Tripping 120Amp

[Andy144]

Over at the Florida regional we were tripping our 120 amp's like mad . . . three times I believe. Each time they would be smoking significantly enough that the judges would be able to smell and it was so hot that we could not touch it without burning ourselves. Does anyone else have any expierence with those things tripping because eventhough we are getting incredible traction with our treads I was pretty sure that we would be tripping the 40 amp's alot but I never would have imagined that the 120 amp would trip. :ahh:

[sanddrag]

Unless that breaker is defective there is something seriously wrong with your robot. The only time you should ever trip that is after running nonstop for more than 10 minutes and getting hit hard by another robot. Drawing enough current to trip that is not good for the battery. Check for binding in your drivetrain and be sure the system isn't geared to high. Another thought, is the compressor constantly running if you use one?

[Andy A.]

Tripping the 120 is pretty hard through 'normal' operations (if there is such a thing), but not impossible, especially with tracks. Still, you may have an underlying problem aside from just drawing to much current through your motors. More detail about the drive train, what the 'bot was doing when it tripped or any thing else you might remember about it would help a lot.

Check for grounding to the frame with a multimeter from the battery terminals to the frame at several points.

Make sure all the 6 gauge wire connections are tight, clean and insulated. They come loose fairly often and should be checked frequently.

Check the breaker blocks- take out all the breakers, check for a loose fit, dirty contacts or anything out of the ordinary.

Also, check to see if any of the breakers are warm to the touch. If you pull enough to trip the 120 amp breaker then you should expect that the others be at least warm. Feel the red battery connector (assuming you use it), is it warm to the touch? Is the Battery warm?

At worse it's a fault to the frame or a defective main breaker. In any case, try to find a replacement for the breaker- if it smoked and smelled like it was melting then it is not reliable anymore.

-Andy A.

[Andy144]

The 6 gauge was melting and I am suprised we never had to replace it. Also supposedly our team could hear the 40 amps tripping in the stands, which if those things did trip that much it could explain the tripping of the 120 amp. The funny thing was that the battery was never really hot ... it got warm, but not even as close to hot as anything else. Everything was wired properly we were using the proper gauge wire for everything so I don't think that the actually wiring could be the issue. Anyone else have any expierence tripping these bad boys?

[Josh Hambright]

Nothing really sounds funny about this situation...
Your either drawing some serious current...or something is wired incorrectly.

Sounds like the advice stated above might help... What motors are you using for drive and how are they geared?
That seems like a probable cause.

[sanddrag]

Originally posted by Andy144
The 6 gauge was melting and I am suprised we never had to replace it.

You shouldn't be melting that wire and if you did you should replace it! Even though you did everything to FIRST standard doesn't mean it's good enough. For the kind of drive systems we are building now, the FIRST electrical system is not up to the task. That is why we use 4 gauge welding cable for our battery, 120A breaker, and ground block, and 8 gauge to our four drive motors (and breakers to speed controls), and a battery connector rated for 120 Amps. I advise any teams at LA to drop by our pit and check it out.

[Andy144]

The larger the gauge the more likely it is to melt . . . that is what they do. . But we had the atwoods and the drills geared up together on a single shaft using the gear boxes that First had provided us with. I believe the ratio was some where around 3:1 or 7:2, don't quote me on that because it seems like a long time ago since we were working on the gear ratios. But whatever it is it works fine because the two motors aren't grinding themselves out at all, and also there is no problem when we run the robot up on blocks it runs just fine, leading me to believe that the motors would have to be geared properly. We were using green brexoflex treads, so if anyone else was using those tell me if you had any problems with them, because it is my belief that those things were sticking to well and the atwoods and the drills were pulling so much power!

[Josh Hambright]

Running it on blocks wouldn't really show you a problem if you were drawing to much current...because there is almost no load on the drive system when its up on blocks.

We were noticing problems with our drive in cleveland and Mr Noble from team 71 suggested that we run the robot up against a wall and see if we can here the breakers pop, we did. So we found out that our drive had some serious issues.

Try doing that and maybe you can find a problem.

[Rickertsen2]

If your other breakers are not tripping it sounds like you may have a short somewhere. Unplug the bttery and check the resistance between each battery terminal and your chassis.

[AJ Quick]

Yup.

We tripped the 120 amp a few times, with the judges, and the Innovation FIRST guy coming over all the time to make sure the robot wasn't going to blow.

Apparently you could hear it tripping in the stands as well.. it became very hot, and was smoking a bit. I don't think we had very many matches where we didn't end in a smoking robot due to the 40 amps and motors.

We replaced the 40 amp breakers about 6 times at the UCF regional, because they became shot quickly.

[Al Skierkiewicz]

Andy,
At first I thought you might just have a small problem (i.e. a defective breaker or loose wire)but after reading all the posts, this is what I think you have...
1. Your drive system is drawing so much current that you have to redesign it to run in the future. I have heard so few teams claiming they are tripping the 120 AMP breaker that I thought we finally have a workable device. Sorry to be the bearer of bad news on this one. You didn't mention your speed but I am going to guess you designed for maximum push and are not a high speed robot. I am guessing from your description that you are tank tread design as well and with few details about the match in question, I would bet you were doing a lot of turning. High power + treads + lots of turning + two motors on a shaft = the highest current draw of any design = breakers tripping, wire melting, motors failing.
2. Replace not only the melted wire but the 120 amp breaker and the smaller breakers as well. They may not test bad now, but they have been stressed beyond normal and will certainly fail in the future. (Murphy's Law "Anything that can go wrong, will, and at the worst possible time.")
3. The larger the wire, the less prone it is to melting, not greater.
4. Just because the battery didn't feel hot to the touch does not mean that the internal temp hadn't skyrocketed. It takes time for that heat to migrate to the outside case. It is likely that some of the fragrance you smelled was venting of the battery as well as hot wires. Everything that got hot did so because of the high current. That current was also passing through the battery as well.

[Andy144]

Do you think that if we were to use a differnent tread, one with a lot less traction, we could avoid this problem in the future? Because I do agree with that our robot was desinged perfectly to trip 40 amps and so on, but there was a siginficant difference between day 1 and the end of day 3 were our robot was working incredibly we were turning perfectly and nearly all problems vanquished (except for smoke which didn't have any impending reprecussions as far as we know). This leads me to think that just getting reds will be fine in the future or sanding down the treads after we buy them.

[rbayer]

As most of the people at UCF can probably testify to, we were one of the fastest 'bots there, and the more I think about it the more I think that may have been our problem. We could easily stall the motors by putting our robot against a wall and we occasionally managed to do it while turning.

As Andy144 said, we had many fewer problems by Saturday. My guess is that our treads finally got worn down enough to the point where they would start slipping before we reached stall torque.

As a side note, it got to the point where we replaced the 40A breakers after every round, just to be on the safe side. This eliminated many of our problems, but we did still trip the 120 once more. Unfortunately, our 120 wasn't exactly in an easy to replace position, so we couldn't really change it out easily.

Side note #2: does anyone have any data on how much current it takes to trip the 120? Last year I remember FIRST releasing a spec sheet on the 60A stating that you could easily put 110A through it for a short time without tripping it. Was there something similar this year?

--Rob

[Josh Hambright]

Look around here on the forums...i know the specs for the breaker have been discussed.

As far as working better one day as compaired to the next...your drive system probably just got broken in and everything meshed into place like it was supposed to be. But you might want to consider doing some redesign work in the post season or in between competitions (It sounds like a big job but starting tomorrow we are completely redoing our drive assemblies in chicago, its gonna be SUPER FUN!)

[Al Skierkiewicz]

Originally posted by rbayer
We could easily stall the motors by putting our robot against a wall and we occasionally managed to do it while turning.
My guess is that our treads finally got worn down enough to the point where they would start slipping before we reached stall torque.
Side note #2: does anyone have any data on how much current it takes to trip the 120?

--Rob

Rob,
I agree that the motor stall is a serious issue, both the drill motors and the chalupa's have over 100 amp stall current ratings. Four motors in stall is 400+ amps.
Worn treads likely helped in turning where significant side loads cause current to skyrocket. At least one team at GLR had an adjustable center bogey wheel that helped in their turning. I asked them about current draw and they replied that the adjustment made a significant improvement but it was still much higher than in forward motion.
The 120 amp breaker specs are not nearly as good as last year. (documentation that is.) The breaker can handle short over currents of up to 300+ amps. As the breaker sustains each of these high current conditions, the trip point degrades a little. The internal temperature of the breaker has a lot to do with this and eventually if the breaker has reached a critical temperature, it will trip off with less than 120 amps. If you are drawing 200 amps or more, I would plan that the breaker can only handle that for about 15 seconds before it trips off. At that point you better change strategies and run straight or stop to let the breaker cool down.