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  #31   Spotlight this post!  
Unread 05-13-2017, 10:09 AM
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GeeTwo GeeTwo is offline
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Re: 775 Pro active cooling

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Originally Posted by gerthworm View Post
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.
Yes, high temperature causes low current, by increasing resistance of the coils. This is the primary reason that power output on the "peak power test" falls off with time (pre-failure).
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Last edited by GeeTwo : 05-13-2017 at 10:13 AM.
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Unread 05-13-2017, 10:35 AM
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Re: 775 Pro active cooling

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Originally Posted by Kevin Ainsworth View Post
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.

One last thought, we were running our motors at full power in reverse...
Didn't Kevin just point out the huge flaw in reasoning we may all be making here?

1108's robot this year didn't even have a 'front'. Gear on one side, balls pickup on the other. We run the motors 'forward' half the time and 'reverse' half the time. The motors on the right side of the robot are probably running CW, while the left side is running CCW. So we know that the fans inside the 775pro are sometimes blowing out the radial holes and sometimes blowing out the front. And I think this is about 50% for our 'bot.

We had people talking about blowing air through vs. sucking earlier, which I think is pretty much moot because the pressure difference is so small. In electronics cooling I've done, you always blow because you don't want hot air to burn up your blower motor over thousands of hours of use...not really a concern in FRC.

If we're going to do this blow-through-suck-through cooling right, we'll have to have a system that switches the direction of the air when the motor changes directions. Not sure I've ever seen a COTS fan that can do that. Also, this means axial propeller, because blowers don't suck when you turn them backwards. Or you could design a 'flapper' into your cooling manifold to switch the direction. But oh-so-complicated.
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Unread 05-13-2017, 10:46 AM
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Re: 775 Pro active cooling

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Originally Posted by hrench View Post
The motors on the right side of the robot are probably running CW, while the left side is running CCW. So we know that the fans inside the 775pro are sometimes blowing out the radial holes and sometimes blowing out the front.
Actually, Kevin is correct.

The fans are blowing out the the radial holes whenever the speed is high enough, regardless of which direction the motor is spinning. That is the way radial blade centrifugal fans work. In single spin-direction applications, the blades can be curved so that preferred direction of rotation blows harder than the other. But these motor fans have straight blades.
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Unread 05-13-2017, 10:52 AM
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Re: 775 Pro active cooling

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Originally Posted by Richard Wallace View Post
Actually, Kevin is correct.

... these motor fans have straight blades.
thanks. didn't know.
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Unread 05-14-2017, 10:02 PM
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Re: 775 Pro active cooling

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Originally Posted by Kevin Ainsworth View Post
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.
Who else has observed a 775 failure and opened it up afterward? Did it fail in the same place?

If the failure mode is in the brush holder arm, blowing (more) air through the motor might not be sufficient to prevent the failure. If the cross section of the brush arm is as small as Kevin is describing, it's ability to transfer heat to the surrounding air will be very limited and moving more air over it probably won't increase the heat removal by much. Does anyone have a photo of the end cap of the motor with the brushes attached? It is likely that the brush arms are right up against the plastic end cap so it will be difficult to get significant air flow over the brush arms without (illegal) modifications to the motor.

The thermal mass of the brush arm would also be so small (and the thermal time constant so short) that it would likely be futile to try to use thermal sensors on the outside of the motor to protect it.

The only protection scheme that would have any certainty of working would be a current limiting scheme, as Kevin and others have described.
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Unread 05-15-2017, 07:42 AM
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Re: 775 Pro active cooling

I have a few observations to add here...
1. Many teams will block the vents in the output shaft end of the motor where cooling air is intended to enter to cool the armature.
2. Much of the heat that is generated at the brush end is due in part to current losses but brush friction and commutator arcing contribute to significant temperature rise.
3. Of the motors I have opened, electrical failure has resulted from a breakdown of the motor wire insulation breakdown. At higher temperatures, the insulation will become plastic and the sharp edges of the armature will be forced through the insulation and short to the armature laminations. It seems that this motor could benefit from additional insulation on the armature structure.
4. High temps at the fan will cause the fan to disengage with the armature resulting in a failure to move cooling air through the motor.
Many of the issues with this motor are the same we experienced with the fisher Price motors.
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Last edited by Al Skierkiewicz : 05-20-2017 at 01:16 PM.
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Unread 05-16-2017, 07:13 AM
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Re: 775 Pro active cooling

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Originally Posted by Al Skierkiewicz View Post
1. Many teams will block the vents in the output shaft end of the motor where cooling air is intended to enter to cool the armature.
On an actuator or something that only works 10% duty cycle, I could see this working, but on drive train, wouldn't this fail in the first match?
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Unread 05-16-2017, 08:19 AM
BoilerMentor BoilerMentor is offline
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Re: 775 Pro active cooling

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Originally Posted by hrench View Post
On an actuator or something that only works 10% duty cycle, I could see this working, but on drive train, wouldn't this fail in the first match?
If any team treats a 775pro drive like a CIM drive they're very likely going to have a bad time open front vents or not.

I'm hoping to have better data on this from testing over the summer on a purpose built testing rig. We'll be primarily using it as a test bed to develop a drive train current management program.

Here are the highlights. I'll start another thread specific to the project at some point when we have the physical configuration totally nailed down.

Suggestions welcome.
  • 8x 775Pro (4 motors per gearbox)
  • Two stage, non-shifting gearbox current tentative target for ratio is ~14:1
  • 4" Colson wheels
  • 6 wheel drop center
  • #25 roller chain for power transmission
  • direct driven rear wheels
  • Provision for cooling of individual motors, likely FDM housning and 40mm fan
  • Electronics system laid out in a planar configuration for the purpose of thermal imaging analysis and ease of troubleshooting
  • Provision for bringing test chassis to competition weight
  • Provision for monitoring/logging of motor temperature, possibly just external motor chassis temperature
  • Provision for monitoring 120amp breaker temperature
  • Robust data collection and analysis provision
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Unread 05-20-2017, 01:21 PM
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Re: 775 Pro active cooling

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Originally Posted by hrench View Post
On an actuator or something that only works 10% duty cycle, I could see this working, but on drive train, wouldn't this fail in the first match?
You would think, but I think our experience shows that this motor is actually somewhat tolerant of abuse. Since the motor is turning, the fan does move air, just not very efficiently.
As a guess, about 75% of motor failures I have seen that were not due to a lock condition were due to overheating caused by blocked ports on the motor. I obviously do not hear about all failures.
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