It has been well documented that for an AC synchronous motor such as installed in Thorens, Rega,  ProJect, Linn Sondek etc, to run smoothly and quietly it must have the correct value phasing/run capacitor.

It has also been shown that the capacitor chosen by the turntable manufacturer may not be the correct value for your particular motor.

The motors are mass produced and tend to have quite a wide variation of coil winding capacitance and/or resistance.

If you wish to ensure that the chosen capacitor on your motor is near to its correct value a simple method involves removing the motor from the turntable and powering it up while holding it in your hand. Different value caps are tried until the motor feels smoothest, or has the least vibration, or you electrocute yourself.

After a bit of Googling I have discovered a more scientific method.

With the motor still installed in the turntable different value capacitors are tried and the voltage measured across each winding. The ideal is to arrive at both windings being at the same voltage. This unfortunately still relies on 'trial and error' at this stage.

When this is achieved the voltage value of one winding, or the average of both, is multiplied by 1.41 which should be the same value measured across both windings. This will confirm that the windings are phased at 90 degrees apart.

For example, on my Thorens motor which I have powering my Oracle, I have arrived at a capacitor value of 0.2uF, achieved by running two 0.1uF X2 caps in parallel. The coil being fed directly measured 90volts. The coil being fed via the capacitor measured 92volts. The measured value across both coils measured 128volts.

91 X 1.41 = 128.31. Close enough for me !!

This is because on a right angle triangle if the two sides adjacent to the right angle are the same length, the length of the side opposite the right angle will be 1.41 times that length. Simple huh.

We can apply this bit of trigonometry using the coil winding values instead of lengths of the triangle sides.

My brain hurts

That's pretty much what I did too, on the Rega Planar 3.

The dropper R also can be fine-tuned to 'balance' the voltages across the 2 windings - see this snippet from a Premotec 9904 datasheet.

From memory, I ended up with a big 13k dropper R + 0.2uF slightly smaller phase split cap.

DarkLantern blog - darklanternforowen.wordpress.com

(R & C values adjusted as above post.)



Split Phase Capacitor motor - basically, a cheap way of getting an AC mtr to work on a 1-phase mains supply, utilising also the 90deg phase shift of the cap to rotate the mtr in a forward direction.

DarkLantern blog - darklanternforowen.wordpress.com

I've been fiddling again and discovered I could reduce the motor cogging ( vibration ) even further by reducing the operating voltage down to 77 volts. At this voltage the motor and platter will still start without having to be pushed. Anything less and it was struggling to start. This voltage was achieved by replacing the 1.5k resistor with a 3.3k on my 100v Geddon Clone. This however introduced a slight complication in that it upset the balance of the measured voltages between the directly fed coil and the capacitor fed coil. It turns out that when the supplied voltage is reduced, to maintain this balance the capacitor value also has to be reduced slightly. Trial and error substitution of different value caps resulted in a final value of 0.17uF providing 78 and 76 volts with voltage across both being 109 volts. Close enough. Obviously this value cap is not available in the X2 range so I had to parallel up a 0.15uF and two 0.01uF to achieve this value.
The motor is now almost dead quiet and maintains perfect speed on the Oracle.
I am now incorporating this capacitor selection process on all my turntable rebuilds.