Any limit to how far away a motor should be from its controller?

I see that Vex sells 36" 2-wire extensions and it makes me wonder if there is some safe limit to how far away a 393 motor should be from its motor controller.

Do line inductances, capacitances, etc. start to become a problem at some point? Is there any reason why a robot can’t have all of its motor controllers fairly close to the cortex and yet control motors that are 6-9 feet away via 2-wire extensions?

For our robot last year we had well over 6 feet of cable and we were fine. Provided you put the motor controllers near the Cortex and the majority of the extension is two wire, you’ll be fine with any amount of cable you’d ever use in VEX.

This is not our experience. Our robot had about 30 feet of cable between the motors and the microcontroller, and we found that it wouldn’t work well unless the majority of that extension was comprised of three wire cables. We had to have the power for the motors come from a battery expander which was within a couple of feet of them. If we tried to run the power over a long distance, the motors wouldn’t work.


On long wire runs with DC current, voltage drop due to resistance becomes an issue before inductance or capacitance. Motors at the ends of long wires won’t be as strong as those closer to the controller. The power loss will be linear with distance.

That said, 9 feet isn’t very far from a wire-run standpoint, so you may never see a noticeable impact. Perhaps you could do an experiment to measure how the torque of the same motor is affected by adding more wire.

One of the rules of Vex restricts the type/manufacture of cables. I suspect this is largely to prevent teams from substituting long runs of larger gauge wire in order to work around the torque loss at distance problem.


The wire VEX uses is 22 AWG, so about 16mΩ per foot. A 12 foot extension will add 24 (out and return) x 0.016 = 0.384Ω resistance. A 393 motor is about 1.5Ω so in theory a stalled motor (4.8A current) would have about 5.4V across the terminals rather than a nominal 7.2V. This is a much bigger problem for the motor controller than the motor as this low voltage can cause the MC29 to reset.

30 feet of cable will be almost 1Ω ( for the total 60ft round trip, and probably more when you add in connectors etc.) and will certainly be a problem. Using a power expander was a good solution.

Okay… so… um… I think what this might mean is that it’s best to keep the motor controller close to the Cortex and then use a 2-wire extension for the longer run. Is this correct?

And, if possible, use a power expander as close to the motors as is practical?

Many thanks for your input!

Best solution is

Long three wire cable to power expander (and battery), short three wire to MC29, short 2 wire to motor.

Next best (good to perhaps 12 feet)

Short 3 wire to MC29, long 2 wire to motor.

Bad (if more than 3-6 feet)

Long 3 wire to MC29, short 2 wire to motor.

Awesome. Thank you. This is solid gold information, especially considering how long these wire runs are becoming as the robots extend so high. :slight_smile: