Turntable Speed Control

The next stage in development would be to go back to Ian's question regarding driving the motor phase winding.

There are two identical windings on the reversible synchronous motors and one is supplied via a capacitor and depending on which one, the motor rotates clockwise or anti-clockwise.

The capacitor produces a 90 degree phase difference and if the alternating current voltage supplied is in the form of the (expected) sine wave, then the voltage to the phase winding must be the cosine - the same but stepped a quarter further on.

The logic can be stepped to produce this difference, and so the controller will require a second output, and therefore a semi-parallel copy of the output stage. This will require a larger box and will therefore be more expensive to produce.

A number of commercial controllers do not bother with this second output, but purists would argue of its necessity, in which motor vibration would be under better control.

With just one output phase is dependent on the motor capacitor but at different frequencies the phase angle will be different to the expected 90 degrees, and it is argued this leads to vibration.

Calculations show that if an average of the two frequencies being used results in a half-way capacitor that the phase will be 16% out at both frequencies. Is this bad?

Mains capacitor tolerances are usually +/-10% and so might vary a total of 20% if many are sampled.

A motor I tried using phasing components (a resistor and capacitor) recommended for 60Hz produced noise at 60Hz and none at 50Hz. What will it do with the phase generated by a second output? We wait and see.

One bit of useful information this electronics guy of 40 plus years experience didn't know - because I do the electronics to make things work, usually to a brief - unless it's audio which I understand...

And I don't do electrical stuff like motors...

Sync motors are not ALL self starters. Some are - some not - don't ask me why.

It would seem that the motors like the one Rega use are made to be self starters, and a few others, but mainly they don't!

Therefore I've been doing a lot of messing about for nothing.

Shame the data sheets don't tell you!!! Shame on those who write them!!!

The internet is quite useful in digging things like this up if you search day and night. Books are bloody useless!

So now you know. And the motors which do self start have some additional "items" which make them start, like the sync motors used in clocks. Those in washing machine controllers use a clock spring...

Ernie wrote: A Shaded pole which is a copper ring around one of the poles, creates a weak rotating field (current is induced into the ring slightly out of phase to the main winding, which is fine if the required starting torque is low. You’re basically using inductance rather that capacitance to shift the phase angle.

Understood, but don't you think the data sheets should mention the inclusion?

It has become rather an expensive job buying all these motors to find some self-start whilst other types just sit there vibrating (correct rotation capacitor fitted).

And here's another observation: the higher the power rating the greater the hand assistance to get them to spin the platter.

Apart from the few turntables which are produced in quantities of a few a week, what else could use these motors - the ones which don't start that is?

And what are the techniques used to start them. This seems to be an official secret!

So far it has been ascertained that the Airpax 9904-111-31104 and the Pemotec 9904 111 31813 self start, but all the others tried do not start, including some Hurst motors BAK helped me obtain.

There is nothing on any of the data sheets to suggest they will self start or not. This seems to be a bit of a rip off!

The capacitor is there to cause the sine wave to lead or lag (depending on the way you look at it) one coil before/after the other, which sets the direction of rotation. In an induction motor this also starts the motor, but not with synchronous motors, but some do. So does that suggest some have one of these poles (the copper ring) and some don't?

I would have thought it was important for the data to state this? Or perhaps the distributors don't have a clue?



Edited by Graham Slee - 30 Jan 2018 at 8:31am

I wonder what motors Analogue Works (Divine Audio) use in their turntables. They are always on with platter start and stop done by hand.