I’ve done some basic testing of the dynamic torque and mechanical power of an 11W V5 motor. After 2 full tests using the RPM reported by the internal motor chip averaged over a period of about 0.5 seconds when the motor has reached its highest velocity under each load. The load is produced by hanging masses on a chain driven by a large sprocket. The mechanical torque in Nm is then just the weight of the mass in newtons multiplied by the sprocket radius in meters. Below are the graphs I produced from this test with the same 11W motor and battery, once starting at 100% charge and the second time starting at 80% charge. Whilst this is by no means a perfect test, the maximum mechanical power output I am seeing is only about 8 watts, much lower than the average of 13.1 watts reported by PROS during the second test at 0.565 Nm and 13.4rads-1. The stall torque (this is a 200 rpm gearbox) is listed as 1.05 Nm on the vex website, but my stall torque is about 0.8Nm or slightly less (I only have 100g increments to test with). I havent calculated the expected uncertainity in my experiment but I can see no reason why it would be as high as ±10% for the power or ±28% for the stall torque. Any ideas?
Here are my initial results for the 5.5W motor. Stall torque is no greater than 0.47Nm (only 6% off the stated 0.5Nm on the vex website). Maximum mechanical power is very close to 5.0W which is almost 10% off the stated 5.5W).
This makes sense for the 5.5w seeing as it is literally a motor 393 with an IQ encoder.
Just spitballing here, but are you measuring and calculating for the inertia of the large sprocket? At small power levels like this, those masses can be significant. Same goes for the chain.
I’m not particularly familiar with inertia, but from my limited understanding, moment of inertia is the resistance to angular acceleration, however setup is purely analysing the torque output at constant velocity once the masses have been accelerated up to their maximum speed. If I’m looking at this wrong feel free to correct me. The inertia wouldn’t affect my result for stall torque either as far as I’m aware since there is no acceleration or velocity involved once the motor is stalled. Is it possible you could give me a hint as to what equations or technique I would need to use to account for moments of inertia in my experiment? For reference, here is the setup:
This is interesting from 1) an academic standpoint and 2) practical.
At such small power levels, items you would typically neglect can be significant. In a more perfect setup I would want to see ball bearings.
I’d also run some tests on 1) the sprocket alone 2) sprocket with chain to account for chain friction.
If I were running this experiment… I’d put a weighted flywheel, on a direct drive shaft, and measure rpm vs watts vs time. This would remove (except for bearings) the friction from the equation.
Given the large heavy-looking weights… I’d say you’ve got some significant frictional losses in a) the chain links as they pivot around the sprocket and b) the chain links as they slide on/off the sprocket’s teeth.
It will make a MESS, but you could try powered graphite on the chain/sprocket and see if your numbers change.
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