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 think you have it a bit backwards. Generally the motor chosen is related to the intended drive system. Plus there is a time-related effect. With regard to my first point, direct-drive motors pretty much must use some speed corrective mechanism, e.g., the well known quartz-linked servo. There are many ways of implementing this kind of circuit, and I am certainly no expert. The science of motors per se is amazingly arcane. However, just about all DD motors are either 3-phase AC synchronous types or DC types. Even within those two categories, there are many different ways to build each, which also will affect how the tt "sounds". Modern belt and direct drive turntables are still using the same technology, only with perhaps faster electronics. As regards my second point, 1960s and earlier turntables tended to use cruder single-phase or two-phase motors, e.g., Thorens, Garrard, and Lenco. But these were idler-drive and the motor is suited to that application. I just mean to say that you have broached a very complex subject and that in general it is not accurate to generalize on what type of motor per se is "best". You would not use a Pabst motor in a direct-drive turntable, for one example. There is more that I don't know about motors than what I do know. That much I am sure of. There are websites like this one where eggheads chat about motors 24/7, for example. Their jargon is at least as obscure as the one I work with all day long as a molecular biologist.

How do TT bearings in motors from the 50s differ from those used now?

Ketchup,
TT bearings of the 50is are bigger, more stable and usually in good TTs of a much superior built quality than those of most modern concepts.

best and fun only

There are also crappy bearings in turntables both old and new.
As a lover of vintage turntables for perhaps irrational reasons, I want to agree with you, Thuchan, but a modern machinist would probably tell us that with the digitally controlled cutting equipment (CAD) and laser technologies available now, it is possible to create bearings that are at least the equal to anything ever made in the mid-20th century and for much less cost (because less human labor is needed). But whether this leads to BETTER bearings in modern top of the line turntables, I do not know. Anyway, we are talking about the creme de la creme of modern turntables, not those middle of the road products that "only" cost $5000. Your Continuum is said to have a spectacular bearing; have you examined it closely?