Tripping breakers

[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