Suicide Cables Safe for Motors?

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.

Our team has been controlling motors for prototyping with suicide cables for awhile now - I haven’t noticed many motors die when they are not stalled - that is where the problem could occur, and you could release magic smoke. But a regulator like you are describing could be useful for speed control - a la thrifty throttle plus pwm motor controller. This can help when prototyping a shooter, where speeds may be very specific.

We have something we call our “motor tester” which uses an old speed controller hooked up to a chip that generates a pwm signal with a potentiometer to adjust forward or backwards at differing voltages. The main power to the motor controller is provided by a regular robot battery which we have hooked through one of the normal 120A breakers and another bus unit (I think from 2009 or earlier) with one of the 40A breakers used on the PDP.

I’m a hardware guy, so I can’t really provide more details on this, but it seems like not the greatest idea, safety wise, to use direct voltage on the motor. I’m not sure there’s much of a problem from the motor’s perspective.

Yes, FRC motors can cook themselves, under some circumstances. Mostly, when they stall (don’t move) while being powered. With some motors, this can even happen on the robot before the breaker trips.

It’s all a question of heat dissipation. Moving that much current through a motor will cause it to generate heat. Some motors, like the CIM motors, have significant thermal mass, and can absorb a lot of heat - stalling them out for short periods is usually ok, which is why they’re almost always used on the drive train. Other motors, like your typical 500 or 700 series (Banebots 550, 775, for example) don’t have that thermal mass. Instead, they rely on built in fans to draw air through the case to dissipate the heat. Those fans only move when the motor is moving, so when you hit a stall condition and are still generating heat, there’s no where for it to go… it builds up quickly and poof, your motor is dead.

Make sure whatever potentiometer you’re using is rated to carry significant current- most of them aren’t! A better (and much easier) solution is to get an old drill. Take it apart, remove the motor and hook up an Anderson connector on one end and whatever standard connector you use on your motors on the other. You can also stick a breaker in the middle somewhere. Then you can easily plug in a motor, control it’s speed dynamically, and even switch between forward and reverse! Plus, the internals are going to be able to handle the current in a vast majority of cases.

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.

I think safety is the biggest issue with suicide cables. As others have said, a stalled motor could draw dangerously high current. Depending on how to cables are connected, a wire short hazard may also exist due to how easily the exposed wires could contact each other. Having a breaker would not only protect the circuit, but also allow you to turn it off quickly and easily should you need to do so. As for the potentiometer, definitely make sure it can handle the current you intend to run through it. I once put a small potentiometer in a circuit with a smallish 12V fan as an attempt at speed control. The device that originally had this pot ran on 12 volts DC, so it seemed like it could work. There was nothing else on the circuit. I might have connected something incorrectly, because I don’t think the fan worked, but there was some magic smoke from the potentiometer.

I’m not sure how all these teams are stalling their motors in prototyping. 115 has used direct battery-motor connection cables via Powerpole connectors for years without issues, to test shooters and other mechanisms. Having a speed controller like the Thrifty Throttle would make it easier to test different speeds, however.
As far as competition goes I haven’t seen a team every connect motor to power without a speed controller in between. That seems like a recipe for disaster.

I meant testing motors on broken mechanisms in the pit. Sorry for being unclear.

GRT takes the motors of 12 volt drills and install leads with Anderson connectors. You can add a fuse to both leads. This gives you a variable voltage reversible polarity power source. We we test motors and mechanisms with this. We have Y cables and can run one side of our drive train with them. One of the advantages of this design is if a mechanism is binding it will not break due to the low power output.

For 99.9% of mechanisms/people I would say this would be safe for the motor, although not necessarily safe for the people around it.

We have done this, though always where we were confident that something would slip before things would break or blue smoke would be released. We actually used something like this on our air cannon for a game early this football season, but we did have circuit breakers (a “Stinger” 4 circuit MAXI fuse block) and switched the motors with pushbutton automotive starter switches (which are difficult enough to operate that they serve as a built-in dead man switch). As light as our air cannon is this year, it would require a gifted act of sabotage or multiple failure modes worthy of Wiley Coyote or the stooges to stall our drive motors. Whether the switches would smoke or the breakers would trip first is probably a valid question.

Edit (addition):

Sure, all the time. Spike relay modules to switch automotive & PG motors on and off.:]

My team made a video a few years ago on how to make one of these.

Nice video. We have been making then for close to 20 years know. We use the batteries that come with the 12 volt drills. This makes them potrable and low powered. This reduces the chance burning the controls in the drill housing. It also may not matter and we just keep doing the same way.

The drill motors are great for projects. We use them off many off season projects. We add anderson connectors and you can guess the powersource.:smiley:

Can confirm this. During one of 3467’s 2015 practice sessions, we stopped mid-match before we were about to place a six stack + can and noodle while leaving the robot enabled. It turned out that our software was still holding a position slightly above ground level, hence stalling our two-miniCIM elevator for more than two minutes. It did not trip the PDP breakers. After that, those two miniCIMs were cooked for good (also the shop smelled considerably of burning CIM internals :P).

The breakers used in FRC don’t really protect the motors. They self reset very quickly letting current through. If you were using 40 amp breakers with the mini cims, it is possible to stall the motor with current sufficient to fry the motor but less than what it takes to trip the breaker.

This is our setup for running 775 Pros for prototyping. :ahh:

Just kidding! We’re using this to run our polycarb line bender.

Our air cannon also uses two batteries (for long run time, not high current draw), but we just put two SB-50As in parallel off of our main breaker, saving 2 connectors compared to this setup. Or do you use the Y adapter for other things as well?

It’s actually wired to run the two batteries in series. The 775 Pro’s just don’t spin fast enough for our liking at 12V. In all seriousness, we needed 24V to run higher current through the Nichrome wire to get it warm enough to bend polycarb. Just something we hacked together last night, no other uses yet.

A suggestion to everyone is to buy a small 12v battery for testing. A lot less dangerous, and easier to handle.

Also, wiring a small switch box with included fuse is a good exercise for your electrical team.

FTC batteries are perfect for this purpose, and they even have an inline fuse.