Hi
I have set up a winch to a 393 and am measuring the stall torque. If the stall torque published for the 393 is 1.67Nm and my winch has a radius of 1.25", then should the weight that will stall the motor be:
Torque = F * D
D= 1.25" = 0.032-m
Torque = 1.67 Nm
F = 1.67-Nm / 0.032-m
F = 52-N = 11.6-lbs
So theoretically speaking I should be able to suspend a 10-lb weight from the winch and the 383 could raise it correct? Assuming I’m supplying 7.2-Volts and approximately 5-amps.
The problem is that my motor driven winch won’t lift this weight at the 7.2-Volts.
I can think of a couple of things that might be affecting your test. First, there is the question of how you set up your winch. If you simply stuck a shaft into a motor and the things mounted to that shaft are not supported with plastic bearings, then you might be exerting excessive forces on the motor’s internal bearings and so the friction is very high.
Also, when lifting a 10 pound weight, you would be operating very close to stall anyway, so you might try testing the motor closer to its mid range and see how that compares with older test data.
Of course, you want to be sure your 393 is actually geared for torque, and there is always the chance that motors vary quite a bit from whatever the average values were in the tests that were done some years ago.
I’ve always wondered what the variability is on this type of torque-speed curve. I think others have noted that the motor PTCs vary in how quickly they will trip out when pushed to their limits.
Thanks for the thoughtful reply. I’m wanting to experimentally measure Stall Torqu and Stall Current, as well as Free Current with no load attached to the winch. I have all the correct power supply, force sensor, multimeter, wiring, etc set up. I’ve checked the motors intetnal gearing and it is indeed geared for torque. The problem is that the motor can’t pull the specified 52-N or even close to it.
The PTC will trip before you can get useful readings for stall torque, even if it allows some motor movement it will heat up quickly and the increased resistance will effectively reduce voltage across the motor. There’s some information in this thread. https://vexforum.com/t/motor-torque-speed-curves-rev2/21868/1
I guess the question for building vex robots is then, how much torque CAN one get out of the 393 before it trips and stops providing power? Should we assume that it only really works west of the peak power (=1/2 stall torque and free speed RPM)? Thanks!
It’s all about the current draw. My general rule of thumb is that the motor (with torque gearing) needs to run at around 70rpm or greater when sending maximum control values continuously, this correlates to about 1.5A. Obviously you can go above this for short periods of time but the PTC is somewhat unpredictable (although we have tried to build a mathematical model of it) so there is no definitive time you can run at these elevated torque levels. In addition to the motor PTC we also have the cortex or power expander PTC to deal with, some performance measurements on that one here. https://vexforum.com/t/ptc-performance-measurements/20872/1
Wow interesting. I understand that you don’t want to run the “nameplate” rating (Watts, Hp) of a motor for very long but it’s critical from a design/build perspective that if my kids are building a lifting mechanism that will be operating for say 2 to 3 seconds, you can’t count on more than 1.5-Amps or around 0.5-Nm of torque at the motor shaft, correct? I understand we can jump off from there with gearing to play with speed and torque, but the fact remains that we can’t count on much above 0.5-Nm of max torque at the motor. Or am I not thinking correctly? For example, let’s say I needed to lift a 5-lb/22-N weight continuously at a decent speed with a moment (torque) arm length of 0.25-m. Would I assume a torque at the motor of 1.67-Nm or 0.5-Nm when calculating my gear reduction?
jpearman is the expert on this sort of thing, so maybe he’ll disagree with me on this, but I’ll jump in and suggest to you what I suggest to my kids: look at the power curve on the 393 graph and see where it peaks. Also, look at the efficiency curve. As a rough rule of thumb, I tell them to aim for operating their motors around the 6 in-lb to 7 in-lb range and assume they will be running about 60 rpm at those torque levels. That’s for doing things like heavy lifting, etc. So I tell them to gear everything with those numbers in mind. Two of those 393 motors working together, geared with my rough rule of thumb in mind, can lift an impressive amount of weight in a reasonable amount of time, even with some friction thrown in.
Power is roughly a product of the torque and speed, so your best bet is to run the motor in the middle zone and gear everything from there to keep it in that zone.
Then I guess the trick is to use driver skills and/or some sensors and programming to try to avoid any prolonged stalls as much as possible.
One of my lessons is to have the kids generate the power curve themselves. We’ve already covered the explanation of the curve and optimal efficiency values (7-lb-in & 60rpm), but I’d like them to generate the curves themselves through lab experimentation for:
Torque vs Speed
Torque vs Current
Input power, output power, efficiency
But you need to measure stall torque and stall current, even just for a second or two to generate the curves. How does Vex test the 393 to stall so they can publish their values of 1.67-Nm at 4.8-amps? Do they not have the PTC installed?
I don’t know how Vex actually measured their torque-speed curve, but I think it would be acceptable for your students to measure the curve where they can, then extrapolate their data to report the stall condition. It’s possible that’s what Vex did, too. It would be a way of creating a line out of data that, in reality, is probably somewhat non-linear. By publishing the free-wheeling condition and the stall condition, Vex makes it possible to plot two data points, then the kids can presume a linear relation and make a quick graph. Not a perfect method, of course, but practical, hopefully.