Turntable speed accuracy

This is an interesting discussion! Thanks all who have participated. I'm particularly interested in the role of the type of motor as it relates to speed stability. It seems that DC motors require a lot of fancy/expensive control mechanisms to achieve speed stability, many with a feedback loop of some sort for control. And as with any feedback loop, there is an opportunity for oscillation, which is certainly a problem.

On the AC motor front, you have synchronous motors, but perhaps the best performance came from the 3-phase motors, like the old Papst pancake motors that a number of high-end turntable makers used to use. I think technically they are eddy-current motors, and hugely inefficient. But their design with the spinning outer rotor and inner coils gave the motor a bit of flywheel effect regardless of the type of platter being used. It seems to me that the Papst motor is what is in the turntable that Atmasphere was raving about earlier.

Are there other 3-phase eddy-current turntable motors still in use today? I know that Papst ceased production of theirs, and it was problematic in several areas (the vanes make a whirring sound, for instance, and the bearing technology is 1950's era).

What other types of motors are best for speed stability?

I found a review posted by 'grimagog'? for a Mark Kelly controller applied to a Papst 3-phase motor. His claim was that having the ability to "tune" the current via the conroller leads to a quieter motor and (back on topic) improved speed stability.

Mark Kelly posts over at the DIYaudio forum occasionally and appears to be extremely motor-knowledgable. Jim Hagerman was toying with the idea of a new 3-phase motor controller as well.

I've got to believe that a multi-phase AC motor should be easier to design for speed stability compared to a 2-phase or single phase motor. More continuity of pulse per revolution, for sure. Certainly much of heavy industrial motors are 3-phase, so I think the math is pretty well-understood. But since most of us live where the power coming in over the wires is not 3-phase, some kind of controller is needed.

The Papst motors discussed above use a 4uF cap to provide the appropriate phase delay, to "trick" the motor into sync. I wonder if atmasphere has any tricks for improving the motor performance on his tables?

I've had good success with some of those grand old tables by micro-polishing the bearing balls, and improving the sleeve's ability to retain oil via fine cross-hatching at 1500 grit. The Rabco tables were interesting in that they had a spiral groove to "pump" oil back up the sleeve to keep a nice continuous lubrication film. With the right lube viscosity, these things would spin for several minutes down from 78rpm. I think the Empires were the same way.

So a nice even motor and nearly frictionless bearing = speed stability? (carefully avoiding the drive question here)