Cogging!

And a coreless motor helps most of all.

Its a new dance try it.

The facts are that almost all motors/drives cog to some extent. Its caused by the finite number of torque pulses delivered to the platter. Its similar to the smoothness of a car with a 4 cylinder motor vs a car with a 6, 8 12 or 16 cylinder engine or even better an electric motor. Generally the more cylinders/poles, the smaller the torque pulses and the smaller the resulting variation in speed of the platter/car that are felt as vibration or lack of vibration or smoothness.

This is not accurate.  Cogging is caused by the change in variable reluctance as the rotating permanent magnet rotor passes over the metal pole pieces and air gaps between them.  The drive signal is not "pulsing", it is a continuously smooth sine wave and the rotor synchronizes to the rotating field;  if there were no metal pole pieces in the stator (coreless motor), there would be no cogging.

Adding more poles to a motor does not reduce cogging, the attraction between the rotor and the stator poles remains the same.  It may feel "finer" in a 300 RPM motor vs 600 RPM, because there are twice as many poles, but the motor spins at half the speed so the vibrations will be identical if the motors are the same power rating.  This was measured and discussed here:
https://www.diyaudio.com/forums/analogue-source/309925-hurst-motors-300-rpm-vs-600-rpm-upgrade-myth.html

The metal poles of the stator concentrate the magnetic field developed by the stator coils.  The higher the drive signal, the higher the magnetic field and the stronger the attraction to the rotor and the higher the cogging.  The vibration in an AC synch motor is directly proportional to the power consumption, not the number of poles.  Reducing the drive voltage will reduce vibration, but it also reduces torque, this is only a band-aid and can start to have a negative effect on the sound.

Skewing the angle of the stator gaps can greatly reduce cogging as the rotor is never completely aligned with the poles or the gaps.  Most 3 phase BLDC motors are constructed this way.  When driven as 3 phase AC synch motors, they exhibit little or no cogging and are as smooth as coreless motors.

@phoenixengr
 
Well stated. Nice to see you here again.

The best way to demonstrate cogging is to spin the platter by hand,  apply stylus to vinyl, and notice the sound. Then engage the motor. If the sound deteriorates in clarity or shows increased high frequency content, you have demonstrated cogging. Obviously engaging the motor will improve pitch consistency.

If you can't demonstrate cogging, it may be because of bearing noise - the bearing is so noisy that it swamps cogging noise. An air bearing TT is capable of resolving even tiny amounts of cogging, which I can hear clearly with a 1.8W multiphase motor driven by an adjustable, precision supply. For the significance of this, read Phoenix above.