Some people might know that Celt-X went with an 8x 775 pro drivetrain this year, building off of Pwnage’s successful experiments in 2017. We were still a little unsure about burning out drive motors going into the season though, so we played it safe. “Tipsy’s” gearboxes were single speed, and geared to be traction limited so that we didn’t have to worry about stalling the motors at high power.
Tonight we set out to see if we were overly cautious in our design. Using our practice bot chassis, we programmed the drive talons to current limit each motor to 15 amps (60 amps per gearbox). This limit was chosen because it corresponds to ~4V at stall which according to Vex’s testing, should be good for over 5 minutes. We then stuck a rod through the wheels to stall them, as shown in the attached photo, and then floored the throttle. Using the Talon’s current sensor (and checked with a clamp-on DC ammeter) we confirmed that the max current to each motor was < 16A, and the voltage to each was < 4.2ish V. (The battery voltage dropped to about 10 during this test) Still, after about 30s of this test we could see and smell smoke :(. We repeated the test with just one motor, and sure enough, after a short while - magic smoke. This seemed to be a partial failure - the motor still turned, but its performance was reduced - as I’ll describe in a bit.
My question is this: Have I read the VEX curves wrong and 4V/15A is not a safe stall limit for 775 pros? Or is there some variation between motors that showed up in our testing but not Vex’s? I would love to hear CD’s thoughts on the matter. We won’t risk a higher speed 775-pro drive gearbox until we understand the stall current limits better.