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Circuit Breakers
Does anyone know which specific 60A circuit breaker is included in the kit?
Are they single, double or triple poled? What is the voltage rating, and what type is it? |
SquareD
The 60 amp breaker that comes in your kit is in a grayish box with yellow & blue print on it and roughly 4" x 3 1/2" x .75". There is a picture of it with all the details in the kit of parts section of the rule book, which you can access right on line at the USFIRST.ORG site. You can buy a replacement from your local SquareD distributor. The phone book may list Brands with the yellow page ad for "electrical distributors." Otherwise call any electrical distributor and they can tell you where to get one if they don't have it.
Tim Gates Team #288 RoboDawgs |
So how many teams have tripped it in competition?
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we have, so now we need to turn off parts of our robot before ramming/pulling other objects on the field. :p
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60 Amp Breaker: Shocking News...
In the Elimination Rounds at the Buckeye Regional, a number of teams, including Chief Delphi, had problems with the 60 Amp circuit breaker tripping.
I have been one of the voices saying that teams should not have any problems so I am especially anxious to get the word out on this issue to set the record straight. Here is my story: I knew we should have been okay on that breaker. There is no way we pull more than an average 100 Amps during a match, no almost no matter what we are doing. The charts tell me at 100A the breaker should take 2 minutes to trip. Yet we were going dark about ½ way through the match! What gives? I thought perhaps it was a bad breaker but tests on the breaker we took out of our machine (thinking it might be bad) showed that it did not trip in less than 2 minutes when we tested it at 100A constant. Shock turned out to be the explanation. If we just tap the breaker on the side of the housing with the handle of a screw drive (and not too hard either), the break time is effectively cut in half. This was found to be true at several different test conditions. When we reviewed the tapes of our matches, in every match that we tripped the breaker, the robot took a hit right at the moment the beacon went dead. I think we have found the smoking gun... Bottom line: Teams that can figure out a way to keep their breaker from getting an impact load may be able to extend the trip time of their breaker by a factor of 2. This is a important bit of news that we plan on implementing at the Great Lakes Regional this week. Stay tuned for more results as the story continues... Joe J. |
Our 60A breaker tripped when we took a particulary hard hit during the qualifying matches at VCU.
Michael D. Team 602 |
Re: 60 Amp Breaker: Shocking News...
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hmm, maybe that's why our breaker blew a few times, for now reason at all. well, i think we'll still turn off none vital parts of the robot before we ram/push/etc. other things on trhe field. doesn't hurt to play it safe.
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Joe, we had the same problem in training before ship. I remember someone saying that the breakers should be cable-tied in the 'on' position during matches to avoid tripping from shock, but I'm not sure that would help.
Our immediate solution was to first replace the offending breaker, and second, mount the new one vertically so as to avoid the largest part of the shock when we smash into the goals. It also seems like someone might be clever enough to figure out the internal geometry of the breaker, and mount it so that the forces from the increased current are cancelled out by the forces from the impact, allowing you an instantaneous current peak. Ok, so maybe that last bit was a little far-fetched. One more issue, though - if the failures you're having are not shock-induced, current spikes can trip these breakers in very short time frames. So you don't need to average 100A if you have a 200A instantaneous spike. The spike will trip the breaker magnetically. |
A clip would not make it auto reset since you need to switch it to on then off then on again (and probally against the rules), the best way to avoid shock tripping is to mout the beraker perpendicular to the force ie. not parallel to the floor. This way the shock wont effect the tripping mechinism. Also if you suspend it with taught rubber tubing it should buffer out the major shocks.
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I'm not sure on this, but I think that the breaker will trip even if the switch is forced in the on position constantly. Even if it could be "kept on", it would defy the whole purpose of the 60 Amp breaker, which is to prevent overheating and a potentially major fire hazard.
I hadn't thought about shock affecting the breaker, but it makes sense now that I think about it. Often, the biggest amp load is when the robot runs into something and the motors are pushing hard, which also happens to be a big physical shock. Bad combo for the 60 Amp circuit breaker. |
You can download the technical documentation on the breaker at this site:
http://www.squared.com/us/products/c...56A630064BC8F/$file/QOUcircuitbreakerFrameset.htm Download document 0720CT9401, "QOU Miniature Circuit Breakers and Switches". Tripping mechanisms are shown on pages 1 & 2 - both the thermal (inverse time) and the magnetic (instantaneous). Thr trip curve is the right-hand one on page 14. It shows both the "inverse time" characteristic of the thermal mechanism, and the "instantaneous" magnetic mechanism. Look at figure 1 on page 2. Of course a breaker on the verge of tripping will trip off when "mechanically agitated". It really wouldn't matter much what the mounting orientation is, either. It's ready to trip, it gets hit, it trips. Once you look at, it appears very simple - deceptively so. Many such things will come back to bite you in your engineering career. Beware of using equipment in applications outside the design basis. Very few electrical distribution cabinets move. Even the best of us (i.e., Dr. Joe) can be fooled on occasion. |
Here is Square D's responce about shock impact:
Alexander, There is no technical information on this application. QOU breakers are susceptible to shock or a large impact. Suggest locating the breaker where it won't receive that shock or large impact. Sherry Glass CIC Tech Support |
On the subject of the 60 amp breakers, I do not believe that any team can trip this breaker on current overload. The breaker has been tested and has predictable trips when run at 120 volts AC and 45 volts DC. Even though the battery at full charge is capable of delivering 450 amps, the terminal voltage at that current would be low enough to reset the robot controller before the circuit breaker would have a chance to react. These circuit breakers are very susceptible to impact trips, however. (try it yourself by tapping on the circuit breaker on either side the wires enter, you will be surprised at how little it takes.)They are especially sensitive to shocks from the sides, where the the wires are attached. For this reason, circuit breakers should not be firmly mounted in the robot such that any shock to the robot is transmitted directly to the circuit breaker. Teams have been successful in adding a few inches of extra #6 wire when installing the breaker. This allows the wire to absorb most of the shock and keeps the breaker from shock tripping.
We add a small block of plastic to the breaker that is wider than the body by 1 inch on both sides. (This allows us to tywrap the #6 wires to prevent flex failures in the wire at the breaker.) Then we bend the wires back so that there is several inches before they are attched to a frame member. That extra wire then absords the impacts to the frame. I saw several impact trips at Great Lakes this weekend. Hope this helps, P.S. The clips on circuit breakers (in a distribution panel) are to prevent someone from turning them off accidently. They do not prevent the breaker from tripping on either current overload or impact. |
my robot took numerous hard hits philly to the side the breaker is on and so on, we never blew out once. Is it possible there is just a batch of bad breakers out there?
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No, all circuit breakers exhibit this phenomena. Consider yourself really lucky...so far. Remember though, the circuit breakers are most sensitive to side impact, i.e. where the wires enter and exit.
Good Luck |
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Also, our breaker tripped mainly during sustained pushing matches, such as continuous low current being used to keep two goals stationary in our scoring zone. We eventually had to make the painful decision to disengage the drills from our mobility gearboxes, which has so far solved the tripping problem but has noticeably reduced speed and torque. What seemed to happen sometimes is that one motor would trip its own 30 amp breaker, and while that motor was not receiving power the other two would be driving it, increasing the load on them and exacerbating the problem. -Ian Mackenzie Team 188 Woburn Robotics |
Ian,
Are you coming to Midwest regional? I would really like to take a look at your robot and talk to you about the tripping. You kind of imply that one of the 30 amp circuit breakers tripped out and stayed that way. The 30 amp breakers reset pretty quickly and on a sustained fault have been known to buzz as they repeatedly trip and reset. You did not mention if your controller was rebooting, did that happen as well? Let me know. Good Luck All |
Yeah, and which motors are tripping the 30A breakers?
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Al,
First of all, no, we will not be at the Midwest regional, although we will be at the nationals (and by that time we will have competed in the Canadian Regional, during which we might discover something else). I confess that I don't know which motors were tripping the 30 amp breakers; in the heat of competition, we believed that it was better to make a drastic change (taking out two motors) that would definitely solve the problem than to take our time investigating and fine-tuning individual breakers and speed controllers. The latter seemed to be, as we say, "building to tolerance". Actually, I couldn't confidently say that the breakers were tripping at all; we never really got conclusive evidence of it, so we may very well have been grasping at straws. I don't believe our controller lost power, because I never remember the light turning off temporarily (I have seen the controller shut off during practice back in Canada, usually when our battery was running low, so I think I would have recognized it if it happened in competition). -Ian Mackenzie Team 188 Woburn Robotics |
Ian,
Good luck at the regional and let us know if you do find anything there. Otherwise, see you in Florida. Stop by and say HI Al |
Ditto
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Magnetic tripping
I seem to remember seeing someone post something about large spikes of power tripping the breaker magnetically, not thermally...does anyone know more about this? And if so, since we are allowed unlimited permanent magnets, could a few magnets be strategically placed around the breaker to alleviate the problem?
-Ian Mackenzie Woburn Robotics www.team188.com |
Ian,
It is doubtful that magnets will help with the trip charachteristics. Effectively there is a little electromagnet inside the breaker. When the current gets to the trip point the magnet attracts an armature that is connected to a release mechanism, which lets go and the spring in the breaker pulls it open. Adding magnets may even make it trip at a lower current. Remember, these breakers are designed to be stacked up next to each other inside an electrical panel so stray magnetic fields are present all the time. Good Luck |
Team 547's solution to the CB problem
With this years game being a contact sport, we prepared for this. We obtained a piece of 35mm DIN rail (legal - extruded aluminum) which the breaker snaps onto. We took two of the shock mounts which were supposed to be used for the compressor to mount the DIN rail to the robot frame. The #6 wires are not tied down near the breaker. This is very forgiving.
We have been hit hard and only have dents but no breaker trips. Hope this helps, (looking for pics) Mike See our robot at: http://www.chiefdelphi.com/forums/sh...&threadid=3479 |
With all of the input on this subject and observing robots firsthand, I still believe that the majority of breaker trips are impact but I now think overcurrent trips are a possibility with the other variables in place. Although the spec sheets provided in FIRST documentation do not show trip derating curves, the Square D website does at...
http://www.squared.com/us/products/c...5661300555d9d/$FILE/2496.pdf The trip current is predictable and decreases with high current over time. In looking at the curves, a 60 amp breaker can be expected to withstand six times it's rated current for a few seconds but that rapidly falls if the overcurrent remains. This is due to heating of the thermal parts within the breaker. If a robot design is such that the breaker receives heat during operation, you would expect that the trip point would decrease rapidly as well. In many of the robots that I have observed, the breakers were enclosed within the body of the robot, wiring to and from the breaker was very short or the breaker was mounted near heat generating components such as motors, transmissions, speed controllers and circuit breaker panels. In the majority of teams that reported tripping the 60 amp breaker they were also tripping the 30 and 20 amp breakers. These breakers (burned fingers to prove it) get very hot if repeatedly tripped. Add to that heat generated in loose contacts (such as the screw terminals on the breaker or the battery connector) and a serious problem occurs. Fudging the numbers and looking at the curves will give you a headache but you can guess that a breaker running about 30C and 180 amps non-continuous current will trip anywhere from 50 to 75 seconds into a match without drawing the battery below 8 volts. Some teams have reported freeze spray lets them run a longer period before trip. The reality is anything you can do to weigh the variables of temp, vibration and current on your side will help. Check the temp of the breaker, the #6 wire, and area surrounding these components when the match ends. Try using software to slow down the motors that draw high current. By changing the duty cycle of the heating current, either in software or driver training, you may be able to get a full two minutes of match time with only a small loss in speed or power. Check out the other posts about efficiencies in transmission design and drive system losses. Please post any other input you might have on this subject. Good Luck All and See You in Florida. |
Mounting oreintation
1 Attachment(s)
I did a little analysis on the circuit breaker. Please refer to attached x-ray analysis. The top picture is a control sample in the the "on" position prior to an inpact. Main areas of interest are a) on and off switch, b) thermal unit and c) breaks contacts. Not labeled on the photo is the moving component at 1 oclock on the yellow circle.
The sample was then subjected to an impact which popped the breaker. It appears the "thermal unit" moves when the impact happens, which is followed by moving component swinging down to the right and pop............... end of match. Conclusion Mount the circuit breaker with "ON" position down and "OFF" position upwards. With this oreintation the thermal unit's gravity works in our favor. Stevo |
Stephan,
Great Photo, it helps a lot. I had a team open one for me and I think what you refer to as the thermal unit is actually the magnetic unit. I think the thermal is at the top of that assembly, there is a factory adjustment screw at the top accessable behind a label on the breaker. (There was no identification of the parts on the Square D site so I am guessing here.) It is easy to see that the two parts visible in the yellow circle will be forced together with impact on one side, tripping the breaker. Hitting on the opposite side keeps them apart. Please note that any heat conducted into the breaker through the wires, (especially the left connector) is tied directly to the thermal unit through the internal metal parts. BTW, we mount the circuit breaker horizontal, the same orientation it was designed for with the #6 wires used to shock mount from the frame. Hope this helps give teams a clearer view. Good Luck All |
We took a similar 40 A Square D breaker apart to examine the innards: the "thermal unit" in the Xray has a frame (in a 'J' shape) (call it 'J') attached to the wire terminal, and the thick piece (call it 'A'), just beside 'J' going into the "C"-shaped bottom of the 'J', carries a recess into which the long piece from the handle fits. 'A' is springy where is it welded to the end of the 'J' near the terminal, and swings toward 'J' with any force applied. When the thin loop hanging from the 'C' piece, which is bimetallic, starts bending (at its end near the 'C') it bends, and because it is attached to 'A' it pulls on the 'C', which pulls 'A' and the recess away from the handle-piece.
When rapped smartly along the long axis of the body, the cold breaker snaps open. The connection from the breaking contact on the side near the other terminal, is made by a piece of thick stranded wire welded at both ends. We saw no evidence of any magnetic coils, nor of any mechanism to trip magnetically on theflow through the conductors as arranged. |
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