VEX Motor problem

Hey guys,

I currently have a tank drive with two 393 motor (high speed configuration) and a gear ratio of 1:3 for speed. I’ve been having some problems when running it and after a while, the motors won’t move. I think it might be a stalling issue.

Check out this video:

After the turn at 0.33, both motors wouldn’t move. Each time the robot moves, both motors are at full speed, either forward or reverse.

What could be the problem?

The motor’s breakers are probably tripping. Either add more motors to the drive, gear down the robot, or make it lighter (which doesn’t look all that realistic since it is bare bones.)

Also make sure the axles that spin the sprockets for the tank drive spin freely. If they don’t that can add a lot of unwanted resistance.

By motor’s breakers, you mean the motor controller 29? I’m not too familiar with some of the terms.

There’s no problem with the axles of the sprockets.

The motors have an internal PTC that will trip when the motor becomes too hot, keeping the motor from running for a few seconds (usually around +/-10 seconds) in order to keep the motor from causing internal damage to itself. Here is a quote from the 2-Wire Motor 393’s VEX Wiki page:

A 1:3 ratio, even on a tank tread drive using the 15-tooth tank tread drive wheel, seems a bit fast for only two 393 motors. I would suggest either changing it to direct drive (you could swap the 393 gears to put it into the high-speed configuration, which would give you a 1:1.6 gear ratio) or adding another set of motors, to have four.


Even with our fully aluminum chassis, 3:1 and 4 393s are too much. I’d recommend adding 2 more motors and gearing down to 2:1, which has worked for us quite wonderfully.

What if I had a resistor to the power line of each motor to help limit the current? Could that work?

If you are trying to maximize performance,
adding a resistor to waste power before it gets to the motor seems like a bad idea.

What do you advice me to do?
Right now, I’m considering adding two other motors. Not sure if it’ll still be prone to the stall problem though.
Not thinking of changing the gear ratio just yet.

First calculate how much current the motors are using by measuring their speed when you are driving and using the data here. With this information you can decide if a change of gearing or adding motors will help. It also seems odd to me that you lost control of both motors at the same time, what is driving the motors? Perhaps the PTC in the controller is tripping.


To my eye, the speed of this robot does not indicate it is radically over-geared. As a suggestion, try moving the position of the track tensioners so that the tracks have less tension on them. This can make quite a difference to the load on the motors. If this does not help, then upping to 4 motors should do the trick.

Good luck, Paul

What ports do you have your motors on? If they are both on 2-5 then move one of them to 6 or higher. That will distribute the current load between the 2 internal breakers of the Cortex instead of just 1.