too many amps?

We are having the same problem again this year. our motors are overheating and tripping during driving practice. we moved out drive motors to two 393 on port 2 and two 393’s on port 7. also have a 393 in port 4 and another in port 6 for the arms. finally a last and 7th 393 in port 5. while driving the drive train appears to trip a circuit. what can I do and what should I be looking out for?

Thank you to all of you amazingly brilliant experts.

Are you guys using a power expander? You can really only run 2 393’s on a single PTC, Maybe 3, but that is a bit much to ask if you want to do any real work.

I believe your problem is lifting while you’re driving, causing an excess draw of current and tripping the current limiter in the cortex.

With the power expander, you would have 2 drive motors on one breaker (cortex), 2 drive motors on the other breaker (cortex) and the rest of your motors on the expander. This way, your drive motors have 2 breakers all to themselves.

Your last 393, which I’m assuming is some sort of pick-up mechanism, can run all it wants during regular driving. And since you probably won’t run the manipulator and the lift at the same time all the time, you won’t run into any stall problems.

Also, consider the issue of scrubbing. If your motors are working excessively hard just to turn, this will also cause current problems as well.

There is no silver bullet and the answer is not as simple as limiting the number of 393’s on one PTC. My testbed pushbot has 4x393s on its drive and they can all run from one cortex bank without a problem because it weighs almost nothing.

Lets review the torque-speed graph that I’ve been on my soap box about all summer.


I added some reference lines on this version. The green line corresponds to the safe current at which the 393 PTC should not trip, follow this line across to the blue torque-current line and then up to find the speed of the motor, around 85 rpm. The orange line shows the 1.8A point at which the PTC will trip after a few seconds, this corresponds to a speed of about 65 rpm. What this graph shows is that motor speed is everything, if you send full power to the motor and it runs slower than 65 rpm then you are operating in the critical area and the PTC is guaranteed to trip.

The cortex is seeing the sum of the current going to all motors connected to a protected bank, two motors running within the safe zone should not exceed the 4A (or perhaps slightly less) limit for the PTC. My little pushbot only draws about 0.5A per motor when running so can easily run from one cortex bank.

Several things can slow the motors down. The two biggest are

  1. Friction
  2. Weight

So I have some questions for you.

First, how long can you run the robot before you have PTC issues? If the answer is 20-30 minutes then probably don’t worry about it, matches are short, with very long practice runs motor heating is almost inevitable.

How much does the robot weigh? If it is 25 Lb then you have a problem.

Post pictures of it so we can see the construction. There are many things that can add friction which slow the motors down and cause your issues.

We are using a power expander. I didn’t realize it had it’s own breaker.

I did this about excessive work. but can’t find any obvious challenges. If the chain on the drive wheels is very tight could that pose excessive friction? Maybe we add a link and find another way to keep it intact.

I also think our bot is a bit heavy but not excessive. we don’t have a scale but I have had a robot with a lot more mass.

I’ll keep you posted with pictures after school today.

Thanks for the help.

Any more ideas what yo look for?