This is offered as a low cogging DC motor as it it ‘ironless’ – the rotor has no steel in it and therefore does not ‘detent’ when the steel aligns with the permanent magnet in the stator.
The first spike at 276/138Hz (1500/750rpm) for the rotation axis is the 11 commutator segment frequency. Not sure what the large envelope above/around 500Hz is – possibly bearing noise as it is a ball bearing motor. First small two spikes on the Z axis is likely to be rotor imbalance. Whilst the plots appear to be comparable to the best Papst B112, the rotor diameter is much smaller (less mass and radius of gyration) thus for it to create similar amplitude signals, the actual speed variation/torque ripple is presumably higher. Hence the need for direct on shaft speed measurement (now done). Whilst better than the Philips/Linn motor by a factor of 10, it is still 4x worse than the Papst B112 and has next to zero inertia. Any energy from the record playing process (stylus drag, dynamic load etc.) will have to come from some form of control loop and depending on the design of that, will introduce further speed variation.
Maybe it should not be so surprising that the commutator frequency is so clear. A fair amount of energy is also present in the Z-axis due to stray flux/bearings.
1500rpm
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750rpm
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.Speed Variation per revolution – 11 cycles due to 11 commutator segments
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