Computer fans?

My team has already smoked a pair of 775s while stalling them, so we would like to strap a computer fan to them. We were considering just hooking them up to the VRM, which means they would run even when the robot is disabled. Is this allowed?

R36 and R73 seem somewhat related but we’re still unsure.


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Start with R34, and make sure you use legal fans.

Computer fan won’t do much - they havae neither the flow rate nor static pressure to move an appreciable amount of air through the 775 to do anything remotely resembling preventing smoking the motors except for a narrow band of cases. Making sure the front vent holes aren’t blocked and current limiting is usually the right move.


Maybe instead of trying a band-aid solution, we should talk about fixing the root cause of why you’re burining up motors. What application are the motors in? What kind of load are they seeing at what speeds? What gear ratio are you using?


We smoked the 775pros when we were tuning the PID. The i term was building up when it was stalled and they got to about 60% output when they smoked.

We have 2 775pros hooked up to our elevator with a 16:1 reduction and a ~1.5" spool radius. The elevator weighs ~20lbs probably

I get that the best way to not burn out 775pros is to avoid stalling them, but we wanted to add the fans as a safeguard to help cool the motors

Quite frankly, you need to reconsider how you are gearing your system, or change your motors. Choosing motors and gear ratios shouldn’t be a random process. JVN’s Design Calculator is a great tool for preventing things like this from happening.

For example, if you plug your situation into JVN, you can see the problem. I took the liberty of setting your weight to 30lbs due to the likely drag in the system and the fact that, at least in my experience, everything is heavier than you think.

As you can see, your elevator is moving unreasonably fast, and the motors are being pushed to 30amps. This shouldn’t be enough to kill them instantly, but cycle after cycle, they will get hot and they will burn brushes. The second this system jams however, those motors are toast.

If we add another 2:1 stage however,
We half the speed of the elevator and we drop the 775 pros down to a safe 15amps.

Another thing you need to do with a mechanism that has the ability to stall 775pros is verify you have extremely solid limit switches. Those motors will burn our very fast (2 seconds at 12 volts), and if you don’t have a means of preventing those motors from running against themselves (a limit switch), you are going to kill motors like crazy.


The heating occurs in the rotor of the motor. The centrifugal fan on the motor shaft draws air in the arc-shaped slots on the mounting face of the motor casing so it flows over the rotor and expels the hot air from the wide slots near the motor terminals. If the arc-shaped slots are blocked, the probability of burning your motor rises dramatically.

Fans blowing air over the outside of the motor will not remove much heat from the rotor inside the motor casing. The types of fans allowed (axial) do not move much air when working against a high pressure such as when air flow is restricted. They are best at moving a large volume of air over a large surface. If one tries to funnel the air from an axial fan through the motor casing, the pressure rises dramatically and the air flow will drop to near zero.

What is needed to cool the rotor is a centrifugal fan/blower. They are much better at pushing air at higher pressures through a restrictive path. They also have a much smaller outlet than axial fans of similar cfm ratings so it is easier to funnel the air through the motor casing. Unfortunately, I don’t believe that any legal fans are of this type.

Long story short, your only real option is to avoid stalling the 775’s with high applied voltage. There is a long history of them burning up under stall conditions.

The rope radius does not exceed 2".

This means that the current draw should only go up to 20 amps even with 16:1 which should be safe, right?

Remember the difference between radius and diameter. JVN is diameter. Your measurement is radius.

I can’t like this post hard enough. Analysis and conclusion are spot-on.

FWIW - we run 2x775s through a 16:1 VP with a 1in diameter spool (same as last year) with no issues.

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Sorry, what I meant to say is that the diameter does not exceed 2"

The minimum diameter is about 3/4" and then it wraps over itself up to about 1 7/8" diameter

Also we burnt out the 775pros when we had a 5.33:1 reduction, and since then we have added a 3:1 stage. We wanted to add the computer fan to help even more, but it sounds like it won’t do much. I am still interested in understanding the legality of wiring fans to the VRM though

A few of us had discussed this back in 2017. Conclusion: Thermodynamics is fun, and you’ll need something waaaaayyyyy bigger than a computer fan to make a dent in the 100’s of watts dissipated by a 775 pro at stall.

Also, for the record, I would like to state that I am a fan of computers.


The relevant rules are R36 and R54.

You probably could power a fan through a VRM but there is little need for doing so. The VRM is meant to give components such as the radio immunity to short dips in the system voltage. Such dips won’t have much effect on the air flow produced by a fan.

Under these rules we do use a small PC fan to cool our compressor during the match.

I would like to point to R34 please.
Hard drive motors or fans that are: included in any Kickoff Kit, distributed via FIRST Choice, part of a legal motor controller (including manufacturer provided accessories),or part of a legal COTS computing device.
As pointed out above, the 775 pro motor has an integral fan to push out heat developed in the rotor due to electrical IR losses and to brush electrical losses and friction. The fan suck air through each end for cooling. Do not block the ends. Additionally, this fan is only efficient at moving air above a certain speed. No matter how you load the motor (but particularly if you heavily load it) the fan will not move enough air and the motor will fail. Be sure to get the motor into the efficient region of the motor chart or higher, to give the fan a chance to do it’s job. If you couple the motor to transmission, be sure the interface has the ability for cooling air to enter the motor. Some teams will mount the motor blocking the air slots as pointed out above.

That’s fine. The compressor only needs air blown over it, not forced through it.

Not to mention the dissipated energy is many time smaller than a stalled than a stalled 775 pro.

Probably too late now, but you might want to use CIMs instead of 775s simply because of the added power (and maybe adjust gears and ratios, as stated in others’ posts). Unless you have size restrictions on your bot that prevent you from using anything larger than 2.5 inches in diameter.