lighter robot though a heavier robot is more likely to cause the motor to go to a stalled condition.
Generally when the motor is stalled for a prolonged amount of time is the only way to cause this. If the robot got stuck or the gearbox got jammed this would be sufficient.
However, if the gearbox was jammed then this would cause the in-place rotation, and a hot motor would not necessarily indicate a dead motor.
You’re might be geared too high and causing the motors to overheat due to working too hard or stalling.
Personally I don’t think I would ever use less than 4 CIMS on a drive system (except maybe if all the wheels were low traction), especially with a toughbox and/or kitbot chassis.
With the default ratios, and assuming you’re direct-driving a wheel and not using any additional reductions, a drive system using the stock toughbox with only one motor on each side only has a pushing force of ~50lbs at 40Amps with 4in wheels (it gets worse if you have larger wheels). At the full pushing force of ~180lbs you’re pulling roughly 100Amps per motor.
Not only will this put you in breaker-popping territory, but it will also make your motors get very, very hot if done repeatedly, which consequently, could have lead to your problem. Ideally you want your robot to be traction-limited (or as close as possible) to avoid motor damage unless you have some other way to mitigate current draw (there are some teams that use very high gear ratios but use current sensing or other means to prevent drawing too much current in high-torque situations).
tl;dr: Use 4 motors minimum on your drive system and check your gear ratios, you won’t regret it.
Overheating motors are usally the result of gearing your robot too fast. Do you know the effective drive train gear ratio between your motors and wheels?
Put another way; how fast does the robot drive in a straight line?
Some photos of your robot drive train would be helpful.
CIM motors are fairly robust, as electric motors go, but they are not invincible. It’s entirely possible to roast them, causing irreparable damage. In addition to shorting windings, you may damage the brushes, commutator or even weaken the permanent magnets. If you think you might have damaged a motor, replace it. They’re fairly inexpensive and most teams have piles of legal motors from previous seasons they’re willing to give away.
See this thread.
What methods do teams use to cool down their motors between rounds, especially in the playoffs where the time between matches is so short.
Do the CIM heat sinks that Andymark sells work well for helping with this?
Andymark also sells temperature stickers for the CIM. Do those work and at what temperature should we be getting worried?
We are switching from 5:1 to 4:1 on each of our gearboxes and want to be careful to not burn up a motor if we make it to the playoffs.
Tagger,
There is no ceramic resin lining the motor casing. The ceramic is magnets.
As to the high temp on a CIM motor, there are a variety of issues that could cause that. One of the motor controllers could have shorted causing the motor to drive against other motors in the same drive train, you could have shorted one or more windings on the armature (the parts that spins the output shaft) or the motor in question was running opposite the other motors.
I’ve seen a variety of solutions to cooling CIMs between matches, most are just various types of small/portable fans. Using a can of compressed air upside-down can provide some quick cooling if you’re in a hurry, but use with caution as it may damage the motor (can crack the magnets, or something).
For active cooling while the robot is running, the most effective thing I’ve seen is to simply use one of the PWM PC fans that come in the KOP/FIRST Choice to blow cool air across the motors (this also helps with other motors and pneumatic compressors that get hot), if you wire them directly to the PDP/VRM they will run whenever your robot is powered on (regardless of if it’s enabled) and cool your motors. The CIM coolers can certainly help, but IMO should not be used without a fan, especially if air circulation around the motors is already an issue.
Finally, keep in mind that switching from a 5:1 reduction to a 4:1 reduction will increase output RPM, reduce Torque, and likely make your motor work harder.
Switching from 5:1 to 4:1 will make your motors run hotter, not cooler.
Yes, we realized that when we made the decision to switch, which is why I am looking to make sure we are cooling the motors between matches or at least monitoring the temperatures.
If anyone knows at what temperature we should get concerned, then I will probably buy the thermal stickers from Andymark and put one on each motor so that we can at least monitor it and pull out the fans if necessary or even switch back to 5:1
Are we still talking about CIM motors on the drive system here, or something else? All motors are not created equal.
My rule of thumb is basically “if you touch it and it feels like it’s about to burn your hand it’s too hot”, but I can’t really quantify that for you. However, it is also possible that depending on the specific motor and/or situation, the motor may stall and fail before you have a chance to measure it’s temperature.
Yes we are still talking about CIM motors on the drive system. I’m not concerned about these motors stalling due to the type of wheels they are driving (omni). The wheels should slip before they stall the motor.
It is definitely possible to stall the motor pushing omni wheels. It really isn’t much harder than stalling solid wheels. Omni wheels slip when lateral forces are applied, but are fairly close to solid wheels in the intended drive direction.
If you are having issues already, I would not recommend changing the gearing the way you are talking about unless you add another motor. You may be a little quicker in a match but it may not be worth it to blow a motor and be out the next, especially since you only have 1 per side as it is.
I don’t think we have an issue with it currently. Part of what I’m trying to determine is how hot can it get before we should be concerned.
After our 3rd semi-final match at the last event the driver noticed that the motors were a little hot. My guess is that is normal since we had quick turnaround between matches.