Tripping breakers

[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.