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Unread 07-09-2018, 11:10 PM
JarJarTheJedi JarJarTheJedi is offline
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Fan for 775pros

I am working on a drive gearbox that uses 3 775pros. On the Vexpro website, it says that the motor is primarily used for applications that don't have long periods of stall. Have people used 775pro gearboxes as drive gearboxes without major issues? Do you need some sort of fan near the motors cooling them down?
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Unread 07-10-2018, 12:01 AM
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Re: Fan for 775pros

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Originally Posted by JarJarTheJedi View Post
I am working on a drive gearbox that uses 3 775pros. On the Vexpro website, it says that the motor is primarily used for applications that don't have long periods of stall. Have people used 775pro gearboxes as drive gearboxes without major issues? Do you need some sort of fan near the motors cooling them down?
I would recomend checking out PWNAGE's gearboxes from the past. They have had some good success with them. Remember to keep the air vents exposed on the motor! You dont NEED a fan but it is a good idea to keep them cool. I would just zip tie or make a mount for a fan to be used on the back or side of the motor.
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Unread 07-10-2018, 12:24 AM
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Re: Fan for 775pros

Not sure how effective fans would be for cooling motors to prevent a stall from burning them out. 775s burn out in just a few seconds in a stall condition, the amount of heat the generate in that short period is quite impressive.
775s do have internal fans, so as long as they don't reach a stall condition, they generally self-cool fairly effectively.

That said, I have played around with the idea of using pneumatic air pressure to force lots of air through the vent ports of a 775 motor rapidly during a stall condition so that the forced air serves to make up for the lack of airflow from the internal motor fan (since the fan isn't moving during a stall). Not sure if this is legal (technically, the motor wouldn't be a pressurized component, and you'd basically just be venting the pneumatics into the motor) or if it would be practical, but it seems like something that would be interesting to try. In theory, this might also be an effective way to internally cool CIM-type motors if used in conjunction with vented mounting screws.

To answer the first part of your question, my team used a pair of 775s and Decimate gearboxes on each of our two 3-CIM ball shifters this year, along with a pair of MiniCIMs on each gearbox (8 total drive motors). Never had any problems with the motors (aside from some gearbox mounting issues that were later resolved with Loctite). Overall, I'd definately be open to using 775s in a drive again, though it's worth noting that this game also didn't lend itself to the heavy-defense situations that you'd generally worry about a stall, so if we end up with another open-field high-defense game (ala 2014), it might be a harder call to make.
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Unread 07-10-2018, 01:24 AM
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Re: Fan for 775pros

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Originally Posted by cbale2000 View Post
That said, I have played around with the idea of using pneumatic air pressure to force lots of air through the vent ports of a 775 motor rapidly during a stall condition so that the forced air serves to make up for the lack of airflow from the internal motor fan (since the fan isn't moving during a stall). Not sure if this is legal (technically, the motor wouldn't be a pressurized component, and you'd basically just be venting the pneumatics into the motor) or if it would be practical, but it seems like something that would be interesting to try. In theory, this might also be an effective way to internally cool CIM-type motors if used in conjunction with vented mounting screws.
If you were to set up a small stack of legal fans, and then 3D print a duct to fit to the vent port, you could probably get a similar effect. Or, better yet, use another 775Pro with a shrouded fan on it through a reducing duct...
I can't say for sure how legal that would be from a safety point of view, or from a pneumatics point of view, though I do recall a past Q&A that would seem to allow such a thing (though it would need to be asked again to be valid for 2019).
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Unread 07-10-2018, 07:21 AM
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Re: Fan for 775pros

There was a thread on this a while back. The conclusion at the time was "you need a pretty big fan to make a dent".
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Unread 07-10-2018, 08:27 AM
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Re: Fan for 775pros

We ran a single speed 775pro drive this year with 4 motors per gearbox. While machining the gearbox we made sure the motors could still get air by giving them a bit of a standoff. (you could also just machine openings in the plate) I'm sure current limiting helped also.
It would be cool to see the results of less motors!

Here is a snip of what we did.

https://drive.google.com/file/d/1ZHm...ew?usp=sharing

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Unread 07-10-2018, 08:34 AM
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Re: Fan for 775pros

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Originally Posted by gerthworm View Post
There was a thread on this a while back. The conclusion at the time was "you need a pretty big fan to make a dent".
I'd say "to make a dent in the middle of a match". Teams like Stryke Force have made adapters for their shop vacs so they can bring the internals down to ambient temperature quickly between matches.

Consider VEX's testing of the 775pro, especially the locked rotor stall test. As people will tell you, stalling at 12V is insta-death. It takes getting down to 6V before you can really get some breathing room, and even that has a pretty significant drop-off in torque over the time.

You do have to take care of 775-class motors more than you would a CIM or Mini CIM, but it's not impossible. Designing to run the motors on the high-speed side of the power curve, planning to run with voltage and current limits (so get your programmers studying the Talon SRX) well below peak (how far below depends on weight, wheel size, and CoF), and ensuring you've got enough power after all that nerfing to be effective (which is why a lot of single-speed designs use a fourth motor).

Do some deep searching--I'm time-limited this morning, but there are some top-notch threads on "775s on drive".
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Unread 07-10-2018, 09:04 AM
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Re: Fan for 775pros

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Originally Posted by EricH View Post
If you were to set up a small stack of legal fans, and then 3D print a duct to fit to the vent port, you could probably get a similar effect. Or, better yet, use another 775Pro with a shrouded fan on it through a reducing duct...
I can't say for sure how legal that would be from a safety point of view, or from a pneumatics point of view, though I do recall a past Q&A that would seem to allow such a thing (though it would need to be asked again to be valid for 2019).
It might be different with the FRC-legal fans, but I actually tried something similar with an 80mm computer fan a few months back and found that if you try to reduce the air channel smaller than a certain point (less than about 50% the diameter of the fan), the airflow simply "leaks" back out around the fan blades and doesn't go into the duct.

Now that said, I could see a fan mounted on the back of a 775 with a 3D printed duct that forces air through the rear vents as having some potential (perhaps in a "pulling" configuration), and it could augment the existing internal fan without interfering with it, though I still think pneumatics may be the only way to force enough air through fast enough to prevent (or delay) a stall burnout.
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Unread 07-10-2018, 01:20 PM
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Re: Fan for 775pros

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Originally Posted by cbale2000 View Post
It might be different with the FRC-legal fans, but I actually tried something similar with an 80mm computer fan a few months back and found that if you try to reduce the air channel smaller than a certain point (less than about 50% the diameter of the fan), the airflow simply "leaks" back out around the fan blades and doesn't go into the duct.

Now that said, I could see a fan mounted on the back of a 775 with a 3D printed duct that forces air through the rear vents as having some potential (perhaps in a "pulling" configuration), and it could augment the existing internal fan without interfering with it, though I still think pneumatics may be the only way to force enough air through fast enough to prevent (or delay) a stall burnout.

Axial fans like computer fans work best at moving air in low back pressure applications like ventilating a box. Centrifugal fans give higher air flow than axial fans in applications with higher back pressure like forcing air along a long heatsink with a tight shroud around it or through a motor like the 775's. Centrifugal fans also tend to have a smaller outlet so there is less need to reduce the cross sectional area of the air path so there will be much less of the effects you are describing in your first paragraph.

The testing my colleagues did showed that at some point, increased airflow actually decreased the amount of heat removed from a heat sink. If I recall correctly, a high air speeds, the air molecules are not in contact with the heatsink for enough time to absorb the heat. You will need to find a way to install temperature sensors on your motor to get meaningful, quantitative results.
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Unread 07-10-2018, 03:11 PM
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Re: Fan for 775pros

My team ran 8- 775 Pros on our drivetrain and got them warm, but never to the point of stalling/ destroying. We used current limiting to help protect the amount of current that was allowed through (acting as a protector).
On our robot cart (inspired by Stryke Force's 2017 and 2018 cart), we installed 2- electric car radiator fans that would move a ton of air across the motors to cool them. A motor almost too hot to touch when placed on the cart was cold by the time we walked back to the pits.
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Unread 07-10-2018, 04:18 PM
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Re: Fan for 775pros

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Originally Posted by Type View Post
My team ran 8- 775 Pros on our drivetrain and got them warm, but never to the point of stalling/ destroying. We used current limiting to help protect the amount of current that was allowed through (acting as a protector).
On our robot cart (inspired by Stryke Force's 2017 and 2018 cart), we installed 2- electric car radiator fans that would move a ton of air across the motors to cool them. A motor almost too hot to touch when placed on the cart was cold by the time we walked back to the pits.

The part of the motor that really needs to be cooled is the rotor. Moving air over the motor will mostly only cool the outside can of the motor. The rotor may still be hot when the can feels cool.
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Unread 07-10-2018, 04:40 PM
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Re: Fan for 775pros

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The part of the motor that really needs to be cooled is the rotor. Moving air over the motor will mostly only cool the outside can of the motor. The rotor may still be hot when the can feels cool.
May, but testing has shown a fan blowing on the motor after hard running makes a meaningful impact. Maybe not enough to totally cool a CIM-based drivetrain between finals matches, but an 83% drop in cooling time for the air inside is significant in my book.

A 775-class drivetrain should be able to cool even faster, since it has less thermal mass, and I imagine big radiator fans cool a bit better than a 120mm KOP fan. (5402 used those in testing in 2017 mounted axially with respect to the CIMs, and they made a difference but didn't reach to the outside edges of the cans so there was definitely meat left on the bone.)

So I suppose the lesson is "let the fan run beyond the time the outer can is cooled to ambient", but how long is a matter for more instrumented testing (and then better data logging than Aren's post, though I appreciate him sharing what he has).
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Unread 07-11-2018, 06:21 AM
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Re: Fan for 775pros

We tried these heat sinks on our robot last year. We did not do any testing to measure the impact, but they did look pretty cool.

We still smoked a lot of motors, so take this with a grain of salt...
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Unread 07-11-2018, 11:03 AM
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Re: Fan for 775pros

Last summer in our testing of our 775 pro chassis that we ran this competition season, we took some thermal images of the motors after high load activity. It's apparent looking at the photos that the primary heating in the motors is occurring in the region of the brushes and brush arms. Active cooling will do little to protect these regions because of how quickly heating occurs and how little thermal mass the components in question have.

Instead of active cooling, I would strongly suggest considering imposing an artificial current limit in your design space. Since the standard in FRC drive design is to target a traction limited situation, make sure you cross your traction limit at a lower current. This means more gearing (larger reduction) and a lower limit for peak robot speed if you're targeting a single speed drive. JVN's calculator outputs the current information natively.

For reference we were geared for 21ft/s theoretical with imposed limits of 14 ft/s in auto and 18 ft/s in teleop with a full weight robot on black 6" hi-grip wheels in a drop center six wheel configuration with eight motors in the drive. Our theoretical traction limit occurred at ~40% rated stall torque. In changing traction materials for off season play we discovered that by increase our available traction our drive was no longer traction limited, because we were exceeding the capacity of the battery to supply the required current. It's a new limit, and something we're developing a method to instrument for testing purposes.

If you have any specific questions, please reach out. People seem to be afraid of these drive trains, and there are considerations, but the threshold for safe use of 775's in the drive is not so daunting as often advertised.
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Unread 07-11-2018, 11:23 AM
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Re: Fan for 775pros

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Originally Posted by BoilerMentor View Post
Last summer in our testing of our 775 pro chassis that we ran this competition season, we took some thermal images of the motors after high load activity. It's apparent looking at the photos that the primary heating in the motors is occurring in the region of the brushes and brush arms. Active cooling will do little to protect these regions because of how quickly heating occurs and how little thermal mass the components in question have.

Instead of active cooling, I would strongly suggest considering imposing an artificial current limit in your design space. Since the standard in FRC drive design is to target a traction limited situation, make sure you cross your traction limit at a lower current. This means more gearing (larger reduction) and a lower limit for peak robot speed if you're targeting a single speed drive. JVN's calculator outputs the current information natively.

For reference we were geared for 21ft/s theoretical with imposed limits of 14 ft/s in auto and 18 ft/s in teleop with a full weight robot on black 6" hi-grip wheels in a drop center six wheel configuration with eight motors in the drive. Our theoretical traction limit occurred at ~40% rated stall torque. In changing traction materials for off season play we discovered that by increase our available traction our drive was no longer traction limited, because we were exceeding the capacity of the battery to supply the required current. It's a new limit, and something we're developing a method to instrument for testing purposes.

If you have any specific questions, please reach out. People seem to be afraid of these drive trains, and there are considerations, but the threshold for safe use of 775's in the drive is not so daunting as often advertised.
I agree that active cooling is of limited utility during the match, though I would venture a fan setup for after the match is more valuable in bringing the motors down to ambient temperature.

I'll ask a couple about your particular setup: How did you implement your speed limits? Did you replace any motors during the season, either due to failures or as preventative maintenance?
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