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Turntable Speed Control

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Graham Slee View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 30 Jan 2018 at 5:34pm
@Chris: Amazingly the two motors will self start one of those replacement acrylic platters on a Rega Planar 3. The smaller standard motor does it within 15 seconds and the larger 3.5W motor in 7 seconds. And by increasing the load slightly by applying a finger to the edge of the platter they still start but it obviously takes longer. They should easily accomodate a heavier platter. There must be something about these motors which assists self starting but it would be a rather expensive do buying one to destructively take it apart to find what?

@Ifor: Using a proprietary hand start turntable loaned to me for R&D purposes the standard motor is a Philips 9904 111 32311. I had to fit an identical replacement because it was originally using a dropper resistor intended for the Premotec 9904 111 31813 which gave it 138V instead of 110V. It could have been that the voltage was too much for it, especially with no breeze to cool it... (no names, no pack drill).

@both (and all): Fitting a 2W Hurst 110V sync motor to the proprietary turntable the result is far worse, taking a considerable spinning up effort. Otherwise the motor sits there vibrating, and changing phase capacitors between 4uF down to 0.8uF (Mfrs value 0.25uF) results in no improvement apart from vibration being much less the lower in value the capacitor goes.

Conclusions:

1. the capacitor has little (or no) effect on starting except for reducing vibration.

2. the capacitor fulfils the requirement for rotational direction and little else.

3. given the above, a separate (cosine) output to drive the phase coil would seem to be wasted effort.

4. it is suggested that a sync motor might be started by ramping up from a very low frequency and in such case transformer coupling can be ruled out. It would have to be a 315V p-p amplifier for a 110V motor. Also there would need to be feedback to detect if the motor was actually ramping up in revs. This precludes quartz control which is fixed frequency and the oscillator's final frequency being RC defined would be subject to large tempco fluctuations; unless it switched over to quartz control after confirmation of being up to speed. Comment: very complicated to achieve and therefore very costly.

5. spinning up by hand using a sync motor which is "happy" to remain in a stalled condition without resulting in damage or danger seems to be the best option. Here again this is something the data sheets do not delve into.

6. or use one of the motors other manufacturers have found to self start from empirical investigation (because the data sheets never say).

7. In either of the cases (5 or 6) ensure controller power isn't wasted as heat; which means: a, rewiring the
Premotec 9904 111 31813 to operate on 110V instead of 230V (UK, EU etc) where the 10k (6k8 on data sheet) resistor wastes power; b, run hand spun motors within their voltage rating and not at elevated voltages which investigation suggests causes motor damage.

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Graham Slee View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 31 Jan 2018 at 1:16pm
Give an engineer ALL the information and he can do his job.

Tell the engineer that these motors are el-cheapo 24-pole can-stack construction permanent magnet stepper motors, and he can design something suitable that cuts through the great hifi misinformation society (to the tune of "the self preservation society" please... something seems a bit criminal here like £77 for an el-cheapo stepper motor).

Like I have said before, the data sheets tell you nothing.

What told me exactly what they are was taking two motors apart. What a pity that the customer/user has to do this??

What was found is pictured by this drawing at http://www.ibiblio.org/kuphaldt/electricCircuits/AC/AC_13.html


image reproduced here for educational purposes (I will take it down if objections are raised)

The reason why the larger diameter motors self start (using a capacitor to step 90 degrees'ish) is probably down to the finger spacings alone.

Photo to be uploaded when my camera battery charges.

Steps back 27 years and I hear a manager saying "I thought you being an electronics engineer you would know that?", to which I replied "how many branches are there to electronics engineering?". What I would have liked to have said was: "you are BSc in electronics engineering so why can't you design all these broadcast audio circuits you get me to do!?"

The above being mentioned in case anybody reading this thinks I'm a bit thick.
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Graham Slee View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 31 Jan 2018 at 1:43pm
Now, having said the above, can we really gain anything by driving these stepper motors as stepper motors?

At 50, 60, 67.5 or even 81 Hz they are "join the dots" and I can tell you now, having used CNC XYZ positioning using precision stepper motors, there will be vibration - yes, even by applying the correct phase intervals. The maximum torque being applied when the motor is stood! And for good reason in locking the position for a process, usually in the X dimesion, to complete. Think of a CNC printed circuit drilling machine using a 0.4mm tungsten carbide drill?!

Put simply, they cog, and the belt plus platter inertia smooths this out.

So all the things hi-fi sync motor "control" does is really a job of cheating the application. For example: nudging the phase difference to lower vibration; making one phase voltage different to the other; reducing phase voltages altogether.

An original case of "it'll do" back when it would do, now being touted as a great solution because it gives manufacturers things to do to make money.

All hail Technics...

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Graham Slee View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 31 Jan 2018 at 7:15pm
Here are the views of the insides of the Premotec 9904 111 31813 (larger) and Philips 9904 111 32311 (smaller) stepper motors.

inside stepper motors
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Graham Slee View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 07 Feb 2018 at 9:29pm
Quite incredibly the 3.5W 220V Airpax motor I bought and fitted to my old Rega Planar 3 turntable is now running at 89V!

And even more miraculous it goes from stop to synchronous speed in 3.5 seconds!

Why bother running it at 220V when it behaves perfectly at 89V?

Or did they badge up a different motor wrongly?

It can be obtained at less than half the price of the Rega motor... Wink
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