Trying to make an AC 3 phase motor run as smooth as possible at low speeds.
Now that I know that your building a turntable, I have some tips, although not necessarily an answer. Instead, I will expand on Charles answer above.

To minimize wow and flutter (turntable terms), you need to minimize torque ripple (a motor term). Besides the motor, the drive electronics and platter play a big roll.

What you want is a motor that is magnetically optimized for a sine-wave drive. Many (maybe even most) brushless motors are optimized for a trapazoidal drive, and they will likely have too much torque ripple.

Then, your electronics need to provide a sine-wave drive. That means that you can't depend on hall sensors for feedback, but need an encoder for high enough resolution. But I assume you have the encoder already, since you need it to phase-lock to.

Another way to produce the sine-wave drive instead of the encoder is to do what Sansui did in their high-end direct-drive turntables (look at the SR-929 and SR-838). They added windings to the motor to sense the rotor position in an analog fashion, and using the sine-wave sensed from those windings to drive the motor windings. It had very simple drive electronics, but the motor was custom.

Increasing the number of poles raises the frequency of any torque ripple that you may have left, and that makes the rotational inertia of your platter more effective at damping any speed variations caused by that ripple. As you know, the heavier the platter, the smoother the ride.