The controller for EV-3 will attempt to keep the two motors in sync using an internal PID algorithm and will stop when either of the two motors reaches the limit you set. In your case the controller has likely give. Motor "C" all there is (100% power) and scaled motor "B" back the max amount (what power that is, I don't know), but that was not enough to get the two in sync.  So, when motor "B" hit 2.8 revolutions, the controller stopped them both.

Likely reasons for this (in my opinion of order of likelihood)
     1) A heaving discrepancy in tightness between the "B" side and the "C" side
     2) A heavy weighting of the robot to the "C" side (putting more load on the "C" motor).
     3) a defective "C" motor


You can rule out #3 by swapping the motors and seeing if the problem follows the motor or the robot. Also, running the two motors with nothing attached in free air, and by running them with an identical load should also be able to shed some light on it.

And as usual, since this has nothing to do with constructing the robot, and everything to do with testing materials, you can help as much as you want - you can't do more then 100% of course, but that has nothing to do with the rules :-)

Brian

Sent from my iPhone

On Nov 1, 2013, at 9:37 AM, Rahman Rasulzada <[log in to unmask]> wrote:

> <Presentation1.jpg>
> Here is the what we are having problem
> 
> 
> On Fri, Nov 1, 2013 at 8:30 AM, Rahman Rasulzada <[log in to unmask]> wrote:
>> Hello folks , 
>> 
>> We are  having a trouble with new Ev3 large motors. Robot wheels not going straight however they put same values (rotation or degrees) but robot degree values change automatically... My students reset , and updated firmware but still having same issue. Motor values keep change ........
>>  
>> Any suggestion ? Really appreciated
>> 
>> 
>> -- 
>> _________________________
>> best regard,
>>  
>> Rahman Rasulzada 
>> Pinnacle Academy ,
>> IT Manager & Computer Teacher
>> School Website 
>> Personal Website
>> Computer Classroom Website
> 
> 
> 
> -- 
> _________________________
> best regard,
>  
> Rahman Rasulzada 
> Pinnacle Academy ,
> IT Manager & Computer Teacher
> School Website 
> Personal Website
> Computer Classroom Website
> 
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