I’ve always been told that spinning a motor when power is not applied to it is unhealthy for the motor, yet I’ve always wondered: why? since either the rotor or the stator will always be an electromagnet, when power is not applied, the rotor should be able to just spin freely as it usually does when moved by the stator.
any help is appreciated.
Short answer: That’s an old wives’ tale. No damage is done by spinning a motor.
Longer answer: If you spin it faster than it’s designed max RPM (over voltage) or force it to spin really fast while shorted (over current) then maybe and I would worry more about the controller. Motors are generators when spun. Some circuits even come alive from the power generated by the motors when pushing a robot around. A Prusa printer will light up its screen if you move the table fast enough. If you think about it, we spin disconnected motors all the time and don’t think much of it. A motor not hooked to anything is better yet.
Also, in FTC and FLL, some of those motors and servos have relatively fragile gearboxes integrated in the motor and repeated spinning of those / hitting the servo hard stops seems more likely to damage something than an FRC motor sans gearbox.
Yeah, this is what my mind went to. The gears inside an FLL motor are all plastic and much easier to damage than the steel or aluminium gears in FRC.
Super good point. When it comes to the smaller stuff, especially the servos, the gearboxes are easy to break. We should be more cautious when forcing motion on a servo or high ratio gearbox.
The motors in the FLL motor assemblies are quite small compared to the motors used in FRC, even the smallest ones, so they don’t resist being back-driven very much. After over 12 years of heavy involvement coaching/mentoring many teams are volunteering at events as a (head) judge or head referee, I have not yet heard of anyone damaging one of the motors. The only damage to any Lego parts I have ever heard of were the result of the application of “extreme brute force”.
You see, when you’re incredibly lazy, sometimes you don’t put the robot on the cart and just push it around the event, perhaps even riding it like a kick scooter.
With enough speed for a sustained period, the speed controllers will power up and boot. And maybe, perhaps, some of those speed controllers are set to brake mode.
this is painful
All the answers so far are correct.
To dig a bit deeper:
Yes, except for magnets!
When rotating a motor shaft by hand, you’re still causing a magnet and some coils of wire to rotate relative to each other. This means Maxwell’s Equations to sprint to life, because you’ve created an electric generator.
The fact that new electric currents are present in the system are often the root cause of the old wives’ tale you’ve heard. It’s not cuz of the motor, but cuz of things around the motor.
Still, think about any time you’re sprinting down the field, and then let go of the joysticks and the robot coasts to the stop. During that coast, it’s the same situation as if you were pushing the robot yourself - no input to the motors, but external factors are causing it to spin anyway.
For FRC applications, nope, almost never an issue.
Ooooh, I’ll point out the one I know. Air compressor blow gun on a Victor 884 fan (884 is 15v max, fan went up to 30). Magic pixies did angry things and that controller was never the same again.
I’ve had a friend fry a motherboard integrated PC fan controller doing fundamentally the same thing.
ooooh very fair.
I should say: no issue as long as you stay within the RPM range the motor itself can achieve with 12V applied.
qualifiers are important 
. In this case though, interestingly, it’s still the stray/unexpected voltage generated by the motor’s spin, nothing about the motor itself, that causes the issue.
The more I think though, the above statement should be generally true for any system that isn’t… really bad. The reason is that all motors should be able to be spun up to their max RPM, then you should be able to turn off the supply to the motor and let them “spool-down” to no speed. If that spooldown period breaks things, you won’t have a useful system for long.
Back in the mid-2000s era of IFI robot controllers, there was a backup battery connected that would maintain radio communication and safety features in case of main battery brownouts. The system would remain running for a short while after the main breaker was opened, but it would not power on again without voltage on the main power input.
Pushing the robot when it was turned off would sometimes generate enough voltage to bring up the controller, even with no main battery installed. We actually exploited that once in order to reprogram the robot without a main battery installed. (The automatic no-power shutdown did not happen while the RC was in program mode.)
On the back drive powered motor drive… I did not see this when I was brake testing the NEO/SparkMax combination. They don’t self-power. You only get braking when there’s a live battery, which you always have during a match
So if this is reference to an FRC recomendation for not back driving, then that has a little to do with the 883 controllers and the first generation of 884s. Those vintage of controllers did not have a transorb on the input of the controller. In those controllers pushing a robot would put some voltage on the power rail. This could have significant spikes but not much higher than twelve volts. However back driving unpowered electronics of the time could cause some incidental damage. Later controllers have the transorb built in so higher spikes do not make it onto the power rail.
This is a bit more involved when the motor is connected to an H-bridge. This applies to most modern FRC motor controllers, including the brushless ones (where there are three power wires to/from the motor). Essentially, it’s likely that the motor will produce whatever AC voltage/s between different pairs of wires – as pointed out, it’s good if this is limited to around +/-12V.
When being backdriven, an H-bridge has diodes (generally not the body diodes but dedicated flyback ones) that will conduct the reverse current. Fortunately, the ways things work out, this will result in the A/C voltage being routed such that it does not wind up applying opposite polarity voltage to the power rails. It could be bad for the electronics, except that engineers design for it.
It’s unlikely you will rotate it perfectly round. Given that, it’s likely you’re applying load that is less than helpful on components that less than appreciate it
its cause the Neos are brushless. A brushed motor you “Break” by shorting the 2 leeds together. A brushless needs an ESC to properly energize the right coil at the right time to do anything. (drive or break). Hence you can take a brushed motor and hook it up to a battery or use it as a generator (if it is permanent magnet) whereas if you hook a brushless up to a battery directly you most likely destroy the motor.
The only time it might not be ok for a motor to be backdriven (turned) is if it has a gearbox. Now if its a conventional one (like a toughbox mini) or a planetary one backdriving is ok as long as you do not exceed the torque rating of the gearbox. Now if the gearbox has a wormgear inside like for example the window motors then backdriving/turning the outputshaft will likely damage the wormgear. Hence dont use window motors where they can be put in a backdrive situation with or without power like extending something beyond the bumpers where they can get into contact with another bot or field element. A gearmotor (PG series which has a planetary) will survive a window motor (With a worm) will not
thanks for answer
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