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Unread 05-12-2017, 01:09 PM
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Re: 775 Pro active cooling

Or you could use a CIM?
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Unread 05-12-2017, 01:14 PM
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Re: 775 Pro active cooling

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Originally Posted by Kyle Serrano View Post
Or you could use a CIM?
That mindset is parallel to the idea of just buying a COTS drivetrain.

Sure, it's totally doable. But how much does it teach you?
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Unread 05-12-2017, 01:24 PM
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Re: 775 Pro active cooling

I run into these type of heat transfer problems in my daily job. There's more to cooling than just matching the total heat flux of the two systems.

The discussed scenarios are talking about stalled or locked rotor instances. When the rotor is locked, the air will not swirl through the motor and evenly distribute the cooling and therefore create local heat zones where the air flow is blocked. I'm certain that even with enough total flux to remove the total heat generated from the motor, the local areas will exceed the temperature limits and still smoke the motor.

If the rotor is still spinning, forced air injection would help remove more heat, but a locked rotor is still at the mercy of its material limitations.
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Unread 05-12-2017, 01:25 PM
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Re: 775 Pro active cooling

So, the highest static pressure rating I've seen on a tube-axial 40mm fan is 27mm h2o, which is an astonishingly high static pressure rating for a 40mm fan. This fan isn't legal (unless CTRE or somebody decided to sell a motor controller cooling accessory that uses it, if I'm reading that rule correctly <R32>)

Unless we're considering a pretty novel approach here, I'm assuming we're talking about pulling air through the motor, because a) air flows in the front of the motors and out the sides and rear and b) a solution to pressurize the motor vents on the front isn't straight forward or really all that accessible in most cases, but it could be done.

In the case of the fan and air flow configuration described above we're actually talking about decreasing the static pressure in the motor case by about 2 Torr with no flow, so we'd see a temperature decrease of an astonishing .056C. It's really pretty negligible, per what I can only assume are my exceedingly inadequate calculations. (I used 760 Torr as standard pressure and 21C as room temperature, in case anyone is wondering)

Unless somebody wants to find a dimensionally accurate model of the internal geometry of the motor and beat it to death with some wicked CFD work the only conclusion I can come to is:

There is no question a duct and fan running constantly to move air through a 775pro over the course of the match will help with heating, but It's not going to let you run them 1 for 1 in place of CIMs. Some other effort to reduce individual motor loading is likely necessary, be it using more motors, imposing current limits, or some combination of those and other ideas.

The potential exists for some super scary single speed as well as shifting drives next season.

The potential also exists for eight 775pros worth of smoke to come billowing simultaneously out of a robot, which could be kind of cool to see, but I'm sure would be a heartbreaking experience for the team involved.
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Unread 05-12-2017, 01:28 PM
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Re: 775 Pro active cooling

Quote:
Originally Posted by Deke View Post
I run into these type of heat transfer problems in my daily job. There's more to cooling than just matching the total heat flux of the two systems.

The discussed scenarios are talking about stalled or locked rotor instances. When the rotor is locked, the air will not swirl through the motor and evenly distribute the cooling and therefore create local heat zones where the air flow is blocked. I'm certain that even with enough total flux to remove the total heat generated from the motor, the local areas will exceed the temperature limits and still smoke the motor.

If the rotor is still spinning, forced air injection would help remove more heat, but a locked rotor is still at the mercy of its material limitations.
Isn't locked rotor the ultimate worst case scenario here? Brushes are first to die in this use case, right? I'm not sure I fully understand which pieces of that beautiful motor data provided by VexPro are actually applicable to this conversation.
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Unread 05-12-2017, 02:39 PM
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Re: 775 Pro active cooling

Quote:
Originally Posted by philso View Post
I doubt that Ike is advocating using a compressor. A fan will increase the air pressure but with commercially available fans, we have never noticed a measureable increase in air temperature due to the air pressure rise when working with our heat sinks i.e. fans running but no output load on equipment under test.

Having said that, it would probably take something like a compressor to get significant air flow through the 775 motor but then you run into other problems (see my previous post).
Correct. I am more referring to giving its current fins in the back a bit of a boost by sucking air through the motor that way. IE, the 775 draws air in near its nose and exhausts near the wiring end.

I was not sure what the internal temps were. With the cooling circuits I deal with on vehicles, a few degrees (say 3F), is difficult to measure, but can make a roughly 3% difference in cooling effectiveness.

Overall though Dr. Joe has the right idea. When going down that route, it is best to stay on the more efficient side of the hill.
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Unread 05-12-2017, 03:23 PM
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Re: 775 Pro active cooling

Quote:
Originally Posted by IKE View Post
Correct. I am more referring to giving its current fins in the back a bit of a boost by sucking air through the motor that way. IE, the 775 draws air in near its nose and exhausts near the wiring end.

I was not sure what the internal temps were. With the cooling circuits I deal with on vehicles, a few degrees (say 3F), is difficult to measure, but can make a roughly 3% difference in cooling effectiveness.

Overall though Dr. Joe has the right idea. When going down that route, it is best to stay on the more efficient side of the hill.
The 775Pro actually ingests air through the vents on the mounting face and the brush cap, opposite side where the wires enter. The hot air is exhausted out the vents on the outer surface of the can. You do not want to force air in through the radial slots as this is where the fan blows the hot air out. The failure point of all the 775Pros (non-drive train related!!! for all the trolls out there) we have seen this year is on the brush arms. See the link below for pictures. The brush arm seems to overheat where its cross sectional area is reduced, where the brush is inserted into the brush spring arm. Either this or there isn't great contact between the brush and brush arm and the heat is generated at the contact point which therefore melts the brush arm. Older style RS775 motors had brushes with braided copper wire and torsion springs which then lead to failure at the windings, not the brushes. I would recommend adding the cooling into the brush end cap vents where the wires enter the motor, this would be most likely to cool the brush/spring arms which is the main point of failure.

I think that time would be better spent limiting the current/voltage based on motor speed or some safety controls scheme. You can run full battery voltage when the motors are spinning freely and the system browns out without it at low speeds, so you will need this anyways. And on the flip side, a fully stalled motor will live a very short life no matter what cooling you add.
If one zip tie, rivet, nut, etc kicked up from the field gets into your gearbox, no cooling feature will protect the system.

One last thought, we were running our motors at full power in reverse at Miami Valley for the first few matches. The result during a pushing match, while we were driving backwards, was that the main breaker tripped. We ran those same motors the next three events without changing them out. This shows to me that the available battery power across 4-8 motors limits the voltage/current to each and pushes the failure point to the main breaker, and therefore not allowing the motors to fail. I think we should lock up one of the develop gearboxes and give all four motors full power to see what happens with a robot battery and a main breaker directly attached to the motors. Do the motors smoke or does the main breaker trip?

Refer to "Death of a 775Pro..."
https://www.chiefdelphi.com/forums/s....php?p=1667123
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Unread 05-12-2017, 04:29 PM
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Re: 775 Pro active cooling

The following blue box (or reasonable facsimile) has been in the rules for a few years, this year on R82:

Quote:
Originally Posted by blue box
The following devices are not considered pneumatic devices and are not subject to pneumatic rules (though they must satisfy all other rules):
  • A. a device that creates a vacuum
  • B. closed-loop COTS pneumatic (gas) shocks
  • C. air-filled (pneumatic) wheels
It doesn't need to be a fan to create a vacuum - and that opens up some possibilities for more than an inch of water.
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Unread 05-12-2017, 06:11 PM
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Re: 775 Pro active cooling

Quote:
Originally Posted by BoilerMentor View Post
Isn't locked rotor the ultimate worst case scenario here? Brushes are first to die in this use case, right? I'm not sure I fully understand which pieces of that beautiful motor data provided by VexPro are actually applicable to this conversation.
There are two high heat scenarios: Peak Power Test and Locked rotor stall test. http://motors.vex.com/vexpro-motors/775pro

Peak Power = rotor still spinning = even distribution of airflow
The motor lasts ~ 80 seconds in this condition according to the test data.

Locked Rotor = rotor not spinning = zero airflow
The motor lasts at different intervals depending on supplied voltage. ~ 3 seconds for 12V.

So this conversation revolves around the stall condition, since we know the motor will last ~80 seconds if the rotor keeps spinning.

It becomes very difficult to provide uniform cooling without swirling air with a locked rotor and forcing air through the motor. While it may not be impossible to prolong the life of a motor with forced air cooling at a stalled condition; it's a very difficult engineering task to save a substantial amount of motor life. There will be internal blockages (fan, brushes, windings, etc.) of air flow where the dead zones won't transfer enough heat away from the material to keep it operational. When the rotor is spinning, it swirls the air and evens out the heat load.

I agree that your best bet, as others have suggested, is to design from stalling or limiting the current/voltage.
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Unread 05-12-2017, 08:53 PM
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Re: 775 Pro active cooling

Quote:
Originally Posted by Joe Johnson View Post
It actually isn't that hard to write code to estimate the temperature of the armature windings given Voltage, Speed, and Current.
For the RS775 Pro I came up with:

T = 20 + 254.5*((11.17*V-0.007128*N)/I-1)

... where
T is coil wire temperature in degrees C,
V is volts,
I is amps, and
N is motor speed in rpm

I'm not sure how to design a test to validate this.

Maybe run a 775 Pro at max power until it heats up, note the voltage, speed, and current, then immediately depower the motor and measure the coil resistance?



Last edited by Ether : 05-12-2017 at 09:04 PM. Reason: added color highlighting for clarity
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Unread 05-12-2017, 09:18 PM
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Re: 775 Pro active cooling

Quote:
Originally Posted by Ether View Post

I'm not sure how to design a test to validate this.

Maybe run a 775 Pro at max power until it heats up, note the voltage, speed, and current, then immediately depower the motor and measure the coil resistance?


The challenging step is measuring the resistance accurately and quickly. Thermal time constants for motors this size are very short. You'll need a good (e.g., 6-1/2 digit) meter capable of 4-wire resistance measurements, and a DPDT relay to switch rapidly from powering the motor to measuring resistance at its terminals. Then you'll need to record measured resistance at regular intervals (best to automate that, for motors this size you'll need a data point every five seconds; more frequent points will give better results) and plot them vs. time, so you can extrapolate back to the instant the motor stopped. This method is often called "rise by resistance" and is documented in NEMA standard MG-1.
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Unread 05-12-2017, 09:30 PM
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Re: 775 Pro active cooling

Quote:
Originally Posted by Richard Wallace View Post
The challenging step is measuring the resistance accurately and quickly. Thermal time constants for motors this size are very short.
How about this then.

Validate the model on a CIM instead.

Run a CIM under load till it gets hot.

Note the voltage, current, and speed and then immediately stall the motor and measure the current.


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Unread 05-12-2017, 09:40 PM
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Re: 775 Pro active cooling

Quote:
Originally Posted by Ether View Post
How about this then.

Validate the model on a CIM instead.

Run a CIM under load till it gets hot.

Note the voltage, current, and speed and then immediately stall the motor and measure the current.


That will work with the 775 also. However, the problem is the same. The current and voltage measurements will be changing rapidly because the armature coil temperature is changing rapidly. The CIM has a larger armature, so the time interval at which you'll need to record measurements can be a little bit longer.
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Unread 05-12-2017, 09:44 PM
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Re: 775 Pro active cooling

Quote:
Originally Posted by Richard Wallace View Post
That will work with the 775 also. However, the problem is the same. The current and voltage measurements will be changing rapidly because the armature coil temperature is changing rapidly. The CIM has a larger armature, so the time interval at which you'll need to record measurements can be a little bit longer.
If you've got the current, voltage, and speed on DSO, all you have to do is stall the motor? Should be able to do that pretty quickly?


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Unread 05-13-2017, 09:29 AM
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Re: 775 Pro active cooling

Quote:
Originally Posted by Ether View Post
T = 20 + 254.5*((11.17*V-0.007128*N)/I-1)


Question,

Why does I show up in the denominator? Intuitively I would think large currents would cause higher temperatures. However, I am not at all versed in the physics described here... perhaps this is tied to how current/voltage/speed are connected anyway? I will continue to ponder....

Update:
Pondering may have gotten me there....

In a simple model of the motor, winding resistance will be impacted by temperature. Higher temperatures imply higher resistances (in most materials). Given an unchanged applied voltage, and unchanged Speed (which implies an unchanged back-emf), a higher temperature will mean a bigger winding resistance, which would lower current draw. Once could deduce then that if a motor's speed and applied voltage remain unchanged, but the current decreases, the temperature of the winding coils must have gone up.

Last edited by gerthworm : 05-13-2017 at 09:35 AM.
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