Quote:
Originally Posted by rtse
My team decided to ban suicide cables for connecting motors directly to batteries for safety reasons and to extend motor lifespan. Our electronics team is building a regulator with a potentiometer and a fuse to go between batteries and motors so that we can test without a RoboRIO and/or speed controller.
I have seen many teams use suicide cables to spin motors at competitions, and as far as I can tell, connecting motors directly to batteries seems to be the norm.
What is the purpose of such a regulator?
It seems to me that supplying a 12V motor with 12-13V from FRC batteries shouldn't cause the kind of physical motor problems due to excess voltage that I read about (saturation).
Can FIRST motors really draw enough current from FIRST batteries to cook themselves? Or is there another problem that I am missing?
Disclaimer: I am a clueless software guy and I don't know much about this stuff. I just want to understand the science behind why my team is doing this.
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To answer your most pertinent question: Yes, FRC motors when stalled can draw dangerous amounts of current. The stall current on a full size CIM is about 130A. While there is a risk you would damage the motor internally, there's also a risk that you'll cause a fire hazard by overheating the wires.
The safest way to test motors is to use something similar to what MechEng83 suggests: have a main breaker to protect the battery, 6 AWG connections between the beaker, battery, and power distribution board, and a 40A (or other suitably sized breaker) to protect the motor wiring. Fuses and breakers protect the wiring, which is what typically presents the largest safety hazard.
As an aside, if your team uses a potentiometer as you mention, that's going to have to be one big potentiometer. Certainly not the sizes that would be used for sensing or what you would find at radio shack. Even under nominal loads, like a CIM motor in a drivetrain on carpet, motors will draw >10A with much higher peak currents when starting. Bear in mind the power rating of your potentiometer when selecting it.