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Unread 04-07-2003, 21:59
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
 
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Adam,
Read carefully when you are checking the spec sheets. A fully charged battery has an internal resistance of 10 milliohm. By Ohm's Law I=V/R=12/.01=1200 Amps. Now asume some variables creep into the calculations like not fully charged battery, some resistance in the wire (#10 is .001 ohm/foot) and a high resistance connection on the wire doing the short and 400 amps is pretty easy to get. Let us remember that the current through the internal resistance causes a voltage drop and that lowers the terminal voltage at the outside of the battery. A stalled motor is still a higher resistance than a dead short and therefore lower current when at stall. Teams stall motors all the time during competition and do not damage batteries or motors unless the stall condition continues for a great length of time.

The brush assembly on the drill motor can easily be damaged and the brushes can be out of alignment without any obvious damage. I came across several in Houston where the brushes were not touching the commutator or were severely misaligned enough to be be damaged by the wire contact at the edge of the commutator.
Sorry for the double post, it had something to do with the spell checker and I am still trying to work that out.
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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.
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