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Unread 04-29-2018, 05:21 PM
Orion.DeYoe Orion.DeYoe is offline
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Non-proportionality in Motor Stall Torque

Disclaimer: I might be an idiot.

I'm working on a spreadsheet to do some calculations for motion profiling on our robot and I'm basing my motor data on Vex's testing data. I notice that in their stall torque test (where they let the motor run at stall until it fails, basically) that the (initial) stall torque does not decrease proportionally with decreasing voltage. In other words, the stall torque at 6 volts is not half of what it is at 12 volts (it's actually more than half).

What is the reason for this? I would expect it to be proportional but maybe I'm missing something. Is there a reliable way to model the torque based on angular velocity AND voltage based on data provided in Vex's tests?
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Unread 04-29-2018, 05:58 PM
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Re: Non-proportionality in Motor Stall Torque

My guess is it's due to error within the estimates. Most motor specs can be +/- 10% on their numbers and there's a number of ways that error could have been introduced in Vex's tests.
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Unread 04-29-2018, 06:15 PM
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Re: Non-proportionality in Motor Stall Torque

Quote:
Originally Posted by ClayTownR View Post
My guess is it's due to error within the estimates. Most motor specs can be +/- 10% on their numbers and there's a number of ways that error could have been introduced in Vex's tests.
The CIM datasheet only says +-10% on RPM, as does the Vex site for all the other motors. Is it generally true for other specs? I'd like to know if I'm only getting 81% of the power I paid for.

A little bit of a tangent:
People quote the 10% number a lot. How true is it in actuality? A brief perusal of the Vex site doesn't tell me whether they tested multiple motors for the RPM data. Would someone with access to a large number of fresh motors, power supply, and encoder be willing to test a handful and see how they stack up?
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Unread 04-29-2018, 06:31 PM
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Re: Non-proportionality in Motor Stall Torque

Quote:
Originally Posted by Orion.DeYoe View Post
Disclaimer: I might be an idiot.

I'm working on a spreadsheet to do some calculations for motion profiling on our robot and I'm basing my motor data on Vex's testing data. I notice that in their stall torque test (where they let the motor run at stall until it fails, basically) that the (initial) stall torque does not decrease proportionally with decreasing voltage. In other words, the stall torque at 6 volts is not half of what it is at 12 volts (it's actually more than half).

What is the reason for this? I would expect it to be proportional but maybe I'm missing something. Is there a reliable way to model the torque based on angular velocity AND voltage based on data provided in Vex's tests?
I suspect this is due to the fact that at high voltages, the locked rotor stall test very quickly heats up the motor and the stall torque begins to fall before the peak torque measurement is complete.
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Unread 04-29-2018, 06:39 PM
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Re: Non-proportionality in Motor Stall Torque

CIM torque is fairly linear with current, compared with many similar size motors I have worked with. Its armature core induction rolls off a little at extreme (12V stall) current, because current in the armature coils creates a magneto-motive force opposing that of the stationary field magnets. This "armature reaction" effect is not very large for a CIM, but you can see it in the attached thumbnail.

As others have pointed out, the Vex data is really all we have to work with until someone tests a statistically valid number of samples. I've never had more than a few CIMs on hand at a time unless my FRC team was building a robot with them.
---------

edit: Jared's suspicion is well-founded. The CIM heats up quickly, or more accurately its armature coils do. This will limit current to less than the published stall value because copper resistivity increases with temperature. It is possible that Vex had trouble making the stall measurement quickly enough.
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Last edited by Richard Wallace : 04-29-2018 at 06:43 PM. Reason: Jared
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