When I plug in the specs of the NEO into a design spreadsheet, it calculates the total output power to be 386w. However, the official REV data says 406w of power. Which one of these is right, and what specs should I use?
Neither is right, and both are right.
I would use the official design data. However, I would assume losses, not 100%–for the JVN calculator, there’s usually an 80% efficiency calculation built in, though it can be changed (that is, you lose about 20% of the power for various reasons). Some quick back-of-the-computer-calculator math says that the NEO is being calculated at 95% efficiency here–while brushless motors are a bit more efficient than brushed motors are, you also have to contend with the gearbox losses. So I’d say around 90% is more accurate as a ball-park number; if you want to be conservative use 85%. “FRC-standard” is about 80% though I think it’s gotten a bit better than when that “empirical guesstimate” became common.
The other factor is that the 386W may be an “as-tested” number. It doesn’t look like VEX has done their testing yet, and I’m not sure if someone else has done that (including with the most recent firmware updates).
On REV’s site they list both theoretical specs and empirical specs, I’d go with empirical but take that with a grain of salt because I’m no expert.
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I’d use empirical, and add a dash of efficiency loss onto that.
In theory, Theory and Reality are the same. In Reality, however…
Slightly off topic - I am surprised that VEX has not conducted their motor testing on the REV motor yet. Although I only heard of a few NEOs getting smoked this year due to stall, we all know that there is a limit where they will fail (just like every other motor out there). I heard a lot of teams stating that they were not using NEOs until they were better understood and all the bugs had been worked out. Those stall it until it fails tests are definitely one of the things that many teams are going to want to see before they adopt these new motors. So, I suspect that REVs sales for these motors were hurt this year by the lack of that data.
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386W is the theoretical max power for the motor assuming that it behaves like an ideal motor (i.e. torque generated is linearly decreasing with velocity). The motor spec values used are taken from the empirical values provided by Vex. You can calculate the theoretical maximum power with P = \tau_s \cdot \omega_f /4.
On the other hand, the 405W is most likely an empirically generated value. So when REV ran the dynamo tests, they recorded the motor speed that gave the highest output power.
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I asked about this but didn’t get a completely clear answer:
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The motor curves are still a calculated curve, but the parameters of that curve were determined empirically rather than based on theoretical data. VEX does similar when developing their motor curves.
FYI this data is already available http://www.revrobotics.com/neo-brushless-motor-locked-rotor-testing/
Let us know if you have any questions about it!
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Thanks for the link! I was not aware of that data being published. Those results look very promising relative to FRC usage. I will be sure to share that with anyone that asks (we’ve had a lot of people ask us about how tolerant the motors are to stall when they saw us suing them on our drivetrain this year).
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