Motor PTC vs MicroController PTC

So I’ll start by saying I’m not even sure yet if this is just to satisfy my curiousity or if there is a legitimate application here, but anyway :wink:

I coach a middle school team. We have a double flywheel set up with turbo motors and a total gear ratio of 23.5 (turbo X 84:60 X 84:7). Each side of the flywheel is 2 X 5" wheels and 2 motors driving each side. The motors are on one shaft driving the first 84 tooth gear. The flywheels will spin up and run at full speed for over 2 minutes without any load. Once we begin shooting balls one side will consistently stall after about 5 balls.

We are going to begin going through the flywheel system again looking to remove any sources of friction - replace the HS gears with LS, check all the bearings for any deformation etc. I’m fine with that.

So at least one of the PTCs is causing the system to stall… either one (or both) of the motors or the PTC at the microcontroller. Through swapping ports I can probably walk the team through figuring out which one is stalling first. That leads me to, so what? Does it matter which one is stalling first? I guess if we go through the whole system and feel we have eliminated any excess friction and it still occurs we’d then suspect that first PTC to stall means we have to replace that component (motor or possibly the microcontroller)?

If one of the motors does go, then the other one will too, thats not a big problem, but if its at the cortex, then you need to split the flywheels onto different breaker which can be hard if you dont have a power expander. Try removing friction with grease in the motors, graphite on the bearing, lining up the bearings and maybe even using 1 5" wheel. also make sure you’re not squeezing the ball too much.

So think through this some more…

Launching the balls is creating a bigger load and so driving up the voltage, correct?

On the side that keeps running that voltage is falling within an acceptable range for the PTCs on both motors and the microcontroller, right? Because if it wasn’t, that side would stall. Let’s call this side A

On the other side, the voltage spike is exceeding the range for at least one of the PTcs, because it is stalling. Let’s call this side B

Then we could say that there are 2 possible conditions

  1. The acceptable upper limit for a spike for side A and side B are roughly equal, but the spike on side B is too high. Spike A < Spike B. The solution here is reduce friction

  2. The spike on side A and side B are roughly equal but the acceptable spike for side B is lower than side A. Tolerance A is > Tolerance B. The solution here is replace the component that has the PTC that is not capable of taking the spike.

I guess there could be a 3rd condition where there is both a higher spike and a lower acceptable limit on side B, in which case both friction must be removed and the component with the PTC has to be changes.

So, is there an easy way to measure the voltage spike on both sides so they can be compared? That would then direct us to situation 1 or 2.

A couple of comments.

  1. We are not dealing with voltage here rather increased current to the motor caused by launching a ball.

  2. The increase in current causes the PTC to heat and eventually trip.

The current flowing through the motor is directly proportional to the speed the motor is running for a given control value. The best thing to do is measure the actual speed of the motor when running the flywheel (but not launching a ball) and see if you have them in the sweet spot of 70%-90% of free speed. If the motors are running slower than this even when not launching then the danger of tripping the PTC will be much higher.

See the motor speed/torque curves.
Motor torque-speed curves - REV2

My middle school team was having the same issue. We changed from turbo to regular 393 motors and can now constantly run without tripping the PTC.

There could also be variations in a number of elements on each side causing the tripping/increased current need - motor build quality among motors, friction on one part of the mechanism versus another, variation in weight between sides, and ability to deliver current over your port configuration all come to mind.

Looking at each of these critically to see if there is variation can be eye opening as you test them out layer by layer. Vex does not offer a current sensor off hand to help you out either but you can frankenwire one in there if you really want to.

A bench test apparatus can test your motors and run them a bit at a similar load to see if it is them. Then look at left and right flywheels with the different motor configurations to see if they operate differently. If motors on the bench test operate similarly, then the friction or build variances is probably the culprit.

Thank you for the correction on current vs voltage.

If I am following you correctly, 70% - 90% of free speed for a motor with turbo gears should be 168 rpms to 216 rpms?

Thank you. All great ideas.

Yes. The graphs were created for a 393 with gears as delivered, output free speed of approximately 100rpm. With the turbo gearing the output shaft will be running 2.4 times faster. Sometimes with a flywheel, reducing the gearing (external or internal) can make the flywheel (and hence the motor) actually spin faster.