Part of my teams strategy has been to have a fast climber. When I say fast I am talking sub 5 seconds. Our biggest factor on that is the thermal breakers.
We plan to use a two 775 pros, each with a 33.33:1 reduction. Using the tall robot size means that it would travel a little over 2ft to reach the top. On the 1.25in roller we are playing to use that ends up giving us a solid climb speed of about 7.5 seconds. CAD suggest that our worst case would be a 100lb robot. Across each breaker that would be 40.48amps assuming a gearbox efficiency of 90%.
I got 90% efficiency from some read I did on planetary gear systems and spur gears. I will admit that I choose that based on some hunch math. I was lead to understand that you would get roughly a 3% efficiency loss from every planetary stage. Using two per side that would equate to 94%. Then from there I plan to reduce using spur gears at a 2.1:1 ratio. It is my understanding standing from my reading that spur gears have a loss of 2-6% so I choose 4% loss for my happy medium. Putting that in to JVN got me my number of 40.48 amps.
Now I might be crazy but I would love a sub 5 seconds climb without adding motors. With a 20:1 gear ratio I would draw 67amps for for seconds, could the thermal breakers handel a spike of the current for that long. Also what would the main breaker risk be?
Risk of the 67 Amps through the snap action breakers, someone else will need to weigh in on that. Personal feeling is not of much concern because they reset.
Main breaker: 1% concern. You can dump 500 amps through it for over a second if I recall correctly. 140 amps can draw for well over a minute. Who has the chart?
See the chart on the bottom right of the first page. You can go for ~5s at ~480 amps (120 * 400%).
Never worked with overloading snap actions before, but the chart on their data sheet shows that it should be ok to run 67 amps through one (167.5% of the rated current, which the data sheets say could be done for about … 5s?
Thanks for making up for my lack of immediate knowledge and vetting my ramblings! I really don’t know where to find most things to link to these days. That’s what crowd-sourcing is for right?
You must spread some Reputation around before giving it to ollien again.
With two 775pros of power, you should be able to get a climb as fast as you describe without drawing anywhere near 60 amps per motor. Is it possible you’re on the “wrong” side of the motor power curve? You might be at the point where your under-load speed will actually go down if you increase your gear ratio.
Edit: Yes, your math is way off. Assuming you ONLY have the 33:1 ratio, and 80% efficiency, you should be drawing ~30 amps per motor and climbing in under 2 seconds with that setup, even with a 150 pound robot. Are you sure you selected the 775pro and listed 2 motors per gearbox in JVN’s calculator?
Theoretically, the OP shouldn’t reach stall on a 775pro if he runs 60A, so stall conditions shouldn’t matter. Still, the risk is there. Just always bring extra 775pros.
Your ratios are what concern me. You should be able to make a sub-5s climb with a single 775Pro, a fully weighted robot, and more aggressive reduction. Have you used the JVN calculator to determine your speeds?
What is the outside diameter of the spool you’re using to climb. As others have pointed out, you should be able to climb comfortably in under 5s with just one 775pro, two should be more than enough.
We’re looking at using a single CIM motor on a 9:1 ratio and the calculations are saying we should be able to climb in roughly 4 seconds and pull less than 30A (though obviously this will change somewhat as the strap winds up and makes the spool effectively wider). Given that a 775pro has about the same performance as a CIM when geared properly you should have no problem getting your climber to work.
I was using JVN calculator. This is all math right now, math that I realize my mistake with. I had used the intake math when I should have used the linear mechanism math.
Check your math. I don’t know what you’ve done wrong, but you’ve done something wrong with your math. If I punch in 2 775 pros, 30:1, 154lbs, 24 in, and 1.25 diameter pulley into JVN, I get 35A motor load, and a 0.82s lift. Which sounds much more like a 2 775pro lift.
Your numbers aren’t even consistent assuming you got the motor load right. 40A = 30% FLA = 70% free speed = 12600RPM / 60 / 33 * 1.25 * pi = 25 in/s.
The snap action breakers we use (including the main breaker) can withstand 600% over current for a few seconds without trip. They can also withstand 125% over current for quite a while. My suggestion is prototype your climber and take some reading to be sure.
