Tripping breakers

[GMKlenklen]

Allright, what's it take to get your cim powered drivetrain to trip a breaker.... and yeild your robot imobil? and how do we keep this from happening? Or is this even a consideration? Thanks

[JoelP]

With the electronics we had last year, we would connect the cim to 40amp breakers, so if the cim drew more than 40 amps you would trip the breaker(s). Usually this happens when you gear your drivetrain too high, so it does not have enough torque. This would result in the robot not being able to accelerate fast enough and therefore the motors would be pulling more than 40amps for long enough to trip the breaker(s). Also, if the wheels you use have high enough traction, but your drivetrain doesnt have enough torque to make these wheels spin, the cims will stall and eventually trip breakers.
And, I've never seen this happen but I think it's possible, if you have 4 cims in your drivetrain and they pull over 30 amps each, you could trip the main 120amp breaker switch before you trip any of the 40amp breakers, which would leave your robot inoperable for the entire match.

[Cory]

Originally Posted by JoelP

And, I've never seen this happen but I think it's possible, if you have 4 cims in your drivetrain and they pull over 30 amps each, you could trip the main 120amp breaker switch before you trip any of the 40amp breakers, which would leave your robot inoperable for the entire match.

The main breaker doesn't actually trip at 120 amps. It can operate at higher current loads for a short period of time without tripping. In the past people have said that number is around 160 amps or so, iirc.

[Richard Wallace]

The data sheet for the 120A breaker includes a curve showing trip level in % of rated current vs. time duration in seconds. The 'curve' is actually shown as a grey area ranging from mininum to maximum trip level; for example, the breaker will open after a 10 second surge of 200% to 330% of rated current, which corresponds to 240 to 400 Amperes. It will trip at lower currents that persist for longer durations. The 160 Ampere trip level that Cory mentioned corresponds to a duration of at least a minute, probably longer.

This helps explain why we rarely see the breaker trip. The batteries we use are rated for 230A max. (5 second) surge current. Anything higher would be the result of a fault; i.e., a short circuit.

[GMKlenklen]

so the morral of the story is give your wheels enough torque, or you may stall a motor!

Now what about in a pushing mach? Would this just depend on how much torque it takes from the motor to spin the wheels against the wall, and if that draws too much curent? I'm thinking it does. Again, don't make the gearing too high, right?

[JoelP]

Yeah, just make sure you have enough torque to spin the wheels against a wall and you should be fine. Gear your drivetrain to give you the balance between torque and speed that you find best suits your design/strategy, but the maximum would be determined by the above test.

[eugenebrooks]

Proper gearing, and the use of a two speed gearbox if the game requires it, will minimize breaker tripping. Although it is true that you don't need any more torque than is required to slip the wheels, some games have involved picking up heavy goals with attendant increases in traction. We will know if this is the case when we see the kickoff this year.

Should your eventual robot design run into a breaker tripping problem, you can sort through your available breakers by testing pairs of them in series, using a NiCd battery or two for power and LEDs to indicate the breaker that tripped first in each comparison. The actual trip currents for the breakers varies a bit and every extra bit of current margin helps.

Eugene

[Al Skierkiewicz]

Originally Posted by Cory

The main breaker doesn't actually trip at 120 amps. It can operate at higher current loads for a short period of time without tripping. In the past people have said that number is around 160 amps or so, iirc.

Actually,
for periods under 5 seconds the main breaker will not trip with currents up to 600 amps. As the temperature inside the breaker rises, this over current trip point is reduced. Think about this, four chalupa motors in stall is 129 amps x 4=516 amps and still no main breaker trip. However, the 40 amp breakers would likely be tripping. Since they are auto reset, the effect is erratic motor operation and a loud clicking from the robot.

[Joe Ross]

The following whitepaper may be helpful as well: http://www.chiefdelphi.com/media/papers/1443

[Jimmy Cao]

If the motor is on a 40 amp breaker, it would take 40+ amps for slightly over a second to trip it. The breakers are slow-blow.

Driving in full power with can get close to the 40 amps required to trip the breaker, depending on your gear ratios and stuff. Pushing gets even scarier. If the drive train wasn't built for high torque and it's used in a pushing match, it's possible for it to easily draw 50+ amps. That, over a few seconds, could trip the breaker.

Good thing is these breakers are auto reset, so they'll reset after 2-10ish seconds.

[Greg Needel]

While many people in this thread are on the right idea, the real trick to this is to design a gearbox based on the current draw. Assuming that you are looking to get maximum torque you should design your gearbox to pull just under 40 amps at maximum torque. This requires you to know the approximate coe of friction with the playing field surface, determining motor torque based on amperage, the approximate gearbox Efficiency, and your normal force.

Assuming a flat surface:

Max Force= Force friction = (Coe of Friction) * (total Weight)

Force = (Torque at the wheel) /

Torque at the wheel = (Torque at 40Amps)*(Gear Ratio)*(Gearbox Efficiency)

So substituting Forces you get:

(Gear Ratio)/ (Wheel radius) = ((Coe of Friction) * (total Weight))/ ((Torque at 40Amps)*(Gearbox Efficiency))


Standard Efficiencies for drives:
Chain drive 95-98%
Spur Gears 95-98%
Bevel Gears 90-95%
Worm Drive 40-70%
Planetary 80-90%

This will give you the optimization of pushing force and gear ratio for a drive train which you can then use to find your maximum speed. If you want to have a greater maximum speed you can decrease the ratio at the cost of torque. Design is all about tradeoffs but this is a good place to start.

[GMKlenklen]

so how's the reseting work? That's pretty cool if you ask me!!

[Mike Betts]

Originally Posted by GMKlenklen

so how's the reseting work? That's pretty cool if you ask me!!

Think of it like a heater thermostat in your house. A bi-metal strip bends when heated, breaking the electrical contact. It bends back to it's "normal" shape when it cools, making the contact.

In our case, the heating is internal to the circuit breaker and is due to the motor current flowing through a small resistance.

Mike

[Al Skierkiewicz]

Originally Posted by Lostmage333

If the motor is on a 40 amp breaker, it would take 40+ amps for slightly over a second to trip it. The breakers are slow-blow.

Driving in full power with can get close to the 40 amps required to trip the breaker, depending on your gear ratios and stuff. Pushing gets even scarier. If the drive train wasn't built for high torque and it's used in a pushing match, it's possible for it to easily draw 50+ amps. That, over a few seconds, could trip the breaker.

Good thing is these breakers are auto reset, so they'll reset after 2-10ish seconds.

The 40 amp breakers have a similar over current charachteristic and obviously they can handle short duration 600% overcurrent spikes since they do not trip on short duration stalls of the drivetrain. Their reset times are in the order of milliseconds though, depending on temperature. When the breaker is hot, trip times decrease and reset times increase. In reality, a hot breaker will trip at less than 40 amps. Now, I can hear a lot of you thinking "why don't we just cool the breaker?" The case of the breaker is significantly insulated from the active parts inside the breaker so external cooling will do little to extend trip times. However, all teams should be aware that external heating of the breaker will affect it's performance. Don't mount the breaker panel above motors or near the compressor and make sure that all connections are tight and use correct wire guage. Heating of the contacts will conduct heat to breaker internals.