Re: 9 motors too much?
 

Ian,
Is it possible you may have tripped due to mechanical stress on the breaker?

For others, the internal resistance of the battery is 11 milliohms. At 500 amps, there will be only 5.5 volts dropped across this resistance which is still enough voltage to power the PD power supplies and keep the Crio and radio up (and therefore your robot). Using smaller snap action breakers does not limit the current to the motors. A CIM will still stall at 133 amps regardless of breaker size. If you use a smaller breaker and then use smaller gauge wire, there can be significant resistance in the wire to limit the current through a motor. The down side to this option is reduced motor performance.
Caveat:breakers that are tripping will heat up. If you hear a buzzing noise from your robot, do not touch the breakers when the robot comes off the field. You will be left with an impression of the breaker size on your finger tip for several months.

Re: 9 motors too much?
 

I disagree, the battery will provide significantly more than 120 Amps. Unhappily, but it will. On what do you base your statement? As Al mentioned, the nominal internal resistance of the battery is 11 mOhm (0.011 Ohm). If I = V/R, 12/0.011 = 1090 Amps. Of course, more realistic would be about half that - for a second or two.:ahh: More than enough to ruin your day.
Hmm, I wonder how he knows that bit of information? :D

Re: 9 motors too much?
 

It is definitely possible. Were playing defense, but not particularly hard defense. IIRC, it was pretty toasty when we found it.

It is near the end of this video. Our robot is the red bumpered ramp shooter. In the final 10 seconds of the match 562 runs back down the field, and we don't move.

Re: 9 motors too much?
 

Ian,
The breaker was warm when you got to it after the match? had it been warm on previous matches?
I should have mentioned there is another failure mode that I have not seen for a long time. Way back in 2006, the KOP contained an SLU terminal for use with the battery wiring and the 120 amp circuit breaker. These terminals were notorious for loosening with vibration. Teams were required by the robot rules to use these terminals. As the robot current draw went up and the terminal became loose on the #6 wire, heat would develop in the connection. Since the 120 amp circuit breaker is a temperature controlled device, this heat changed the trip characteristic of the breaker. The heat from the connection migrated into the breaker. If you check my electrical presentation (WPI site and linked in several threads.) you will see an actual #6 wire damaged (read that melted) due to a loose and improperly terminated battery wire.

Re: 9 motors too much?
 

3: 2 for drive and 1 for arm.

Re: 9 motors too much?
 

I learned early on to observe others and learn from their mistakes.

Re: 9 motors too much?
 

While I don't doubt that it is possible for an FRC bot to trip the 120A breaker, in 8 years of FRC (multiple events per season) I have never once seen my team, or any team at an event we were at trip the main breaker. I'm sure you'll be fine so long as you're geared sensibly. Even if you ARENT geared sensibly, you're much more likely to just kill batteries in no time flat than you are to trip the main breaker. As Don Rotolo and Al Skierkiewicz have been saying, the breaker will happily conduct 600A+ for a few seconds, and the battery will only provide that much current for a few seconds before become fully depleted. I've seen many robots, my own included, that had major issues keeping batteries charged because of incorrect gearing.

Re: 9 motors too much?
 

4 Drive, 4 Manipulators, 1 Compressor (4 pneumatic cyllinders, 3 of which are 3/4" bore and less than 4"; the 4th is single-use for the minibot deployment)

We could easily use 2 less motors, yet they're added in order to balance the load under heavy usage during elims and after competition season. This typically only adds 2-4 amps of usage overall. We may swap the motors on our wrist joint for a dual-cyllinder pneumatic setup depending on driver desires in control.

By using some calculators out there, I've figured that if we're not pushing any other robots and no manipulators experience catastrophic failure, we will use 80 amps IF and only if we run absolutely everything at once. The current moves up by 25-ish amps if we're moving and turning. Of course, the chances that our elevator will run while we move at 13 fps is next to nill in the practicality of our strategy, so this is a worse-case scenario at best.

For fun (and to inspire someone to want to do batteries this year) I can estimate that a single match where we successfully put up 5 tubes on the top row and deploy a minibot will require 24 minutes of recharge time on a 6 amp charger. I think that this is a bigger indicator of whether or not a robot has too much current draw -- low batteries in elims lead to all sorts of incoherent behavior on the field. Again, it's an estimate, but it's useful to know how many chargers and spare batteries we need (and is good fodder for inspiration/training).

Re: 9 motors too much?
 

Yes. It was actually after the robot had been removed from the field and we were done celebrating that someone went over and said "whoah! The breaker's popped!" We were pretty "caught up in the moment" so no one could really recall who had done what, or whether it had been hot previously.

Now that you mention it that loose terminal issue sounds vaguely familiar... but things tend to blur together after a while. :)

Re: 9 motors too much?
 

We're using 10-11 motors.

Drive - 4 motors
Lift - 2 (upgradeable to 3) motors
Arm - 2 motors
Manipulator - 2 motors

Re: 9 motors too much?
 

I feel like we might be under-doing it now... 3 motors. (2 drive, 1 arm) 2 pistons (1 claw, one deploy) I suppose we did keep it super simple. :D

Re: 9 motors too much?
 

You should be fine... normally the big drains...(the drive motors) won't be operating during the lift/arm motion... and even if they do the 120 A breaker can handle much more for short periods...

We are using 8 motors and 8 pneumatic cylinders ...
so far... (not counting the minibot.)

Re: 9 motors too much?
 

It gets worse as you get older or maybe it is just the sixteen years of competition. Yeah that's it!

Re: 9 motors too much?
 

hello yes we are using 8 motors 4 for chasis and 4 for the arm so far we have had no probles with the amps so i would believe you would be okay

Re: 9 motors too much?
 

If anyone is going to trip the 120A breaker this year, it will be us.

We have 9 motors + Air compressor, the important thing here, is that 8 of these motors are for the drive chassis where stall conditions is a possibility, but highly unlikely due to the friction co-efficient of the wheels.