10 Motor current limiting question

Hello all,
I’m currently retrofitting a cortex robot from a past worlds to the V5 system. This robot uses 10 393 motors, but I’ve heard that after 8 motors on V5, the motors get current limited. I have a couple of questions about this. 1. Is this limit still in place? There were some threads a while back that talked about that being moved to 10 motors. 2. How does the current limit affect the motors? I’m assuming it lowers torque and acceleration, but does it affect anything else? @jpearman hoping you can help with this :slight_smile:

current limit on the V5 starts at 8 smartport motors, maximum current is limited, so that will limit torque. V5 does not really support the old 393 motors (at least not well), I assume you are replacing them with V5 motors.


If I have 8 V5 Motors, how many 393’s can I plug in without being current limited? Also, does the current limiting affect Motors all the time, or only when all ten are drawing peak power/current? Thanks!

As I said, only V5 smart motors are considered.
Using 393 motors on 3wire ports is very limited, they are running at lower voltage than the cortex, at most you can use two without running into other problems. A V5 motor is far more powerful than a 393, don’t worry too much about any current limiting, they will still have more than enough power if replacing 393 motors.
(remember the name 393 came about from the power rating, 3.93W, a V5 motor is 11W)


Sorry to bug you again, but I’m retrofitting ANOTHER cortex bot. This one has 6 Motors on its flywheel, and it’s going to be really difficult to fit more than 1 V5 Motor into it (space issue). Can I run more 3-Wire Motors on the V5 3-Wire expander? What problems happen when more than 2 393 motors are connected to the brain? Thank you so much for your help :slight_smile: . Also, is there a Voltage/Speed Curve for the 393? I can find ones with current, efficiency, etc. but never voltage.

Here’s the problem. Below is the theoretical torque speed curve for a 393 motor, notice that stall current is around 4.8A. Now to protect itself the motor has a resettable fuse known as a PTC which will after a few seconds disable the motor, but for at least a short while it will pull lots of current. At maximum motor power, the motor is still pulling over 2A, this is all at battery voltage of the cortex which was 7.2V.

The 3wire device cannot provide 7.2V, only 5V. It also is limited to 2A maximum for all 8 ports combined. You can see that’s it’s not really intended to drive motors. So you could use a 3wire expander, but it will also be limited to 2A.

It sounds like a redesign is needed (you really would not need 6 x 11W V5 motors) or perhaps you could substitute the 393 motors using the smaller 5.5W V5 motors rather than the 11W.

There may be some around from years ago, but you never directly send voltage to these motors, rather, a pwm waveform is generated in the motor controller (MC29) that is proportional to the command speed you send to it. The actual motor speed vs command is quite non-linear for an unloaded motor. You will find some curves like the one in this old post around though.


Also, if you want to see current limit with different numbers of V5 11W motors, the PROS wiki has a calculator based on an algorithm I gave them.

Wiki is here

and a direct link to the calculator.
If you know that some motor will never need maximum current, they can be manually limited leaving more available for the remainder.

Motor current calculator