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Power Supply Issues with Motors in Parallel.

posted Nov 19, 2011, 10:41 PM by Greg Mills   [ updated Nov 21, 2011, 3:13 PM by Nate Delson ]
Some of you have approached me with an issue that occurs while plugging into the table and testing your robot. 

It goes like this:

    I have two motors in parallel.
    I test them with alligator clips and it works.
    I plug into the table with the 9 pin connector and it does not.
    Help me!

There is unfortunately a significant resistance in the table wiring.  About 1 Ohm.  Essentially what is happening is that when you put your motors in parallel they have less of a resistance in your circuit than they would have individually. And since the resistance is at its lowest when the motors aren't spinning fast, aka when you start your robot, it exacerbates the problem. What happens is that the resistance in the table wiring and the reduced motor resistance form a sort of voltage divider and you don't get the proper amount of voltage to the motors. And when your motors receive less voltage they produce less torque and give you that sluggish performance or stall all together.

What do I do then?
     Hook the motors up to separate channels
     Try to reduce friction
     Ask yourself if you can improve your gear ratio to provide more torque

For additional information see writeup on the MAE3 powers supply and electrical debugging.

Nate Delson,
Nov 19, 2011, 10:41 PM