I’ve been going through the posts and documentation I can find on the 11W motor and torque characteristics, e.g.:
Understanding V5 Smart Motor (11W) Performance – VEX Library
If I’m interpreting the graph in the article correctly a 200 rpm (green) cartridge running at 50% speed would be better than a 100rpm (red) cartridge running at 100% speed as the torque of the 100rpm has dropped off significantly. Does that sound about right, or are there other reasons to pick red vs green in this case? The application I’m doing needs a high torque at relatively slow speed which falls in the range of the green and red cartridges.
Thanks,
Nick.
I recommend watching some youtube videos about speed and torque curves in a DC motor. Your interpretation is incorrect: A red cart running at max voltage will provide more torque (in this case, twice the torque) than a green cart at max voltage.
The “right” way to think about a torque curve is that under no load, the motor is able to spin freely, and is therefore providing no torque to the system. Under mid-level load, the motor spins slowly because of the load, but it also exerts plenty of torque. And of course, at stall, torque is maximized. But youtube can certainly do better than I can at explaining this.
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Thanks, yeah I’ve found a few videos and articles, so was trying to relate that to the specifics of the vex motors. What I think will happen to the red cart is would run freely at 100rpm at no load, but given I need around 1Nm it would slow down to about 85%. The green cart has a lower torque capability, but its flat around the operating point of 100rpm so would stay running at speed given they have speed control built in. Of course if i needed >1Nm (or 1.05) then the red would be needed. Testing this on the bench with the green seems to correlate. I just don’t have any reds so was wondering if i needed to.
The red and green motors both run from the same internal motor, as far as I am aware. Then, the red/green/blue gear cartridge will take the internal motor’s motion and use a series of gears to lower the speed/increase torque. The internal motor spins at 3600rpm, and the red cartridge will contain a 1:36 gear ratio which will slow the speed but increase the torque 36x. The green cartridge only increases torque 18x. They both have the same wattage though, and when you run a green motor at 200rpm speed it will be able to supply up to 11W of power. However, at 100rpm it will only use 5.5W of power. On the other hand, the red cartridge will use all of its power at 100rpm so it will be twice as powerful as the green cartridge at half-speed.
point of clarification, 11W (and 5.5W) is the output power, not the electrical power the motor is using. Max power occurs at around 65% of max speed (theoretically at 50% but the above graph treats 200rpm as 100% of max speed), so not at 200rpm.
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It’s a complex subject.
One thing NOT mentioned is heat. Generally speaking, motors run cooler at wide open speed… although this varies depending on how the motor is speed-controlled electronically.
Unless you’re applying the load AFTER the motor is up to speed (like a flywheel) you need to consider torque at start up rpm a.k.a. when it turns on. This will probably favor the red cartridge.
Try both. Time the output/mechanism/etc… see how many cycles you can perform in 2 mins. Check the temps before/afters. Swap cartridges, and re-run experiment.
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The actual motor turns significantly faster than 3600rpm. The part that turns @ 3600 is the gear that feeds into the planetary gear cartridge.
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