Turbo Motors Stalling...Please Help!

Hi! For Vex In the Zone my team is using turbo motors and after some 40 seconds (less if we are carrying a mobile goal) our turbo motors will just stop. We have checked and the turbo motors do not get warm. In addition, our robot battery indicator light turns orange even when the battery is at 7.22 volts. Can anyone shed light on why this is happening? Thanks in advance for your help.

Additional Information:
Two of our turbo motors are hooked up directly to the wheels while the other two are connected to their respective wheels via chain.

Please help as this is becoming a more and more pressing problem. Our coaches told us to come here because they are as baffled as we are. :frowning:

Update #1:
Robot battery light read red-battery was at 7.66 volts?

No need to spam the forum. Post in one place and you’ll get the help you need.

A VEX battery that reads 7.2 volts with no load is a dead battery. Make sure it’s charged, or try another battery. Normal unloaded voltage is more like 7.6-7.8 volts than 7.2.

How big are your wheels, and how many drive motors are you using? If the answer is 4" wheels and four drive motors, you are overdriving you motors. Don’t use turbo gears in that configuration. The motors don’t have to feel warm to be overloaded.

Make sure the chain is not too tight. Tight chains add a lot of friction.

Does your robot visibly accelerate up to speed? That’s a sure sign of gearing that is too high for your robot. A VEX robot should leap to full speed pretty much instantly, and if you can SEE it accelerate, it’s working too hard.

Good luck.

Hi! Thanks for replying! Sorry for posting in multiple places-I didn’t know where I would get help, I have deleted my other posts. We are using omni wheels-three something inches I believe, maybe four. There is a little acceleration but it is for less than a second. Our chains aren’t very tight but I don’t know what you would define as “tight” (You can push them down easily.). We are using four motors for four wheels. What would you recommend? Not using turbo motors? Different configuration? etc…
Grateful Team Member

Don’t trust the leds on the cortex because they are not always accurate. Also if you are doing turbo motors go 6 motors on the base with 3.25 inch wheels. Or I would also recommend using 4 motor high speed on 3.25 inch wheels. 4 motors turbo on 3.25 is not a good config depending on the weight of your robot. The high speed config is pretty fast too. 6 motor torque is not a bad idea either as it is not slow at high battery.

How are your motors wired? (Ports on cortex, expander?)
You’re very likely triggering the PTC fuse (search forums) - 1s to accelerate to speed is quite a long time and likely means that each of the motors draw close to the stall current for >0.5s…

Try speed motors instead of turbo. Turbo isn’t giving you enough torque with only 4 motors.

Thanks again everybody! @Owen, would we get the same torque from high speed motors-enough to carry a mobile goal without stalling. @nenik, sorry I am not the most technical person on my team-we are using a motor controller that splits so we have our two motors on the left side in one port and the two on our right in one port. I think we have them in ports one and ten (their choice not mine) and that has occasionally caused problems with other robots I’ve worked on but I have never heard of turbo motors stalling. Thank you everybody for your suggestions!

Ports two and nine.

How exactly does the PTC Fuse work? Does it power off the motors or just cause them to stall?

High speed motors will have a higher torque and lower speed output. The fuse works by heat. Essentially, the more power the motor draws, the more heat the ptc will give off. Additionally, the hotter the ptc is, the more heat it will give off with the same amount of power. The hotter the ptc is, the more it will resist power flowing through the motor. This is the effect:

  • If the ptc can cool down at the same rate or faster then it heats up, the motor will spin continually.
  • if the ptc can’t cool down as fast as it produces heat, the motor will start to stall because the ptc is getting hot
  • if the ptc is stalled, waiting around 10 seconds for it to cool off will make the motor work normally again

Then why are they called high **speed ** motors? Also, how might the PTC fuse affect the turbo motors? Sorry for my ignorance.

@antichamber is saying that in comparison to the turbo motors. Hi-Speed gears are still faster than the standard torque gears, but have a lower torque than the latter. There’s an inverse relationship between torque and speed for the same max power capacity, and changing gears affect the balance between torque and speed.

Standard > High Speed > Turbo
Turbo > High Speed > Standard

The PTC fuse is designed to protect the motors from damaging themselves when attempting to work against an excessive load. It’s a thermoresistor whose resistance varies with temperature. What happens is when the motors are subject to an excessive load (such as trying to transport a heavy weight or working against a force too great for the motor to overcome), the current inside the motor heats up the fuse faster than the fuse can cool and as a result, the PTC generates more and more electrical resistance until it reaches a tipping point in terms of temperature, at which point it just sends out a lot of resistance that the current is essentially blocked and the motor won’t run. It’s this yellow thing on top of the motor (the actual motor inside the motor casing) in the image below:

Once the fuse trips, the only way for the motor to return to normal is to let the PTC fuse cool down over time. In case you get the brilliant idea to just take the fuse out, let me remind you that that is extremely illegal in the VEX Robotics Competition and if referees at a tournament discover your team doing this, your team will be DQed from the whole tournament, no exceptions. There have been teams at VEX Worlds that got disqualified from the entire event at the inspection table over this.

If you want to keep the PTC fuse happy, you must not subject the motor to a load that it can’t bear. If you want the motor to carry a heavier load, you “downgrade” the motor to Standard or High Speed. The motor will be slower though. Other ways to keep the motors from overheating include adding more motors to the same mechanism (to distribute the load among multiple motors), reducing the weight of the robot or mechanism itself, etc.

I had to change motors before because the PTC in my motor wanted to shut down my claw after using it pick up a cone and put it on stationary.

It’s probably because your claw pressed against the cone too much.

As @FullMetalMentor said in another thread:

I had some 15 dot c channels with standoffs but it also happened with standard Vex claw

Then it’s probably because you’re trying to keep the cone in place by sending motor values even when the claw cannot close anymore. Use programming to keep it in place by having the program send values that are 10. This should keep them from tripping.

You can do the math…

Standard = 100 RPM max.
High Speed = 160 RPM
Turbo = 240 RPM

These will rarely reach their maximum theoretical RPM, so when doing calculations I assume the motor is spinning at 60% of its max RPM.

An exercise for the builder, with 3.25" wheels, direct external gearing, and high speed gearing, what is your robot’s maximum speed?

May I add a question to this thread?

Does doubling the motor count, in effect, double the theoretical output torque?

If a single high speed motor outputs 160RPM and 1.04 Nm, would a pair of them output the same RPM and 2.08 Nm? And three motors would output 3.12 N*m?

If the motors were running at 60% of the max wouldn’t it be something akin to 1.23245 linear feet per second?

What do you mean by a pair @mr.d?

I’m comparing one motor on each side to a pair of motors on each side to a trio of motors on both sides.