in need of another 40A

[lukevanoort]

Ok, so I was "discussing" a six motor drivetrain with my team. Then, one team member pointed out that that we only have six 40A breakers available, and that if we have a high draw motorized arm, we'll have a power problem. Personally I think our arm shouldn't draw that much, but anyway, is it legal to wire a pair of 20A breakers to act as a single 40A?

[Tristan Lall]

You have 40 A breakers for all of the motors that need them, that is to say, the CIMs and F-Ps.

For competition purposes, it certainly isn't legal to run breakers in parallel (or series, for that matter, excepting the 20 A breaker on the compressor spike), and you have to use an equal- or lower-rated breaker for the motors.

Actually, unless it was updated/Q&A'd and not incorporated, the 2004 rule to only use CIMs and (the discontinued) drills on 40 A breakers wasn't specifically listed in the 2005 rules. Nevertheless, an inspector wouldn't likely let you get away with using a 40 A (or even a 30 A) on a little Mabuchi, Globe or a window motor, because of the low current ratings—it would allow the motor to stall and overheat, without tripping the breaker. The van door is a little trickier to judge; it stalls somewhere between 40 and 60 A, depending on the spec sheet consulted. That's too close for comfort, in my estimation, to use a 40 A.*

This rule needs to be put back in the book, for safety reasons. Unfortunately, it's easy to miss this, because all of the breakers look essentially the same, at a glance.

*A little clarification: the 60 A end of the scale should be fine, but there was a little bit of controversy early in the season as to whether or not all the van door motors were identical. Published specs for the Tiagene van door motors in previous years listed stall torque at 40 A, while the current one lists it at 59.4 A (at 12 V). If the specs are right, then a 40 could work, because it will interrupt it before stall; if a team recieved any old stock from FIRST (and some motors had strange markings on them, like "bad" stickers, and different part numbers), they might end up with a motor that will stall before the breaker is triggered. Also note that breakers don't trip instantly at their rated current; the time to trip scales with the overcurrent.

[Andrew Blair]

Well, as was said, I don't think we have more than six motors that are really capable of pulling more than 40 amps, or 30 for that matter. I haven't looked at all the specs, but generally speaking, when we have motors like a window or globe on a 30 amp breaker, they stall before the breaker trips, indicating to me that they can't pull 30 amps at stall.

[coastertux]

You could also buy 40A breakers from IFI for $7.95 each.

http://www.ifirobotics.com/accessories.shtml

[Billfred]

Originally Posted by Tristan Lall

Actually, unless it was updated/Q&A'd and not incorporated, the 2004 rule to only use CIMs and (the discontinued) drills on 40 A breakers wasn't specifically listed in the 2005 rules.

I can't pull up the section in the manual right now (so much time and so little to do...wait a second), but I believe they regulated the breaker size by the wire gauge this year.

[lukevanoort]

Clarification, I wanted to try and make a six motor drivetrain this year, and our only surviving member who did electrical mentioned that these breaker panels only take six 40A breakers, which he didn't want all used. We have plenty of the breakers. (I think the exact words were "No way, Luke.") I had a vague memory of reading a rule that the old FPs were required to be on a thirty, so I thought "if those worked on thiries, ehy can't these", but given my inexperience with doing the electrical, I said nothing and continued my gearbox lecture. I decided that I would check CD for what you guys thought before I disagreed with a more senior member about a subject I've never worked on before. Thanks.

[Tristan Lall]

Originally Posted by Billfred

I can't pull up the section in the manual right now (so much time and so little to do...wait a second), but I believe they regulated the breaker size by the wire gauge this year.

You're right that the wire size was tied to the breaker used, but that would just allow a team to put 12 AWG on a Globe motor, and to connect it to a 40 A breaker—it only specified minimum gauges, not maximums.

I've been informed that there was in fact an incident at the Championship last year where this occurred, and, though the inspector noted the hazard, after consulting the lead inspector, it was determined that since the 2005 rules didn't prohibit it specifically, the inspector couldn't reject it at that stage. (Only if some unsafe operation occurred during a match, could the referees then have had the robot disabled, under <S01>.) As hazards go, this one has the potential to turn ugly.

The 2004 rule was more conservative, and is likely the one that Luke recalls.

[Al Skierkiewicz]

First, you may not tie breakers in parallel. The breaker size is related to wire size so that we don't have robots flaming on the field, but the rules specify branch circuit design on a motor by motor basis. The smaller motors may not have tripped breakers at stall because they are a temperature controlled device capable of maintaining circuit continuity for up to a few seconds when the current exceeds the breaker by as much as 400%. They would have eventually tripped if the stall conditio9n remained. Please remember that with the autoresetting feature, breakers can open and close very fast producing a buzzing sound and the results will be extremely high temperatures at the breaker. If a breaker is warm or hot, believe it was tripping even if there was no other obvious evidence that it was tripping.

[Richard Wallace]

Originally Posted by Al Skierkiewicz

.... Please remember that with the autoresetting feature, breakers can open and close very fast producing a buzzing sound and the results will be extremely high temperatures at the breaker. If a breaker is warm or hot, believe it was tripping even if there was no other obvious evidence that it was tripping.

Big Al, you know whereof you speak. Those who participated in Stack Attack (FRC 2003) may recall that many robots had problems with overheating 40A breakers when they climbed the ramp. This was often accompanied by a buzzing noise coming from the breakers. Breakers that saw this type of overloading eventually degraded and would no longer sustain high (>50A) peak currents; this caused some robots to veer off intended course when the problem was more severe on one side. Extra 40A breakers were a very popular bartering item in the pits that year.

[Sparks333]

This may be redundant...
Once a breaker has tripped once, it will trip much more easily the second time.
In the 2004 competition, our robot found it difficut to climb. If we popped our breakers getting up the step the first time, we were screwed: the breaker would pop early when we would try again.

Sparks

[TimCraig]

Most circuit breakers work thermally. So the first time, the breaker is starting out from dead cold. Once it trips, it's gotten to the critical temperature. It doesn't reset until it cools to the reset temperature which is well above dead cold. So next time it takes less energy to heat up to the trip temperature.

[lukevanoort]

Originally Posted by TimCraig

Most circuit breakers work thermally. So the first time, the breaker is starting out from dead cold. Once it trips, it's gotten to the critical temperature. It doesn't reset until it cools to the reset temperature which is well above dead cold. So next time it takes less energy to heat up to the trip temperature.

That makes sense. I wonder if muffin fans or another cooling method directed at or near the breaker panel would help? Or watercooling using the water from this year's water game.

[Mike Betts]

Originally Posted by lukevanoort

That makes sense. I wonder if muffin fans or another cooling method directed at or near the breaker panel would help? Or watercooling using the water from this year's water game.

Luke,

Changing your gear ratio is the best solution. 40A is a LOT of current on a continuous basis.

JMHO.

Mike

[Al Skierkiewicz]

Originally Posted by lukevanoort

That makes sense. I wonder if muffin fans or another cooling method directed at or near the breaker panel would help? Or watercooling using the water from this year's water game.

I have seen teams use a variety of ways to cool the breakers, from fans to cool spray right before a match. The thing to remember is the metal inside the sealed breaker is at temperature and there is very little thermal conduction to/from the case. I should point out that breakers that trip under load (motors) will arc at the contacts. Over time, repetitive trips cause pitting and therefore high resistance which just adds to the heating problem. Not to mention the role of the arc in local heating of the contact area.