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Unread 05-12-2003, 10:23
Joe Johnson's Avatar Unsung FIRST Hero
Joe Johnson Joe Johnson is offline
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Re: Shifting Gears

Quote:
Originally Posted by Matt Adams
3. If you choose to couple a drill motor and an Atwood motor in single side of a drive train design, (using 4 motors total) your danger lies in pulling more than 120 Amps which would trip the master circuit breaker.

Matt
I agree with 80-90% of what Matt has to say, but the above quoted comment is one that I feel I need to say something about.

In the 2002 season, many of the matches ended up being 120 second pushing matches. In addition, many teams lifted the goal(s) in order to get a higher normal force with the floor (resulting in more pushing force for the same coef. of friction). Finally, FIRST provided 80 Amp household breakers (read: "not rated for mechanical impacts") for the main power.

This combination resulted a number of teams with N motors per side (N>1) having issues with the breaker tripping, especially if they got bumped during or shortly after a pushing match.

Last year, FIRST switched to 120 Amp automotive aftermarket breakers. Between the higher current rating and the designer's expectation that cars see more shock and vibration than typical homes (West Coast teams excluded ;-), I don't know of any teams that had problems with the main breaker tripping last year, whether they had N motors per side or not.

This is not to say that there are not multiple reasons NOT to use N motors per side -- there are many, but tripping the 120 Amp breaker is not one of them (imho).

By the way, I have it from usually reliable sources that given the current breaker and the individual 40 or 30 or 20 amp circuit breakers, it is a very close call as to whether it is even possible to trip the breaker short of a metal bar shorting your main power feeds.

Depending on the state of the charge of your battery and the variation limits of your particular battery's internal resistance and the lengths of your wiring harnesses, it may be almost impossible to trip the main breaker.

Having said I agree with Matt, I will point out one reason for multiple motor drives that he ALMOST points out but does not quite complete the thought.

As Matt points out, in many cases, the 40 Amp breaker is the limited factor on the output of your drive system.

The consequence of this is that if you have 2 motors per side, you have 2X the 40 Amp current limit.

This was the case with our robot last year. In normal driving (i.e. practicing), our robot worked just fine. It had a good balance between top speed and turning/pushing torque. But after a while of "competition driving" the breakers would poop out on us at the whimpiest of pushing matches.

So... ...we retrofitted an extra set of motors per side for the sole purpose of pushing back the limits due to the single 40 Amp current path.

For what it is worth...


Joe J.

Last edited by Joe Johnson : 05-12-2003 at 10:28.
 


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