VEX Forum fun with high strength motors...
 Portal Register Gallery Wiki Members Calendar Search Today's Posts Mark Forums Read Rules

 UNOFFICIAL Answers - Ask the Community! Community Members can provide UNOFFICIAL answers to the questions from the "locked" forum above.

#1
05-10-2012, 04:44 PM
 grammatically_uncorrect Senior Member VEX # 4405a Join Date: Mar 2012 Location: Omaha, NE Posts: 134 Images: 1
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?
#2
05-10-2012, 05:14 PM
 Disciple855 Senior Member VEX # CSM Join Date: Apr 2011 Location: Maryland Posts: 233 Images: 1
Re: fun with high strength motors...

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.
__________________
Even now,” declares the Lord,
with fasting and weeping and mourning.”
for he is gracious and compassionate,
slow to anger and abounding in love,
and he relents from sending calamity.
Joel 2:12-13
#3
05-10-2012, 05:29 PM
 jpearman Senior Member VEX # 8888 Join Date: Apr 2011 Location: Los Angeles Posts: 2,813 Images: 2
Re: fun with high strength motors...

I refer you back to my previous calculations on this.

http://www.vexforum.com/showpost.php...0&postcount=46

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

http://www.vexforum.com/showpost.php...91&postcount=1

You have to consider both the motor PTC and the cortex/power expander PTC.
#4
05-10-2012, 07:08 PM
 4495Arobotics Junior Member Join Date: Mar 2012 Posts: 13 Images: 2
Re: fun with high strength motors...

Is there any chance of a change in the rules to allow a second power expander?
#5
05-10-2012, 07:57 PM
 Warrior5 Senior Member Join Date: Dec 2011 Posts: 513
Re: fun with high strength motors...

Quote:
 Originally Posted by 4495Arobotics 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.
#6
05-10-2012, 10:03 PM
 fretless_kb Senior Member VEX # 1166 Join Date: Jan 2010 Location: Parker CO Posts: 282 Images: 6
Re: fun with high strength motors...

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...
__________________
Kevin Barrett - Mentor - Team 1166

 Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)