What values of free speed RPM, stall torque, and current to use?

**Maybe I just need some 101 on how to read the motor curves.

What percentage of our max stall torque and/or free speed RPM should we run our motors at? How do we determine this? I.e. what is a good rule of thumb?

At said stall torque/free speed, what current will we draw? (How do we read this off of the motor curve?)

In our application we are trying to lift about a 20-25 lb mechanism through a winch. We will have up to four RS-550s dedicated to this task, but we really have no idea how to go about the calculations without the knowledge above. Please apply this to other motors as well!

We will theoretically never stall these motors, or come even close.

Thank you so much,

This might help you:


It’s JVNs’s very useful mechanical design calculator. The “Linear Mechanism” tab would probably be of greatest use to you.

Just some quick math:
100" of travel (random number approximately in the ballpark)
4 550’s
30lbs (just to be safe)
1.5" pulley radius
16:1 ratio
70% efficiency
<punches numbers into calculator>
129lbs stall
0.69 sec lift time
12.13 ft/sec
14.5 amps per motor loaded

So, if you had 4 550 motors it would be really, really, really fast and come nowhere close to stalling the motors

<does more math with only 2 550 motors>
25:1 reduction (next up that Banebots makes)
1.17 sec lift time
101 lb stall
18 amps
7.11 ft/sec

Just to make sure I’m putting in the values correctly,

If we want to use four 550s, we don’t increase the free speed RPM, but the stall torque, the stall current, and the free current all get multiplied by 4, correct?

  • Sunny Gupta

I’m pretty sure you just multiply the stall torque and then the other numbers refer to current per motor instead of overall current

In general for FIRST purposes, what should our current limitations be for mechanisms? I.e. what amount of current should be partitioned to our drive train and what amount should be partitioned to our mechanisms? I know we are limited by the 120 amp breakers.

Normally we just kind of throw our electronics together. It has usually worked, but last year we had a problem of drawing too much current while simultaneously driving and winding our winch spool.

EDIT: Thank you, aparld.