Suicide Cables Safe for Motors?

[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.

[snekiam]

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.

[MechEng83]

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.

[Jon Stratis]

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.

[EmileH]

Originally Posted by Jon Stratis

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.

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 ).

[FrankJ]

Originally Posted by EmileH

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 ).

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.

[sastoller]

This is our setup for running 775 Pros for prototyping.


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

[GeeTwo]

Originally Posted by sastoller

...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?

[sastoller]

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.

[JCharlton]

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.

[cgmv123]

Originally Posted by JCharlton

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.

[Seth Mallory]

Originally Posted by JCharlton

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.

This is why we use 12 volt drills with their battery minus their motors. They come with controls for reversible polarity and variable voltage. Just add fuses and connectors.

[ctt956]

Originally Posted by sastoller

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.

Who needs a (cordless) Dremel when you have two robot batteries and a 775Pro?

Originally Posted by JCharlton

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.

Use a large(as in physical size) main breaker/power switch so it can be turned off quickly and easily in the event of an emergency. An E-stop button may be a good option, but the low priced ones can only handle about 10 amps, so I wouldn't use it for large motors.

[Al Skierkiewicz]

Just to add a little to this discussion...
Stall current is by definition, the current a motor draws when power is applied but the motor is not turning. (Start satisfies this definition.) The large CIM is rated at 131 amps stall.
As stated, no breaker is designed to protect a motor. It is designed to protect the wiring in an electrical failure. That is why the electrical rules relate breaker size to wire size.
Most pots are in the 1/4 to 1 watt rating. At 100 amps, that pot could be asked to dissipate 80+ watts when in series with a CIM at half speed.
The drill option stated in several places, gives a lot of advantages including reversing, variable speed and it likely also has internal over current protection.
An alternative is to use an inline fuse holder designed for the maxi-fuse 40 size when wiring to a robot battery. The inline circuit protection should be placed as close to the robot battery as possible to protect the wiring and load from heat and fire. Use a 40 amp breaker in place of the fuse. The main breaker can also be used but is harder to insulate and protect.

[jee7s]

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.

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.