Yep, motors generate electricity while they are running too. Thatís what keeps them from burning up. Itís back EMF, a current in the opposite direction to the current applied. That reduces the net current.
From your description, I expect that was a permanent magnet motor but it didnít have to be to act as a generator. There would be enough residual magnetism from hysteresis to start generating current then it would start providing itís own field current.
I didnít address this in my previous post
Black as ground - Not just battery operated devices, TVs, radios, computers, aircraft, spacecraft, industrial machines, etc. often use black to signify a ground or return.
Henceforth, please make it clear if you are referring to earth ground
, chassis ground
, signal ground
, or some other kind of ground. It can be confusing otherwise.