Keeping drive motors cool

[Ether]

Originally Posted by MichaelBick

The 6 CIM drive rally helps us accelerate better. We modified the backplate for the WCP DS gearboxes, so we couldn't fit a mini CIM. Bag motors are significantly less powerful than 550s. Bags are about 150 watts. 550s are 250 watts.

What gear ratio are you using? 550s have a free speed of 19,300 RPM. They need much more reduction than a CIM would. Without the additional reduction you are overloading (and overheating) them.

[MichaelBick]

We are running the 550s on a 4:1 I believe(might be a 5:1, I will check later). We chose 550s over the 775s because we didn't want to deal with the case shorting problem.

[AdamHeard]

We cook CIMs in drive practice, we're planning doing fans for the next season.

For the speeds we gear, in back to back elims we also get a little hotter than we like. We aren't failing CIMs, but it'd be nice if they were cooler.

There is also nothing inherently wrong with adding a 550 alongside two CIMs. Many teams avoid 775s because the widespread case shorting issues in 2011. It seems like they're failing at a low rate now, but many teams will likely never use them again.

Be a little less deadset in your own thoughts, and a bit more accepting of what other people are doing.

Originally Posted by Racer26

Am I really the only one?

In 10+ years of FRC experience, I've never found a need to actively cool a CIM.

Their cases are pretty good heatsinks all on their own.

Only once in my FRC history can I remember killing a motor due to overheating, and that was a 550 used as a drive motor (the only drive motor on that wheel) in 2009 at the Waterloo Regional when I was with 1075. It was actually pretty spectacular, watching that robot drive around the field trailing a stream of smoke until the field crew made us E-stop it (the robot was otherwise operating just fine). I should have a look and see if I can find video. (Looking at TBA? I think it must be one of the 4 missing qualification matches.)

To me, the better question is why are you using 550's in your drive, particularly with the advent of the miniCIM and BAG motors in 2013?

[Abhishek R]

Yeah, extended drive practices can really heat CIM's up.

When we run a series of practice matches, or back to back elims, or just trying to repeat maneuvers over and over again, it can take some time for those to cool down. I've heard some teams use ice packs, not the most elegant solution but it seems effective.

[Ether]

Originally Posted by MichaelBick

We are running the 550s on a 4:1 I believe(might be a 5:1, I will check later).

Is there an additional reduction due to sprockets (chains) or pulleys (belts). What diameter wheels?

[AdamHeard]

Originally Posted by Ether

Is there an additional reduction due to sprockets (chains) or pulleys (belts). What diameter wheels?

He is quoting the gear ratio from 550 to CIM, not the entire ratio.

So the 4:1 is a fairly good match in that situation.

[Ether]

Originally Posted by AdamHeard

He is quoting the gear ratio from 550 to CIM, not the entire ratio.

So the 4:1 is a fairly good match in that situation.

OK. That makes sense.

[DampRobot]

Originally Posted by MichaelBick

We chose 550s over the 775s because we didn't want to deal with the case shorting problem.

IIRC, the 775 (when run at the same voltage as the 550) has more or less the same characteristics in terms of power, free speed, stall torque, etc. as a 550. However, since it is a larger motor, it performs much better thermally when stalled and when run over a long period of time. If you're worried about overheating, consider trying out 775s. I really like them as motors.

I can't really speak to the case short issue, as I've never seen a 775 case short, only a AM PG-71. However, I've heard through the grapevine (was this you, Adam?) that since 775s are a less common can size, China has more trouble manufacturing them correctly. Perhaps now they have a bit more experience, they're manufacturing them with fewer case shorts.

[Ether]

Originally Posted by DampRobot

IIRC, the 775 (when run at the same voltage as the 550) has more or less the same characteristics in terms of power, free speed, stall torque, etc. as a 550.

The 775-12 has only 38% of the free speed of the 550.

The 775-18 at 12 volts has 60% more stall torque than the 550.

[BrendanB]

Originally Posted by Racer26

But why would you use 550s for that purpose over 775s?

Most likely from the bad history of the cases shorting out in the 775's in the past few years. We avoid using them in our designs for this reason.