Compressed air for rapidly cooling motors?

[GreyingJay]

Somewhat related question about CIM heat:

The first day we did a drive practice session, we kept going and going and going, swapping out batteries as needed. Nobody was thinking too much about motor heat or letting the robot rest. Eventually we noticed the smell. Someone described it as smelling like yoghurt. I ascribe it to burning plastic. Knowing where it was likely coming from, we turned our robot upside down and, sure enough, the four CIM motors were extremely hot.

At what point has irreparable damage been done? I assume the fact that I can smell plastic is not good news, but the robot still seemed to drive fine after being given a cool down period. This suggests to me that windings have not melted or shorted appreciably.

We will probably want to test the internal resistance of the motors once the practice bot is eventually disassembled.

[Ether]

Originally Posted by GreyingJay

At what point has irreparable damage been done?

Here are some quick tests you can run to assess the performance degradation.

Make a test rig using a well-assembled, lubed, and broken-in dual-motor gearbox with an encoder. And set aside a known-good CIM (I'll call it the "reference CIM" with its leads soldering together. Keep this test rig for future testing and CIM evaluation.


1) Put the suspect CIM in the gearbox, apply full voltage, and measure the free speed.

2) leaving the suspect CIM in the gearbox, add the reference CIM (with leads soldered together) to the gearbox. Apply full voltage and measure the speed.

3) If you don't already know approximately what results you should be getting, remove the suspect CIM and repeat steps 1 and 2 using a known-good CIM.

[GeeTwo]

Originally Posted by Ether

.. add the reference CIM (with leads soldered together) to the gearbox ...

I'd like to suggest a simple tweak (based on an Ether post in another thread). Instead of soldering the reference CIM leads together, simply attach and disconnect the wires; this will allow you to leave the reference CIM in the gearbox all the time. As this is likely to carry significant current, a regular switch is probably not the best way to do this. I would put a power pole connector on each lead (probably both in the same oddball color so I knew this was my reference CIM), and connect/disconnect them to short/open the motor.

[Alan Anderson]

Originally Posted by GreyingJay

At what point has irreparable damage been done? I assume the fact that I can smell plastic is not good news,...

The lasting damage on many motors is when plastic brush holders soften and stop holding the brushes in place reliably. If you can smell plastic, I would call it toast.

[Dunngeon]

This should help answer your question.

[aphelps231]

Originally Posted by GreyingJay

The first day we did a drive practice session, we kept going and going and going, swapping out batteries as needed. Nobody was thinking too much about motor heat or letting the robot rest. Eventually we noticed the smell. Someone described it as smelling like yoghurt. I ascribe it to burning plastic. Knowing where it was likely coming from, we turned our robot upside down and, sure enough, the four CIM motors were extremely hot.

We learned a similar lesson about motors and gearboxes after our first district this year. We used to have a four bar linkage with a U shape on the top formed by two prongs on the driver and follower arms, and the coupler link (I have a close up of the arm in our reveal video). Given how tricky the arm was to operate at our first district, we decided to take it off. Before we had gotten to removing the arm, the drive team was practicing with the robot and ended up burning out the Mini CIM attached to it because one of the limit switches had failed. After removing the arm, doing a bit of research, and testing the arm again, we concluded that the motor had gotten so hot that it boiled (or just heated to a high temperature, I was in the shop when this happened) the lithium grease inside of the Banebots gearbox it was attached to.

Long story short, cooling CIMs is probably a good idea in between tight matches, and CIMs are tough motors, they're pretty hard to burn out.