I now try to make a mechanical calculation based on basic physics.
I measured the cogging moment by a dynamometer, and it is 0.25 N at a radius of the seen wheel of 17 mm. It makes 4.3 mNm = M.
I estimate the moment of intertia of motor. The rotor got a volume of about 22 mm length and Ø16 mm. If you look at it as an iron cylinder it makes 0.066 kg = m.
The moment of intertia I = ½mr^2, r is radius of cylinder, and it is 0.008 m. So
I = 2.1 x 10E-6 kgm^2.
w is rotational speed of radians per second.
dw/dt = M/I = 4.3 mNm / 2.1x10E-6 = 2000 sec-2. It is a speed change of 20 rad/sec each 10 ms.
20 rad/sec = 3.2 revolution/sec = 191 rpm. This change in speed is done in 10 ms by the cogging moment. 100 in diagram is 164 rpm.
Based on this calculation, I find the changes seen realistic. It does not mean, that you also got ripple voltage from tacho generator, but not that much.