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Unread 06-23-2002, 09:23 PM
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How do people get hot motors?

Posted by Elaine Owens at 1/7/2001 3:11 AM EST


Student on team #368, Kika Mana, from Mckinley High School and NASA Ames/Hawaiian Electric /Weinberg Foundation.



Hi, I heard last year that some of the teams had a hard time cooling down their motors and that they kept getting hot. But for our team, last year our motors were fine. They barely felt warm. So I was wondering why some teams had problems with hot motors. Is is beacuse they ran the drill on the high gear....because we used the low setting for our robot(last year).

Elaine


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Re: How do people get hot motors?

Posted by Patrick Dingle at 1/7/2001 9:57 AM EST


Other on team #639, Red B^2, from Ithaca High School and Cornell University.


In Reply to: How do people get hot motors?
Posted by Elaine Owens on 1/7/2001 3:11 AM EST:



If I remember physics correctly, the amount of heat given off by anything is related to the amount of power used. This is also directly related to how fast your battery drains. As Woodie explained at the kickoff, there is a linear relation between torque and speed of a motor. He also explained that the power used is proportional to product of the torque and speed. In other words, the most power is used when there is a perfect tradeoff between speed and torque. Therefore, at a low gear, your motor has a lot of torque, and not as much speed. Since the heat (power) is the product of these two (say torque = 8 and speed = 2), the heat is torque (9) times speed (1), or 9. If there were a perfect tradeoff (torque = 5, speed = 5), then heat = 5 x 5 = 25, which is much greater than 9. The robots that will have the most heat (at least with the drill motors) are the ones that use a medium speed.

I think most times, however, there is not problems with the drill motors. I think most of the times the problems occur with the smaller motors. The problems occur when the smaller motors are continuously draining a lot of power. When designing components of your robot, I think it is a good idea to make sure the motors are geared down as much as possible. That way you get a lot of power w/out eating up tons of power.

Patrick

: Hi, I heard last year that some of the teams had a hard time cooling down their motors and that they kept getting hot. But for our team, last year our motors were fine. They barely felt warm. So I was wondering why some teams had problems with hot motors. Is is beacuse they ran the drill on the high gear....because we used the low setting for our robot(last year).

: Elaine



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Mechanical Power Out Vs. Electrical Power In

Posted by Joe Johnson at 1/20/2001 3:33 PM EST


Engineer on team #47, Chief Delphi, from Pontiac Central High School and Delphi Automotive Systems.


In Reply to: Re: How do people get hot motors?
Posted by Patrick Dingle on 1/7/2001 9:57 AM EST:



Mechanical Power OUT maximizes at 1/2 the stall torque
with the motor running at 1/2 the free speed.

The Electrical Power IN, peaks at the stall torque.

The amount of heat generated is

Mechanical Power Out - Electical Power In

It is this term that makes the motors hot.

In general, the Efficiency Curve :

(Mechanical Power Out) / Electical Power In

Looks like this:


S.....**..................
P....*...*................
E...*......*..............
E...*.........*...........
E..*.............*........
D..*................*.....
...*...................*..
/..T...O...R...Q...U...E..

The important thing to know is it peaks over on the far
LEFT side of the Torque curve.

THE BOTTOM LINE IS THIS:
If you have a certain amount of mechanical work you
want to do in a certain amount of time, you a have
established how much mechanical power you need (e.g.
get a robot and 2 goals up an incline in 6 seconds).

In order to heat your motors up the least while doing
this work is to design a geartrain that loads the
motors in the 1/5 - 1/3 of stall torque range.

Good luck and low motor temps to us all in 2001!

Joe J.



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Re: How do people get hot motors?

Posted by Marjory Stager at 1/7/2001 10:09 AM EST


Student on team #47, ChiefDelphi, from Pontiac Central and Delphi Automotive Systems.


In Reply to: How do people get hot motors?
Posted by Elaine Owens on 1/7/2001 3:11 AM EST:



I am not sure why the motors got hot but I know that we had that problem last year with the motors on our wheels and on the arm. Our wheels would get pretty hot after a big pushing match, or our motors on the arm would if we went up and down a lot. I can tell you that we never really had a problem cooling down, we would just use cooling spray, it was never far from our bot. We took a can out with us every match just in case. Martus might be able to help you when he gets back from NH if anyone hasn't already helped you before then. I wish I could have been of my help.

Marjory
"Frosty"



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Re: How do people get hot motors?

Posted by Jon at 1/7/2001 11:30 AM EST


Engineer on team #190, Gompei, from Mass Academy of Math and Science and Worcester Polytechnic Institute.


In Reply to: How do people get hot motors?
Posted by Elaine Owens on 1/7/2001 3:11 AM EST:



We've noticed that the motors didn't cool properly due to a lack of ventilation holes...

this last year we test-built a forced circulation system so that we could get a little more air in there since our old clamping system prevented air from getting in there. worked alright, we wired it to run anytime the battery was in so that air would move over the motors especially when the robot wasn't in motion.

how do others work against motor heat exhaustion?
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Freez-It.... :o)

Posted by Lora Knepper at 1/7/2001 1:42 PM EST


Other on team #419, Rambots, from UMass Boston / Boston College High School and Seeking Sponsorship.


In Reply to: Re: How do people get hot motors?
Posted by Jon on 1/7/2001 11:30 AM EST:



(note, I'm on a new team this year)

Well....in past years, my old team always had the issue of motors getting extremely hot, though not usually enough to affect preformance in a match, but giving trouble if the matches were close together. I think it was a combo of not having enough air flow around the motors, and not using the muffin fan at all. Let's just say we purchased a lifetime supply of Freez-It )

~ Lora (T419)


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Re: Freez-It.... :o)

Posted by Jon at 1/7/2001 8:33 PM EST


Engineer on team #190, Gompei, from Mass Academy of Math and Science and Worcester Polytechnic Institute.


In Reply to: Freez-It.... )
Posted by Lora Knepper on 1/7/2001 1:42 PM EST:



My crash course on motor technology taught me this:
cooling sprays are ok, but they don't get to the heart of the problem. you're cooling the casing but that's not the source of the heat... the heat is in the armature and whatnot... you can help all that by getting more air in there and keeping it moving...

i like mini muffin fans...

: (note, I'm on a new team this year)

: Well....in past years, my old team always had the issue of motors getting extremely hot, though not usually enough to affect preformance in a match, but giving trouble if the matches were close together. I think it was a combo of not having enough air flow around the motors, and not using the muffin fan at all. Let's just say we purchased a lifetime supply of Freez-It )

: ~ Lora (T419)


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Re: Freez-It.... :o)

Posted by Matt Leese at 1/7/2001 11:01 PM EST


Other on team #73 from Edison Technical HS and Alstom & Rochester Institute of Technology.


In Reply to: Freez-It.... )
Posted by Lora Knepper on 1/7/2001 1:42 PM EST:



The main problem with things like Freez-It and then like is that you risk thermal shock. Thermal shock is decidely a Bad Thing (TM). We consider using it last year and mainly decided that sticking with just air (from muffing fans and compressed air) was the best idea.

Matt

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ok, so you got a tradeoff here

Posted by Peter VanWylen at 1/7/2001 4:48 PM EST


Student on team #107, Team ROBOTICS, from Holland Christian High School and Metal Flow Corporation.


In Reply to: How do people get hot motors?
Posted by Elaine Owens on 1/7/2001 3:11 AM EST:



I'm not sure how many of you saw Woodie's presentation on DC motors at the kickoff yesterday. He did a real nize job of showing the linear relationship b/w torque and power, and then showed the power curve, which had its maximum value at half of stall torque, where the RPM was also half of its no-load RPM. This was really great, but what he forgot to show was the efficiency curve. Efficiency is a measurement of (power in)/(power out). Now, those with a physics background will know about the conservation of energy. We know that all the energy from the battery that goes into the motor goes somewhere. Guess what? That extra energy that doesn't go to the output shaft goes into good old heat. So, as you see energy is still conserved. Well, the point at which efficiency is greatest is also the point at which there is the least heatower ratio.

Basically, on a graph with the torque on the x-axis, we get a power curve which maximizes at half torque. But, the efficiency curve maxes out near 15% of max torque (don't hold me to this, its different for every motor). So, contrary to what Patrick said, the most heat is not put out at the same time as the most power. Talk about one big optimization problem. So, basically you want the torque to be somewhere between the place of max efficiency and the place of max power. On either side of this, both curves go down, and it is a lose-lose situation.

The real question for you is where to put it. If you use a motor very infrequently and need the extra power, then go for it! But remember that a frequently used motor that gets hot, only looses efficiency making things worse. So, for the drivetrain, you must carefully choose a gear reduction that provides enough power, but still stays cool enough for long practice sessions.


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Excellent explanation.

Posted by Matt Berube at 1/7/2001 6:54 PM EST


Engineer on team #49, Delphi Knights, from Buena Vista High School and Delphi Automotive.


In Reply to: ok, so you got a tradeoff here
Posted by Peter VanWylen on 1/7/2001 4:48 PM EST:



:=)
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Re: How do people get hot motors?

Posted by Mark Pierce at 1/9/2001 12:20 PM EST


Other on team #288, RoboDAWGS, from Grandville High School and X-Rite, Delphi, and others.


In Reply to: How do people get hot motors?
Posted by Elaine Owens on 1/7/2001 3:11 AM EST:



There are several things that can cause a motor to overheat. The basic cause is more load than the motor is designed for. Some causes have already been mentioned: Pushing the power/torque limits of the motor and poor air flow.

Another problem can be side loading on the shaft or excess friction in the drive system caused by poor installation or design. This might not show up in practice, but become a problem after things move during the competitions.

Another cause of overheating can be rapid accelleration and deceleration (especially going from full forward to reverse quickly). The speed controllers may have the most problems with this one, but the motors get hot also. Either way, the motor stops for the match.



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Motors will cry uncle before Victors...

Posted by Joe Johnson at 1/9/2001 10:35 PM EST


Engineer on team #47, Chief Delphi, from Pontiac Central High School and Delphi Automotive Systems.


In Reply to: Re: How do people get hot motors?
Posted by Mark Pierce on 1/9/2001 12:20 PM EST:



Except for when we had dead shorted the outputs of a
Victor, I have never had them overheat to the point
that they could not light up a motor like a toaster!

Believe me, the Victors can pump a lot more current
into those motors than they can effectly dissipate.

Don't be trash talking those Victors ;-)

Joe J.



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