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Unread 08-04-2002, 01:33
Unsung FIRST Hero
Al Skierkiewicz Al Skierkiewicz is offline
Broadcast Eng/Chief Robot Inspector
AKA: Big Al WFFA 2005
FRC #0111 (WildStang)
Team Role: Engineer
 
Join Date: Jun 2001
Rookie Year: 1996
Location: Wheeling, IL
Posts: 10,798
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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.
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Good Luck All. Learn something new, everyday!
Al
WB9UVJ
www.wildstang.org
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Storming the Tower since 1996.

Last edited by Al Skierkiewicz : 08-04-2002 at 01:39.