I just watched a video on Facebook by somebody who built a project to run an LED using a 9 volt DC motor scrounged from a battery operated toy. They simply wired the LED to the motor and then used a pulley system to spin the motor.
Here's a link to the video: Creating a generator from a DC motor.
Doesn't a typical DC motor like those used in toys act like an AC generator if you spin it? When you move a magnet past a coil of wire you get an AC pulse out of the coil. I would be surprised if a cheap DC toy motor contained a rectifier diode since it's designed to be a DC motor, not a DC generator.
So my expectation is that a typical DC motor would act like an AC generator.
Further, a 9 volt DC motor spun at a fairly high speed would probably emit around 9 volts AC with a fair amount of current behind it, so I would think you'd risk exceeding the reverse breakdown voltage on a small LED and burning it out.
I think the project in the video in question would need a rectifier diode (ideally a full-wave rectifier) and a current limiting resistor or it would risk blowing out the LED.
Best Answer
A cheap DC motor of the type that has a permanent magnet stator uses brushes and a rotor commutator to continually reverse the current into the rotor coil thus the effect is like feeding AC into the coil: -
If you didn't do this the rotor would spin maybe up to half a full turn and stop. Then it would take too much current and maybe burn out.
When driven as a generator the commutator does indeed work as a rectifier to produce a DC output: -
Maybe you are thinking that a cheap motor uses slip rings. This type of motor requires AC and will produce AC: -