High Strenth Motors, And Their Problems.

On Saturday, my team attended the first round up qualifier in Bridgewater, NJ. We ended up being in the winning alliance and qualifying for worlds, but the high strength motors were giving us problems the ENTIRE time. They were also a problem for the alliance captain, 677. When 4 of these motors were used (on torque settings), the robot would act strangely, and die out, work again, then die out. 677 experienced the exact same problem. When we replaced two of the High strengths with regular motors, the problem was solved. At least for our robot. As I recall, this did not remedy 677’s robot. They ended up doing a base rebuild just before our first elimination match. Luckily, they fixed the problem by using the regular motors. (I think) This was just in time to help the alliance win. Anyway, we found out that the high strength motors were tripping a 4 amp breaker. There apparently is a breaker for 1-4 and 4-8. On the cortex there is one for 1-5 and 6-10. Spreading out the motors across the breakers also didn’t help.
These motors have proven to be problematic, especially at competition when there performance and effect on the electronics is unpredictable. This thread is meant for posting ideas on how to deal with them, share solutions, and ultimately figure out the best way to deal with these valuable but temperamental motors. Post your ideas below!

Were you using the cortex or the standard microcontroller? I am a bit scared to hear about this happening but the good news is that the season has just begun so there is plenty of time to get this figured out.


I was using the standard, while 677 was using the cortex. It seems to be a problem for both microcontrollers, considering that have the same 4 amp breaker which the motors seem to be tripping. It seemed to be more of a problem for 677, who was using the cortex.

I thought high strength motors didn’t draw (much) more current than the regular motors… Interesting. I find it odd that a change to normal motors in the same application fixed the problem. Wouldn’t high loads leading to current trips be closer to a lower torque motor’s stall torque?

Rhys Jones (677) asked the vex pros how to fix it, and they said the high strengths were tripping the breakers. They said to spread the motors across the breakers. This didn’t help, so i created this thread.

Somehow I knew this was thread was going to happen :confused:

For my official Q&A thread: https://vexforum.com/t/answered-high-strength-motor-problems/17412/1

Just to make this clear, even though the power expander has its own 4 amp breaker, exclusively offloading two of the four high strength motors to the power expander while keeping all other motors on the cortex causes the breaker in the power expander to TRIP FASTER than the internal breakers in the cortex would if you were to not use a power expander at all.

Our solution after rebuilding the drive train 3 times was to use only two high strength motors to directly drive the wheels. While we were rebuilding it, we missed the entire time allotted to take part in the driver/programming skills challenges.

Currently the only realistic solutions I see are only using a maximum of two high strength motors on your robot and making sure they are never under a heavy load, or using programming to turn down the maximum power they can output. Either way, it defeats the entire point of high strength motors.

That’s the only thing that worked for us, as well.

In theory, you can mathematically calculate motor current draws under various loads. As long as you keep each motor to about 1 amp of current and pick your motor ports properly it should work. The only reason this strikes me as odd is because a direct change to a lower torque motor fixed the problem…

Just a thought here. The stall current of the HS motors is 1.8 amps so even if you STALLED 2 of them on one of the 4 amp breakers it shouldn’t trip. This leads me to believe that if you spread the motors in such a way that there were no more than 2 per breaker you should be fine an by not stalling the HS motors you should be fine running some other motors as well. I think something else is causing the problem even though the symptoms are consistent with a tripped breaker. When the HS motors stop driving do the standard motors that are supported by the same 4 amp breaker stop running as well?


Everything stops working after the breaker is tripped. ( Or something goes wrong)

Everything as in all motors connected to that breaker or all motors plugged into the microcontroller?


I mean everything. Also, the battery light on the micro controller turns red for the time everything is not working. The batteries we were using were fine, it was not a low battery problem. This Didn’t happen when 2 hs motors were used. After about 10 seconds, the battery light went green again and the robot moved. Again, this only occurs with 4 HS motors.
Very odd, i know!

It’s puzzling.

I think the spec is wrong. Maybe it was intended to be 1.8 amps, but I honestly doubt it. The official tech support response assumed that it was a breaker being tripped as well.

The reason why I doubt the spec is because of how ridiculously powerful the motors are. I mean, on our original base where we had four of them geared 3:1, it could still topple a goal and push it around. It accelerated to top speed in an instant and got across the field in less than two seconds.

I think we might conduct some tests to see if the same thing happens with 4 motors in direct drive.

This sounds like you were causing a brownout in the microcontroller. This happens when you draw so much current from the battery that the microcontroller can’t draw enough current. If you had an old microcontroller that was using crystals, this would signal the microcontroller to reset, causing your autonomous to run again. The backup battery on the Vexnet system should prevent a robot that is using the Vexnet system from rerunning its autonomous code.

We were in fact using vexnet and the backup battery.

DO you mean a burnout??? Or is it called a brownout.

It’s called a brownout. Brownout (electricity) - Wikipedia

My team (254A) had a lot of problems with brownouts at the beginning of last season. We fixed these problems by gearing the drive down so that the motors were drawing less current.

Batteries have a maximum discharge rate. Once you reach this discharge rate, you cannot draw any more current from a battery, causing everything that is running on that battery to stop. I believe that the maximum discharge rate for the VEX 7.2 volt batteries is about 4 Amps.

The backup battery will only keep the VEXnet system alive, it will not run any of the motors. By keeping the VEXnet system alive, the battery prevents the robot from resetting and running the autonomous code multiple times, something that is often crippling during a match.

I think this is something to explore further although this is a bit weird because if i understand correctly using the power expander should help with this because you would be drawing across two batteries but from what we were told the power expander did not fix the problem at all.

Do we know the max discharge rate of the newer batteries both the small robot battery and the 3000mah batteries. If one of them had a higher max discharge rate that may be helpful and could possibly prevent a brownout.

This is indeed puzzling.


Does anyone know when the new 2000mah small batteries and 3000mah batteries are coming out???