# fun with high strength motors...

Hello again everybody…

I’ve noticed some disscussion about the safe amount of high strength motors that one can use on a SACK ATTACK robot.

I’ve heard that the “safe number” is six.

The question is this:

What is this number based on? Has anyone tested the use of many high strength motors in the cortex?

It’s based off of the fact that there are three 4 amp fuses available to you. One for ports 1-5 of the cortex, one for 6-10, and another on the power expander. The stall current for the 393s is 3.6 amps, just under the 4 amp limit. The motors operate at maximum efficiency when at half the stall current. The relationship is linear, so that means at half the stall current, one 393 draws 1.8 amps. You double that because you’re using two motors, so now you’re at 3.6 amps out of the maximum of 4 for a particular fuse. If you strategically place your motors, you can use six 393s at a time (two on each of the three 4 amp fuses). Meaning if you were using all HS motors, you could only ever safely run 6 at a time. Assuming you spread them across the fuses carefully.

So yes, there is a reason people are saying 6 is the “safe” number.

I refer you back to my previous calculations on this.

https://vexforum.com/showpost.php?p=292240&postcount=46

and also some real world measurements made on the 4A cortex PTC.

https://vexforum.com/showpost.php?p=266191&postcount=1

You have to consider both the motor PTC and the cortex/power expander PTC.

Is there any chance of a change in the rules to allow a second power expander?

C’mon, that’d make it too easy! Part of VEX is finding unique solutions to problems with limited resources.

Why does everyone seem to assume all the motors will be running at the same time?
It’s not unlike my house which has 200 AMP service but all my branch circuit breakers can add up to well over 200, because all of them won’t be at max for long.
But I encourage folks to engineer thier power needs and design their systems to work at peak efficiency, and test, test, test…