Tripping 120Amp

[KenWittlief]

If you have 4 motors on 40 amp breakers then 4*40=160 - so yes, you can easily trip the 120 without tripping the 40's.

4 Gauge wire: against the rules specified by FIRST - your bot would fail a critical inspection. Putting heavier gauge wire on the battery or other circuits reduces the resisitance, and will give you an (UNFAIR) advantage over teams that followed the rules.

Wire melting: where is it melting? at the terminal connection? thats a sign of a bad crimp. You might want to use some solder on the terminal and put heatshrink over it.

Two motors on one shaft - not an easy thing to do - the power/torque curves for the two motors are not identical - if they are not geared exactly right then one motor has most of the load.

Are your controls damped in SW? the worst thing you can do to your drive train (besides stalling the motors against a wall) is to go from full forward to full reverse.

If you did this in your car your drivetrain would be left in the street -but for some reason we think its ok to do this to our bots.

You esp need to be carefull in your auton code, backing up then going forward in one step, or going from straight to a tank turn in one step - can throw the motors from 254 to 0 in 26mS.

"she was a Day Tripper!
one way ticket, yeah
it took me Sooooo long to find out
but I found out!" -Lennon/McCartney

[AJ Quick]

Originally posted by KenWittlief
Wire melting: where is it melting? at the terminal connection? thats a sign of a bad crimp. You might want to use some solder on the terminal and put heatshrink over it.

Yeah, that may be a good part of one of the reasons it was overheating and melting. I know once our wire came out of the connector during the build season, it may have not been seated correctly, or came loose.

[Joe Ross]

Originally posted by KenWittlief
4 Gauge wire: against the rules specified by FIRST - your bot would fail a critical inspection. Putting heavier gauge wire on the battery or other circuits reduces the resisitance, and will give you an (UNFAIR) advantage over teams that followed the rules.

http://jive.ilearning.com/thread.jsp...=6177 67#3043

We used 4 AWG wire and connectors rated at 120 amps. Apparently our team and 696 were the only ones to do this.

We never thought of it as a competitive advantage, when we decided to do it, it was because of safety.

[Rickertsen2]

Originally posted by KenWittlief

4 Gauge wire: against the rules specified by FIRST - your bot would fail a critical inspection. Putting heavier gauge wire on the battery or other circuits reduces the resisitance, and will give you an (UNFAIR) advantage over teams that followed the rules.

First off 4AWG is perfectly legal. second of all do you have any clue how little the resistance of the wires is different by. Do you realize how little difference this actually makes. Its completely negligible and not worth worrying about. It provides no power advantage whatsoever.

[Matt Leese]

Originally posted by Rickertsen2
First off 4AWG is perfectly legal. second of all do you have any clue how little the resistance of the wires is different by. Do you realize how little difference this actually makes. Its completely negligible and not worth worrying about. It provides no power advantage whatsoever.

I wouldn't be so sure that it provides no advantage. 6 gauge wire has .47 Ohm per 1000 ft while 4 gauge wire has .24 Ohm per 1000 feet. Assuming a 10 foot length at 120 amps, 6 gauge wire will give you a voltage drop of .564 V while 4 gauge wire will give you a voltage drop of .288 V. That's a voltage differential of .276 V. That's a 2.3% loss of the 12 volts that are normally output by the battery.

Now, I'm sure there are very few people who are going to consider that a serious issue to the design of a robot. Obviously that .276 V difference probably won't be the difference between winning and losing. However, in some applications it can actually be significant. Don't discount the change out of hand. Remember, wires do have resistances (and inductances for that matter) and they shouldn't be completely ignored until analysis says that you can ignore them.

I'd also point out that FIRST has always specified a minimum wire size for safety reasons. At all times, teams have been able to use larger wire with no penalty. FIRST is concerned about safety with wiring in this case, not about competitive advantages.

Matt

[Al Skierkiewicz]

Originally posted by Rickertsen2
First off 4AWG is perfectly legal. second of all do you have any clue how little the resistance of the wires is different by. Do you realize how little difference this actually makes. Its completely negligible and not worth worrying about. It provides no power advantage whatsoever.

Robot Rules...
"3.2 Wiring the Robot Controls
3.2.1 Power Distribution Circuits
...The battery, 120A main circuit breaker/disconnect switch, and the three provided power distribution-circuit breaker panels may only be connected with the AWG #6 wire provided in
the kit. Protect all branch circuits by circuit breakers as shown in the table below."
So, #4AWG is illegal. As to the change in resistance, many teams use long lengths of #6 wire and even longer runs of #10 to the speed controllers and motors. I think that in many cases these long runs add sufficient resistance to keep teams from getting into trouble by limiting the current in the system as a whole. The difference in resistance of one foot of wire is not that much but when you start adding it all up and passing 200 or more amps through it, the voltage drop is smaller in the #4.

[Andy A.]

One thing to note on the rules is that it contradicts its self on this issue.

It does say you must use #6 for main power distribution, but it goes on to say that the stated wire sizes are the minimum allowed.

I take this to mean that you must use #6 between the battery and breaker blocks, but other wise may use what ever you wish (as long as it meets the minimum in the rules).

So, if you want to replace #10 going to a drill motor with #8, then that is legal.

-Andy A.

[sanddrag]

Again
http://jive.ilearning.com/thread.js...3&q=617767#3043

[KenWittlief]

Joe Ross is right, I stand corrected.

The FIRST Q&A link he provided gave the OK for using larger than 6 gauge wire.

:c) -Ken W.

[Al Skierkiewicz]

Originally posted by Andy A.
One thing to note on the rules is that it contradicts its self on this issue.

It does say you must use #6 for main power distribution, but it goes on to say that the stated wire sizes are the minimum allowed.
-Andy A.

There is no contradiction. The battery, the 120 amp circuit breaker and the power distro blocks must be wired with #6 only. The minimum allowed wire size refers to all other wiring. Although I don't know why you would try to attach a blade terminal to a #4 wire to feed a speed controller or motor. The weight of the wire would eventually break the terminal off. (although it makes a dandy heatsink!) Otherwise it adds weight and expense and takes up room in an already crowded electrical system. (well ours anyway.)

[Joe Ross]

Al, There is a contradiction, which is why I asked the previously posted question on the jive forums.

Page 8 of the robot rules says

The battery, 120A main circuit breaker/disconnect switch, and the three provided power distribution-circuit breaker panels may only be connected with the AWG #6 wire provided in the kit. Protect all branch circuits by circuit breakers as shown in the table below.

However, on page 10, Table 3.2 says

Table 3.2 shows the
minimum wire sizes allowed for hookup of the various control system devices.
Table 3.2: Minimum Wire Size and Protection by Device Type
Device Wire Type Circuit Breaker
Power distribution from battery through
120A Main Circuit Breaker/Disconnect
Switch to fuse panels
6 AWG/red & black 120A

In one place it says minimum, in another place it says only. That is the contradiction. In cases of contradiction, I ask on the jive forums and got this response: http://jive.ilearning.com/thread.jsp...=6177 67#3043

I wish that FIRST would have issued an update fixing the contradiction, but they didn't and so I go by the jive forums post.

[Al Skierkiewicz]

Joe,
Oh those guys! We try to stop the confusion and....oh well. As long as you didn't get problems from the inspectors. I wasn't able to get to the forum reference from your other post so I couldn't see your reference. See you in Houston?!?