[archiver]

Posted by Dodd Stacy.

Engineer on team #95, Lebanon Robotics Team, from Lebanon High School and CRREL/CREARE.

Posted on 7/13/2000 8:40 PM MST


In Reply to: Timing is everything... posted by Joe Johnson on 7/13/2000 5:26 PM MST:



Thanks, Joe. Very clear. I hadn't realized that these devices (fuses) were so sluggish in their time response. I would not have thought they had sufficient thermal mass to absorb the I^2R loss above their rated current for so long before melting/vaporizing at the high resistance pinch point.

It's also interesting in the context of a 120 second match. If a 60 amp fuse will carry 120 amps for 100 seconds, perhaps it will carry 100-110 amps for 120 seconds? That would say we can run at a steady average system power of about 1 kw throughout a match, if the battery can maintain a 10 volt or so potential under a 100 amp draw. That's a lot of power.

Seems to me like it would be hard to blow the fuse unless you are stalling one or more motors for a significant period of time. But since each motor is protected by its own 30 amp breaker, and since the stall current on the 'big' motors is so high (I dimly recall), I would think the standard FIRST wiring schematic would provide adequate protection. At least if the drivers refrained from stalling multiple motors simultaneously for 10 seconds or more.

I only know that we've used all the more powerful motors (2 drills, 2 F-P's, and 2 van doors) and never popped a breaker or blown a fuse. Maybe we're not trying hard enough (but the GraceHopper wasn't exactly undergeared as it was).

Dodd