2020 update : JC Verdier La Platine

AC vs DC. While I fundamentally agree with Dover, a few points he raises need expansion.

Not all AC motors lock on to the field frequency. What we should be talking about is synchronous motors.  In this design the rotor follows the rotating field in the stator at the same speed, but not the same phase angle. IOW the rotor is running slightly behind the rotating field in the stator, but at the same speed. There is an angular displacement between the two. No displacement, no torque. This angle is determined by the load the motor is seeing and the motors design. More load equals greater angle, where the motor momentarily slows until the torque it produces equals the higher torque demand. Maintain that higher load and the angle will stay at this higher figure. Reduce the load and the angle will similarly decrease and momentarily the rotor's speed will increase. The rotor maintains an average speed dictated by the rotating field, but when driving a dynamic load, its real-time speed is changing at microscopic rates. Some synchronous motors are very stiff. This means that the increase in displacement angle is very small for any given increase in load, others are comparatively loose where the angle increase with load is correspondingly larger. 

Further adding to the mix, the change in angle vs load is not linear. As the angle increases the motor draws more current to self correct, but it first needs to increase the angle, thus slow momentarily. It has go wrong to correct. It is feedback. The correction is sinusoidal so we can assume that this is more benign. But, as above, the point to point speed can be changing when playing a record. There is no free lunch.

Most synchronous motors used in TTs are 2 phase. We need a way to generate a phase shift between the two phases in order to make the motor rotate. This can be done with something as simple as a capacitor all the way up to sophisticated regenerator controllers. If you are targeting smooth, ultra low torque ripple you want to use a 3 phase synchronous motor. Well designed 3 phase synchronous motors are virtually linear torque devises. Of course the controller needs to be capable of driving such a motor by generating three phases, 120 degrees apart 

If one was designing a TT today, where we were aiming for great dynamic speed stability.  A logical strategy would be to use a 3 phase synchronous motor that is very stiff and of sufficient intrinsic torque to dominate the platter. Then wrap a very high speed, but gentle, feedback loop around this to make the drive even more speed stable. 
 

Cheers 

@ledoux1238  This is good news that there is a option on the table, supported by a very skilled EE.

My thoughts on the the LDA Quartz Lock Controller, is that it was produced to be used in very similar situations, to bring a Vintage TT, into the modern world in relation to Speed Control .

Your own intentions will be very closely aligned to this purpose as well.

I am anticipating a positive outcome and your needs will hopefully seem to be more than surpassed.

@ledoux1238 , thanks! So, basically it regenerates the power and control speed by changing the output frequency! Unlike the SOTA Eclipse, the LDA doesn't have the feedback function, so you have to adjust speed with some sort of probe. I wonder what the adjustment steps are, ie 0.1Hz or 0.01Hz etc.

One of the advantages of the Eclipse is the 3-phase controller, which I found very beneficial for AC synchronous motors. On the other hand, the LDR webpage seems to indicate it would fit the asynchronous motors of vintage idler drive turntables better!  If you are getting either one, I would be very interested to know your findings!

Best,

@thekong I preface by saying I am really quite lost AC vs DC. However, in the case of SOTA Eclipse the Condor motor control is driving a 3 phase DC motor ( ? ). And the Longdog audio LDA is designed to drive an AC motor (? ).  According to Nick, the existing Verdier DC Philips motor cannot be controlled by the LDA, hence a search for an AC motor if I go this route. 

@richardkrebs Richard, given the issue at hand, the search for an after market motor / controller to drive a 18kg platter on magnetic bearing on the Verdier deck, how would you advice? 

ledoux.

The motor I described is what I used in my design of the OMA K3 turntable. I found the constituent parts online after a 6 month search. I had to make the motor from these parts.  Being direct drive, I have optimised the motor for low speed. You need higher speed to suit the Verdier, but the same basic specs apply.  

The controller was also found after a 6 month search. The combined cost motor/controller is massive.  Ultra high count feedback adds $Thousands more. 

But there must be motors and controllers out there that are suitable. Look for 3 phase synchronous motor around 0.1 Nm or more torque, if you are intending to use thread drive and appropriate speed capability. A little higher torque and lower speed if idler.  Good motor manufacturers will specify torque ripple. Less is more.

I haven't researched the Eclipse controller but thekong mentions that it can drive a 3 phase synchronous motor, so that would be a good start.