Dud? right - sadly literally... some are just mechanical duds.
I got myself a few of these dirt cheap things... and some they hum smoothly, some they 'rattle' - you hear a hum overlaid with another synchronous noise of much lower frequency... at higher speeds (attached sound examples are at 1[ms] stepping interval. The volume of the rattling/touching noise is from placing the an empty open tuna can upside down on top the phone and the motor on top). Hold the motor with its back onto something resonating or your ear, and you will hear it whether your motors are smooth humming runners or guard rail smashing drunken drivers (out of the 5 I got, 2 hum, 2 hit a bit, one is stuck until you give it a twist in the right direction.)
The rotating magnet is not always centered and therefore touch the poles. Even a very little it touching just slows them down in getting them moving, and due to the construction, the 2nd step after the initial step just pulls the rotor back in the original position... because it did not reach the 2nd step fully when the poles change again.
Pulling the rotor out you will notice marks where it touches... bending the poles a tiny tiny little bit can solve the issue... but increases the gap and reduces of course the max torque.
Run them on 5 V first to separate the good from the bad one. Running them on 12 V works - in some way the duds even better when started slower and then sped up, but they get pretty quickly hotter than I like them... on the other hand, they have quite nice torque. For a running motor heat dissipation is probably not work... but in applications with duty cycles, heat dissipation may be manageable.
Attached is a shot showing the marks on the rotor - exactly at the position you will notice in the touch.mp4 clip, where you actually can her and 'see' where it touches... (stepping interval - stT = 15[ms]). Attached are too additional - just audio - 'clips' with healthy and unhealthy sound.
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