Help to prevent 775 Pro Burnout

Thanks for the confirmation and specifics of your setup. When we make the switch (you all have convinced us) we’ll be sure to adjust our target velocity in our PID loop. Currently we are using 500 encoder velocity using the CIMs and CIM Encoder, which may be at our upper bounds. We’ll be using a VersaPlanetary Encoder with the 775s. It will be nice to run somewhere below the upper bounds for later in the match when our battery is lower. We haven’t had a lot of testing yet to determine if we’ll have trouble hitting our velocity throughout a full match.

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Thanks for your response. We are using a PID Velocity loop with the TalonSRX. We’ll have to adjust the velocity when we switch motors and encoders.

Be careful if you adjust the velocity, as it depends on where the sensor is located in the system. If it’s not on the final drive, then yes, you’ll have to do the math to estimate the new velocity and tune new PIDF values. If the sensor is on the final drive, it should just be plug and play with some PIDF tuning.

Here is my take. As noted its useful to think about shooters as a motor adding energy into the flywheel which balls then take out. Your testing leads you to a shooter wheel RPM that is best for your shooting. Then the question is how to best transfer energy at that RPM.

VEX has useful motor curves showing efficiency of the 775.

The plot shows the 775 has the best efficiency at very high RPM (~16,000). Also, the internal fan has the best cooling at high RPM. The lesson is run fast to avoid burnout. Design the gear ratios so that the 775 runs at 16,000 RPM and then use a gear ratio to run the shooter at its desired RPM.

Use Neos, instead

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Is this based on reduced chance of burning out? Or just better performance for the weight?

Both 775Pros and CIMs are brushed DC motors. All brushed DC motors will generate more heat when run in near-stall conditions (and when full voltage is applied in general). There is a difference in how these two motors dissipate heat, however. The 775 Pro is a fan cooled motor, while the CIM is a sealed motor. The CIM relies on its thermal mass to “absorb” heat and dissipates it slowly via conduction to attached surfaces and thermal radiation to the atmosphere. The 775 Pro relies on the fan attached to its rotor to pull air through the motor to remove heat via forced convection. Because the fan is attached to the motor’s rotor, the faster the motor is spinning, the more air is circulated through the motor and the more heat is removed. This creates a “double whammy” effect with 775 Pros operating near stall. Not only do they generate more heat when stalled, they also remove less heat.

There are also differences in failure modes between the two motors. 775 Pros tend to fail in what is, for all FRC intents and purposes, an irreparable fashion. The brushes on the motor will physically detach, causing internal damage to the motor as a whole. CIM motors will burn insulation and coatings, and make all sorts of nasty smells, but will generally still be operable to an extent (albeit at a much lower efficiency that may not longer achieve your flywheel’s objectives).

In terms of learning more about DC motor fundamentals and how to gear your motors, there are some great resources out there already. Check out this video featuring WPI Professor and 190 mentor Ken Stafford:

Also check out this page from VEX (already linked a couple times in this thread):

And finally check out the motor specific data for a 775 Pro motor from VEX’s empirical testing:

@donut already mentioned the locked rotor test, but a flywheel isn’t going to be a locked rotor scenario (unless something goes drastically wrong). The test I want to highlight is the peak power test. Even under a 50% load condition (peak power), a 775 Pro cannot operate at 12VDC for the duration of a 2.5 minute match. However, if you relate that peak power condition to the expected current draw (using the motor curves above), you’ll notice that you should trip your breakers before pulling that type of current continuously (~90A). Ultimately, you’re going to want to design your flywheel to be running at 40A or lower (probably lower) continuously. Using the motor curves (and picking the better side of the efficiency curve), you’ll see that 40A translates to about 16,000 RPM. You can then gear down this value to your target RPM for your flywheel (or really a bit over that to allow for PID overhead).

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