Does motor temp affect performance?

[NC GEARS]

We just completed the Michigan State Championship last weekend and during all of the qualification matches we were running top notch. Drive fast, turn efficiently, cross bump with ease etc. Once we got into the elimination rounds, our robot performance started to drop drastically. We kept trying to figure out what was wrong but we really could not find a cause. In the actual Finals rounds we started to feel our CIMS (4 of them 2per side for drive train) and they were extremely hot, literally almost too hot to touch. We were curious as to if the temperature of the motors could have something to do with performance of our robot? If anyone has any info on how CIMs will work/break down, how temperature affects them, etc that would be great! Thanks.

[Joe Johnson]

Originally Posted by NC GEARS

We just completed the Michigan State Championship last weekend and during all of the qualification matches we were running top notch. Drive fast, turn efficiently, cross bump with ease etc. Once we got into the elimination rounds, our robot performance started to drop drastically. We kept trying to figure out what was wrong but we really could not find a cause. In the actual Finals rounds we started to feel our CIMS (4 of them 2per side for drive train) and they were extremely hot, literally almost too hot to touch. We were curious as to if the temperature of the motors could have something to do with performance of our robot? If anyone has any info on how CIMs will work/break down, how temperature affects them, etc that would be great! Thanks.

Hot motors have less output primarily because the have higher resistance in the windings of the motors which means lower stall currents, which means lower stall torque, which means that for a given load you need to operate closer to the stall point, which means you are less efficient at turning electron energy into shaft energy which means you generate more heat which increases the resistance of your windings... ...rinse and repeat.

If you are SURE that nothing bad has happened to your drive train and that you are just overheating your motors, then I fully recommend buying a 24 can case of "Freez it" or similar product and icing the motors before and after every match*.

I recommend keeping ahead of the heat because by the time you feel that the outside can of a CIM is warm the inside is HOT HOT HOT. Freeze 'em early, Freeze 'em often. That'd be my motto. If it weren't illegal, I'd put a small hole in the can of the motor and drizzle the liquid from the can right into the motor body itself that way the evaporating liquid would directly cool the bits of the motor that need cooling the most. As it is, you'll have to cool the case and depend on thermal conductivity to do the rest.

I loved you in Ypsi. Good luck in Atlanta -- buy a 24 case really. You'll thank me (again, assuming you have 100% certainty that your drive system is as it should be and it is just repeated, frequent matches that are causing the problem)

Joe J.

*I know some who argue that the thermal shock is too much for a motor but in the years I have been doing FIRST, I have never seen a motor fail this way and I have used this method on many many robots.

[EricVanWyk]

One effect of heat is that the resistance of the motor will increase, which will decrease the amount of current (torque).

Increasing the winding resistance by 100 C increases resistance by 40%, so a super liberal estimate is that you could lose 30% of your torque. However, this doesn't factor in A) any other resistances in the path or B) that a 100C rise is really really hot.

I'm betting that they got hot -> increased resistance -> decreased efficiency -> got hotter, but the real cause for your drop in power was your battery giving up. Running that hot is not good, but it probably isn't your only issue.

[Tom Bottiglieri]

Originally Posted by EricVanWyk

I'm betting that they got hot -> increased resistance -> decreased efficiency -> got hotter, but the real cause for your drop in power was your battery giving up. Running that hot is not good, but it probably isn't your only issue.

I agree with what Eric has to say here...

How old were the batteries you ran for elims? What were your charging cycles like? Have you ever run these batteries down below 12 volts?

[NC GEARS]

Our oldest batteries are only a couple of years old and they have all been load tested and seem to be fine. All the cables have been reattached so we dont have any loose connections either. I noticed that some of our battery cables were hot when we took them out of the bot as well. Any idea if that may have something to do with it?

[thefro526]

Originally Posted by Joe Johnson

If you are SURE that nothing bad has happened to your drive train and that you are just overheating your motors, then I fully recommend buying a 24 can case of "Freez it" or similar product and icing the motors before and after every match*.

I recommend keeping ahead of the heat because by the time you feel that the outside can of a CIM is warm the inside is HOT HOT HOT. Freeze 'em early, Freeze 'em often. That'd be my motto. If it weren't illegal, I'd put a small hole in the can of the motor and drizzle the liquid from the can right into the motor body itself that way the evaporating liquid would directly cool the bits of the motor that need cooling the most. As it is, you'll have to cool the case and depend on thermal conductivity to do the rest.

Joe,

Is there a noticeable performance gain by cooling motors like this, or is it just a good preventive measure? I was always told that cooling the motors in such a way could damage the motor, but your post has sparked my interest. Maybe I'll pick up a can as an experiment.

Originally Posted by EricVanWyk

I'm betting that they got hot -> increased resistance -> decreased efficiency -> got hotter, but the real cause for your drop in power was your battery giving up. Running that hot is not good, but it probably isn't your only issue.

I would guess that some of it was due to Battery levels as well. In 2008 our robot was a real battery drainer, and as we would go through the elims our batteries never got the full charge time (Close matches, and we only had 2 or 3 batteries) so we would go out to the field with weak batteries. This wreaked all kinds of havoc on our robots performance, especially in autonomous mode.

[Joe Johnson]

Originally Posted by thefro526

<snip>
Is there a noticeable performance gain by cooling motors like this, or is it just a good preventive measure? I was always told that cooling the motors in such a way could damage the motor, but your post has sparked my interest. Maybe I'll pick up a can as an experiment.

<snip>

This effect is real and measurable. In automotive applications, it is often the case that a motor at -40C breaks the actuator because it has more power/stall torque (and the plastic is generally more brittle).

As to FIRST, the cooling effect of Freez It is fleeting in that you do not have an infinite sink of -40C to draw from so your motors get hot eventually anyway but if you start warm, then get warmer, then start warmer get hot, then start hot and get hotter, then start hotter and get boiling, you are fighting a loosing battle.

If you can keep your motors starting a match at just warm or warmer then you can keep from overheating by the end of the match (potentially).

The problem with a brushed DC motor like the CIM is that it is totally enclosed. All of the heat is generated in the windings of the armature. How does that heat get out? Well, a little is conducted away via the shafts and a little is convected away via the air inside the can but that air has to then re-convect (if that is a word) the heat back to the inside surface of the can of the motor for it to be conducted away to the outside of the can and then convected away by the air outside the robot.

All this takes time. If you have time, all is well, but if you don't (as in the elims) the heat builds up.

If you open the can of your motors, you are likely going to smell something that to this day makes me ill, burnt motor insulation.

By the way, your gear ratio is probably not high enough but that cake is already baked. From what I saw of you at the MSC, you guys are doing just fine with the too tall of gearing you have. You just need to make sure your motors are cooled down between matches and you'll do fine -- next year, consider a bit more ratio ;-)

Joe J.

[Ether]

Others have already mentioned increased resistance of the motor windings.

Heat also affects the magnets and magnetic circuits within the motor, decreasing the torque output for a given current.

[Al Skierkiewicz]

I have to add to Joe's statement about Freeze spray is that some products contain other chemicals that can wash lubricants out of bearings. Use with caution. Remember that the majority of the heat generation takes place in the windings that are mainly coupled to the outside of the motor through radiation/convection within the motor. There is very little air movement inside a closed motor. The only real connection to the body is through the bearings and brushes.
The effects of heat in addition to those above is the possible breakdown of the insulating varnish on the windings (eventually leads to failure), the change in efficiency of the magnetic structures in the armature and the expansion of metal parts inside and outside the motor.
If the motors were hot (could not pass the five second finger rule) and the battery wires were hot as well, this is a good sign that you are using a lot of current. You should consider a cooling scheme for the robot in between matches. High CFM fans come to mind.

[JesseK]

In 2008 we lost a severe amount of available torque for turning due to a misaligned gearbox. We took the AM supershifter box shroud off and replaced it with plastic spacers when we got to Atlanta. The plastic spacers allowed us to tighten the top of the gearbox more than the bottom of the gearbox, thus making the plates non-parallel. This led to our gearbox binding ever so slightly and it was enough to be extremely annoying on the field.

I would double check that all of your shafts, plates, gears and other critical parts are aligned properly. This year's highly interactive game may have knocked something loose.

[sanddrag]

Our CIMs get burning hot every time. When running for more than a few minutes with them hot, you'll permanently damage them, resulting in a significant loss of power. Quite often they'll be black on the inside. We have a box of about a dozen CIMs this has happened to over time.

[Dad1279]

Wouldn't the heat also cook the lubricant in the bearing/bushings? I have not had a CIM apart, but some destructive testing may be in order.

[joek]

we've never had a problem with overheating motors, but i've heard of overheating fisher-price motors, but never CIMs (and we're running off ones from '07)

[Al Skierkiewicz]

Any motor provided in the KOP, in any year, can be overheated through misuse. The FP motors are particularly sensitive to operating in an RPM range (low RPM) that does not allow the internal fan to cool the motor.

[Conor Ryan]

Going back to the problem at hand.

Originally Posted by Joe Johnson

By the way, your gear ratio is probably not high enough but that cake is already baked. From what I saw of you at the MSC, you guys are doing just fine with the too tall of gearing you have. You just need to make sure your motors are cooled down between matches and you'll do fine -- next year, consider a bit more ratio ;-)

If you have the ability to do it, gear down your motors a wee bit. All of these high temp scenarios can lead to some ugly consequences in the heat of competition (pun intended). You really are playing with fire (also intended).