At our first competition, we noticed a couple of our CIMs getting pretty hot. So, among other things, we got some of the Virtual KoP heatsinks.
How are you supposed to attach these things? One of the Andymark comments said that they just “Snap on,” but that’s certainly not the case for us. And, unfortunately, the instructions are, well, lacking. We’re going to use thermal paste (same stuff used on CPUs), but I don’t think that will hold it on there sufficiently during a match.
Current plan is to get some metal zip-style ties, but… well, hoping there’s a better solution.
Anecdotally, the CIM heat sink coolers are almost useless. It’s the inside of the motor that gets hot- when the motor case it hot to the touch, it means the inside is even worse. Cooling the outside does nearly nothing to counteract the heat, especially in 2 minute matches.
Instead, prevent heat by stalling out your motors less, and cooling down motors between matches. One common way to do this is to spray air dusters upside down to get the liquid out. I’ve also seen ice water. A difficult way to do this is to get an air fitting on the CIM mounting screw and blow air straight through the inside of the motor. In any case, make sure your motors start and stay cool, since they output significantly less power when heated up.
These are two-sided tapes that are good at conducting heat.
Our team used them this year, as we were pushing our motors very hard. The CIMs ran significantly cooler with the heat sinks than without. CIMs are not designed for high service factors, and have poor thermal design.
High motor temps can cause the insulation on the windings to crack, leading to shorts and therefore poor motor performance. This takes time - typically many heat cycles and many months.
I recommend that you also label your CIMS with the dare of acquisition and use the freshest on your competition bots
While I totally agree with the rest of this post, please do not put water in your robot. At one of our district events this year, a team was trying to quickly cool their motors when their alliance partner (this was elims) rushed over with a bottled water and rag and started applying it to the motors. The look on the FTA’s face was a unique form of horror. Water and robots do not mix.
I recommend obtaining some ice packs (the gel kind are nice) that are self contained, and possibly keeping them in a cooler of some type. I realize this can be difficult at a competition, but it’s probably one of the better ways to cool a motor safely in queue.
If you attend cold regionals/districts (northern Michigan I’m looking at you!!!), you can leave things outdoors to cool them and then place them on your robot, or in case of hot robot emergency, ask the FTA for permission to take your robot outside to cool it down (you may or may not get permission, but the team that originally resorted to water did this next).
Yes. This is the root cause of the motor heating up.
Is there any data to support that these methods actually cool the rotor? Motors with internal cooling fans move much more air than could flow through a hollow screw.
The thermally conductive tape reduces the thermal resistance between the case of the motor and the heatsink. The thermal resistance between the actual hot spot, the rotor, and the case of the motor is going to be much higher because there is an air gap between them. The thermal resistance between the rotor and the heatsink is the sum of the two and is dominated by the gap between the rotor and the case.
Do you have data that shows the rotor temperature is significantly reduced?
There’s also “Thermal Adhesive” which works a lot like Thermal Paste but is much, much more permanent.
Frankly, the main advantage I’ve seen with the heat sinks is more one of allowing the motors to cool down faster between matches to avoid heat building up between close matches that would otherwise dissipate given enough time. As such, I don’t think I’ve ever used them on a robot.
On the note of the hollow center mounting screws, one thing I’ve always wondered is what, if any, benefit there might be to using something like these screws and somehow connecting one to a pneumatic line and forcing a bunch of air through the inside of the motor casing to rapidly cool it. Theoretically, you could do this even more easily with the inlet vents of an air-breathing motor, but I’m not sure how effective it would be in either case. ::rtm::
1296 was one of the pioneers of the hollow screw design, and several other teams have used it. Using 100psi-ish compressed air through those screws cools it down pretty fast. Even if it’s only a 4-minute turnaround, that’s 4 minutes of compressed air being pumped straight through the rotor.
Thanks for the suggestions. To give a little more context, we have four CIMs on our drive train, only two of which (one on each side) were getting hot. They’re all turning correctly, so this is not a case of CIMs pushing against each other (and one winning).
I think the root cause was re-using CIMs from previous years. So, the first thing was to buy four new CIMs. We’re also making some drivetrain changes that should help. The CIM coolers were really an effort in “Ok, what else can we do to make sure thiis problem doesn’t happen again.”
And, yeah, we’re getting some spare air. Also have some small sealed freezer packs that we can put directly on the motor between matches.
Thermal dynamics is not my area of expertise but wouldn’t increasing the pressure also increase the volume of air (SCFM)?
I doubt anyone has actually measured the core temperature, but if the case is cooled and remains cool, that would be an indicator that the rotor is now cool.
Yes, increasing the pressure of the air forced through the hollow screw would increase the volume of air flowing in. The question is “will that volume of air cool the rotor in a significant way?” Has anyone looked at the airflow pattern inside the motor?
As I stated earlier, the rotor is isolated/insulated from the case by the air gap between them.
My questions are based on the issues addressed in the twice yearly R&D updates from the various R&D Departments, including one that designs motors from scratch, that I sat through for 15 years at my last job.
The fact that only one motor on each side is getting hot is definitely indicative of some sort of mechanical or electrical issue, generally gearboxes that use multiple of the same motors should heat relatively evenly.
Buying all new CIMs is definitely a good call, I would also recommend taking a multi-meter to your speed controllers (with motors unplugged) and verify that they are, in fact, outputting the same voltage. Also, check to make sure that all the speed controllers are all in either coast or brake mode (but not a combination of both, also coast is probably better for a drivetrain).
If you want a non-permanent mount, those steel ties are good, but I would still recommend using either the thermal tape that MisterTim posted above, or using regular thermal compound. Either of these options will vastly improve thermal transfer over just mounting the heat sink straight onto the motor.
Why do you think the root cause of the motors being hot is that they had been used the previous year? Do you have data showing the temperature difference from year to year? Have you made changes that affect how the motor is used in your drivetrain? What is the airflow over each motor like and how has that changed over the last year?
This is an engineering competition. Engineering decisions should be based on critical thinking informed by solid objective data. It appears that you are basing decisions on comparisons of single subjective data points taken under different conditions.
Before we had a counter-spring on our robot arm, the motor required constant current and heated up.
Our first reaction was to put am-2865 fins on the mini-cim. We had them laying around the shop from First Choice a couple years ago.
When we added the spring, the heating-up went away, but we left that heat sink on it.
So then we go to Wisconsin. Senior Robot Inspector Big Al Skierkiewicz actually made a point to come up to me and say “you know, that heat sink doesn’t do anything. If you don’t make weight, you should remove it”. Also, “all the heat goes out the shaft.”
Initially, though I know that the heat in a CIM is in the armature, I wanted to argue that the heat from the armature obviously does make it to the magnets and the frame, at least some of it does. I know all of the heat gets out–some way or another. The heat sink can remove that frame heat more easily, over time. But yes, I understand that in a match, this won’t be much time.
Another mentor on my team wisely advised me to not argue with Big Al.
Also, I’ve wiped water on a hot CIM and watched it evaporate immediately, indicating absorption of heat. Heat of vaporization indicates really quick usage of calories. I’ve seen people in this string talk about not mixing water and robots, but if you just wipe the outside of a CIM with a damp towel, you’ll cool it really fast. And I don’t have seven bucks to waste on canned air every match.
I think the difficulty of unmounting and remounting CIM motors in most applications is a big factor on why people don’t do this. It’s also important to remember that using canned compressed air upside-down (so it’s cold) can actually crack the internal ceramic components of a CIM motor, causing permanent damage. Even when we use it on the outside of the motors, we try to use it sparingly.
I saw some teams at WV last weekend with compressed air cans, and flipping them upside down to cool their motors. I dont know how well it worked but it seemed like an interesting idea.