Motor Info

[Andy Baker]

Adam,

This is a good question. For most motors we use in FIRST, the max efficiency is at about 1/2 of the free speed. With this in mind, some designers size their reductions for this speed and have a robot that drives well.

For instance, they gear their 'bot to run at top speed when getting an input speed from the CIM motor at about 2800 rpm, or about 1/2 of the 5500 free speed rpm. Then these teams make their robot, get their drivetrain as efficient as possible, with little friction, and it drives too fast. The top end is too out of control, tripping circuit breakers and the low end is not torquey enough. Usually, they end up gearing down their robot, slowing it down so it is more controllable and doesn't trip the circuit breakers after running at top speed for more than 5 seconds.

From my experience, I use a "working rpm" top speed of about 75% of the motor's free speed rpm. For instance, when designing drive trains with the CIM motor, I assume that I can get 4000 rpm (again, 5500 is the free speed) at the robot's top speed. This difference of free speed and working speed is because of efficiency losses in the drivetrain and rolling friction losses of the robot.

Have fun,
Andy B.

[Adam Shapiro]

Thanks for the help. If I was to calculate using approx. 75% on the CIM and 85% on the drill would it work in practice or would the numbers only be usable in theory? For instance: if I wanted to get to 150 RPM I figured I could gear the CIM from 4200 (1->28) or the drill from 16800 (1->112), now if I was to use this in actual practice would it work well or should I expect torque issues?

[Max Lobovsky]

When using two different motors, you should always make sure that they are geared so their free speeds match. This means you should be using the same percentage of each motor In this case, the ratio between the ratios for drill to CIM should be 19500/5500 or about 3.7. (112/28 != 19500/5500). This is probably a bit hard to understand, but there are numerous posts about this all over the forum with much longer, and well written explanations of why free speeds need to match and such...

[Adam Shapiro]

Originally Posted by Max Lobovsky

This is probably a bit hard to understand

Actually that makes plenty of sense. Now I just have to work on getting numbers that are easy to work with as the drill's no-load speed is something like 19,760 RPM ...

[Kevin Sevcik]

Originally Posted by Adam Shapiro

Actually that makes plenty of sense. Now I just have to work on getting numbers that are easy to work with as the drill's no-load speed is something like 19,760 RPM ...

John V-Nuen has made up a rather extensive spreadsheet to help with all sorts of things, one of which is the combining of different motors. I haven't looked at it recently, but it has gotten fairly good reviews.

Whitepaper