Go Back   VEX Forum > Official VEX Technical Support > UNOFFICIAL Answers - Ask the Community!

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

Reply
 
Thread Tools
  #1  
Old 05-10-2012, 04:44 PM
grammatically_uncorrect's Avatar
grammatically_uncorrect grammatically_uncorrect is offline
Senior Member
VEX # 4405a
 
Join Date: Mar 2012
Location: Omaha, NE
Posts: 135
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?
Reply With Quote
  #2  
Old 05-10-2012, 05:14 PM
Disciple855's Avatar
Disciple855 Disciple855 is offline
Senior Member
VEX # CSM
 
Join Date: Apr 2011
Location: Maryland
Posts: 233
Images: 1
Smile 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,
“Return to me with all your heart,
with fasting and weeping and mourning.”
Rend your heart and not your garments.
Return to the Lord your God,
for he is gracious and compassionate,
slow to anger and abounding in love,
and he relents from sending calamity.
Joel 2:12-13
Reply With Quote
  #3  
Old 05-10-2012, 05:29 PM
jpearman's Avatar
jpearman jpearman is offline
Senior Member
VEX # 8888
 
Join Date: Apr 2011
Location: Los Angeles
Posts: 2,848
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.
Reply With Quote
  #4  
Old 05-10-2012, 07:08 PM
4495Arobotics 4495Arobotics is offline
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?
Reply With Quote
  #5  
Old 05-10-2012, 07:57 PM
Warrior5 Warrior5 is offline
Senior Member
 
Join Date: Dec 2011
Posts: 513
Smile Re: fun with high strength motors...

Quote:
Originally Posted by 4495Arobotics View Post
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.
Reply With Quote
  #6  
Old 05-10-2012, 10:03 PM
fretless_kb fretless_kb is offline
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
Reply With Quote
Reply


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

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Forum Jump


All times are GMT -6. The time now is 01:11 PM.


VEX and VEX Robotics are trademarks or service marks of Innovation First International, Inc.
Copyright © 2002-2013. All Rights Reserved. VEX Robotics, Inc. is a subsidiary of Innovation First International, Inc.
All other product names/marks of others are the property of their respective owners.