Heat sink on Cim Motors?

We’re trying to be able to install heat sinks on our cim motors. There is a space restriction, so the heat sink should be no taller than 3 inches. Any ideas?

Don’t ask why, just give suggestions.

You really don’t want to post this type of question in Chit-Chat, many members block chitchat from their portal and don’t check here often so they may have advice but may never see this thread.

You are coing to have trouble finding an out of the box solution, in general you can find clip on solutions for smaller motors with thinner walls (like the FP and BaneBots motors) but not for the CIMs. If your CIMs are getting that hot you are much better off looking at the performance curves and redesigning to put less stress on the motor. Adding a heat sink wont help that much since the majority of the heat is in the motor internals, by the time you feel heat on the surface (thus by the time a heat sink would start to help) the inside of the motor is already hot enough to cause damage.

If you are not willing to redesign and not willing to give more details so we can provide real advice then you can try to get whatever benefit possible from making your own, either by bending a flat heatsink, or there are a few instructables about making them out of things like soda cans and aluminum tape.

Not a great way to get people to help you

Make them 2.5" tall.


(It is a joke on their name)

Seriously though, look into PC heatsinks. They tend to be not too tall. That being said my real suggestion would be don’t run your CIMS so they get that hot.

… or mount the CIMs vertically. then the heatsinks will never be taller than the CIM.


Are you basically saying to do a barrel roll?

Liquid nitrogen will keep them cool.

The only realistic solution is to fabricate a clip-on, but that sounds like more trouble than it is worth.

Read JamesBrown’s answer, he’s right about internal heat.

Then think about the problem you want to solve - get heat away from the outer case. Um, won’t a fan do that just as well??

Alternatively, blast your CIMs with canned-air coolant inbetween matches.

In 2008 we were burning up CIMs like crazy and I machined some aluminum heat sinks. As it turned out, they were rather worthless. In a controlled setup, they really did nothing to lessen motor heating, even with a large fan blowing directly over them. At about 15 Amps continuous load, after three minutes they were already too hot for comfort. The CIM motor heat is created in the armature and the only path out is through the bushings which is a rather poor heat transfer. It’s not a continuous duty motor. It will heat, and there’s really no great way to prevent it other than reducing the load.

Our team needs a replacement for the CIM for non-competition purposes.

Ideally they would have the same mating as the CIM and be a drop in replacement and be rated for continuous duty.

Does anyone know of such a motor ?

What is the needed power output for this replacement motor? If you don’t need the CIM’s full power, I’d recommend a Banebots 775-18 with the CIMulator. Since the Banebots 775-18 has a fan, it should stand up better to continuous use.

Since I’m not particularly knowledgeable about motors, can someone please chime in and let us know how to find the best spot for continuous operation on a motor. I’ve heard it suggested that, for a continuously-operating motor, the best point to operate at to prevent overheating is usually the point of maximum efficiency. Is this correct, or is there a better metric?

The speed of maximum efficiency, or faster, is a good operating range because the least amount of heat per unit work will be generated by the motor and the cooling fan will be spinning fast.

what cooling fan ?

They were talking about a continuously-operating motor with a cooling system. Something like the Banebots or FPs, which have a fan in one end to circulate air.

^ That. Open-case motors like the RS775-18 generally have a cooling fan mounted on the armature.

Note: This post is only concerned about brushed DC electric motors.

Many motors, including the Banebots 775-18 (IIRC) include a cooling fan. (I don’t know for sure if “fan” is 100% correct, but hopefully you get what I mean).

This fan is attached to the motor shaft and is inside the case (you should be able to see it through the cooling vents – it is a form of centrifugal pump, so the blades are parallel to the shaft, not rotated).

Am I wrong about the 775-18?

I understand that, given the following conditions, a higher speed results in lower heat generation:

  1. Constant voltage across the motor terminals
  2. Motor velocity is between zero and the free speed of the motor given the supplied voltage (this includes direction)

I also understand that, for a motor with a built-in fan, a higher speed results in better cooling.

I also understand that the output power of a motor decreases the higher the speed goes above the maximum power speed (again, I’m assuming it’s rotating the same direction as commanded by the supplied voltage).

Therefore, I guess I should restate my question as follows: Is there a way to determine, given an arbitrary DC brushed motor, the operating point at which the motor’s cooling is sufficient to prevent damage from its heat generation under “usual” operating conditions? If so, how can I do that?

Are you saying that this usually occurs at the motor’s maximum efficiency point?

EDIT: Was slow at posting – looks like I was right about the 775-18 having a cooling fan.


Your best bet to learn that is through testing.

You could construct a, electro-thermal (maybe fluid mechanics, too) model that incorporated the armature fan efficiency and flow rates, heat transfer coefficient vs fan speed & temperature, resistive heating of the armature, heat transfer (convection, conduction, and radiation) from the armature to the case and from the case to the environment, and construct a thermal model of the battery. You would then have to compare the steady-state of this model to whatever temperature either ruins the permanent magnets or damages the enamel on the armature wires, or some other component in the motor that might be a failure point. Doable, but way more work than testing. At my job I have the tools to construct and evaluate that model, but I would choose to test the motors.

If you are looking for a very general rule of thumb: the motor should be OK if operated a speeds higher than whichever is the higher of these two:

a) speed of maximum efficiency

b) 90% of free speed

Ether, thanks for that. That sounds like a pretty good rule of thumb (of course, as JamesCH95 has suggested, testing is the best option if you need more accurate information).

According to the BaneBots website, at 18v, the 775-18’s maximum efficiency speed is 17,040 rpm (which is over 85% of its free speed). Scaling down to 12v, the maximum efficiency speed is 11,360 rpm. At this point, it is producing a torque of 15.99 oz-in at 12v.

Therefore, according to Ether’s rule of thumb, the maximum continuous power for the BaneBots 775-18 (geared for 11,360 rpm at 15.99 oz-in of torque) is 134 Watts at 12 volts.

Ebarker, is this enough power to suit your purposes?

Thanks for the help, JamesCH95 and Ether.

Where did you get the 15.99 oz-in number from? Should be more like 14.2.

The power at max efficiency is 119.25 watts according Banebots specs.


I got those specs off of http://banebots.com/pc/MOTOR-BRUSH/M5-RS775-18

What I failed to notice is that the page I found was for the M5-RS775-18, not the M7-RS775-18.

My numbers are therefore erroneous; yours are correct.