We actually ran into a snag today while testing, and our Cortex stopped working. It turns out we were trying to drain too much power, and it overloaded or something to that effect.
Exactly how many motors are we able to run through the Cortex at one time? Last year, we had 4 393-Speed and 4 269 running all at the same time with minimal problems. What if they were all 393’s?
I’m just trying to understand what all of the information on the motor pages on the VEX store means. Our teacher/coach isn’t explaining it clearly (well, really at all), so if someone wouldn’t mind walking me through what happened and what the capacity is, that would be great.
There are 2 circuit breakers in the cortex (port 1 to 5, and port 6 to 10).
Each breaker has a rating of 4A.
As for 393… the stalling current is 3.6A… which means, it will stop working if you overwork the motor and cause the motor to draw a current higher than 3.6A.
But of course, there is also a PTC (or current limiter) in each 393. The current limiter will cause the motor to trip if it draws 1.8A continuously for about 7seconds.
But that said… if you are plugging in 4 393s in port 2 to 5, and overworking them at the same time, each 393 might draw more than 1A (total more than 4A for 4 393s)… that will be a recipe to trip the cortex
Re-distributing the port assignment will definitely reduce the chances of tripping a cortex.
But a lot depends on how hard you are going to work the motors.
For example… even if you are only putting 2 x 393 on port 3 and 4, but if you are going to drive your motors to draw 2A each (which is possible, even with the PTC), then if you throw in a 269, the cortex will still trip.
Ok, I will give a short answer tonight but plan on posting a full explanation of this in the near future.
The cortex has two paths for current to flow from the battery to the motors. Motor ports 1-5 are on one circuit, motor ports 6-10 are on the second circuit. Each circuit is protected by a 4A PTC (positive temperature coefficient), a type of resettable fuse. I posted a lot of information about the cortex PTC in this thread, although I have not had time to finish the tests.
The motors will use a variable amount of current depending on how much torque they have to provide. The specs for the motors usually give the no load current (maximum rpm) and the stall current (motor is stopped), you can create a graph that plots the current against rotational speed of the motor that is usually close to a straight line meaning that at half speed the motor will draw about half the stall current. See here for some information on this. For the 393 motor the stall current is given as 3.6A so when it is running at half speed the current draw will be about 1.8A.
If you had 4 393 motors running together at half speed then the total current needed will be 4x1.8 = 7.2A, as you can see this is above the 4A that one circuit on the cortex can handle so the PTC will protect that circuit after a few seconds if all the motors were all on one of the two groups. Even if you spread the motors over the two circuits, you can blow the PTC if the motors were to stall and remain so for several seconds.
All of this can get even more complex when you also take into account the fact that the motors each have their own PTC but the following facts will not change.
The cortex can supply 8A for about 30-60 seconds spread over it’s two circuits.
If you use a power expander you can have an additional 4A from a second battery.
You can run the cortex beyond the 4A per circuit but it will be for a much reduced time, for example, tests show that you can pull double the current for about 6 seconds.
Okay, that would be why we could drive for 2 seconds, then crapped out. We had some ridiculous gear ratio set up just to test if we could go at around 10 feet per second.
Thanks for explaining how to safely wire motors. I think we should be fine for next year.
