Quote Originally Posted by CescoAiel View Post
Depending on the stator/rotor confuguration (and whether it is brushless or not) you may even be able to sense the rotor rotation from the outside, without attaching any magnets... Something you could test before even attempting to open it and attach a magnet anywhere...
From the photos, I'm pretty sure it has brushes and permanent stator magnets. I have no idea whether the rotor magnetic fields can be detected outside the motor, but I doubt it, with the steel casing and stator magnets shielding them. Maybe a hall effect sensor could be located inside the motor as close to the rotor as is practical; that might be able to count the passing rotor lobes, however many there are. That might be easier than dealing with the noise in the power circuit.