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Platter mass and speed correction
Platter mass and speed correction
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Old 5th November 2021, 10:35 PM   #31
wiseoldtech is offline wiseoldtech  United States
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Quote:
Originally Posted by Arch Stanton View Post
In the direct drive motor the stretching part is the magnetic field.

Quite possibly so.


However, in a well-designed system, the pulses of the driving coils are controlled in intensity so that any deviations to rotation are beyond trivial.
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Old 6th November 2021, 04:40 PM   #32
Pyramid is offline Pyramid  United States
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Quote:
Originally Posted by wiseoldtech View Post
Quite possibly so.


However, in a well-designed system, the pulses of the driving coils are controlled in intensity so that any deviations to rotation are beyond trivial.
The phase angle between the rotating magnetic field in the stator coils and the rotor is how the motor produces torque; it is a necessary component if the motor is going to do any work.

Any change in load whether from friction, groove modulation or other outside forces will necessitate a change in torque requirement (higher current draw) from the motor which will manifest itself as a change in phase between rotor and stator fields if the platter is to remain at a constant speed.

Therefore Arch Stanton's analogy is correct: There is a compliant element in the drive train even with a direct drive motor (this effect occurs with belt drive and idler drive motors as well, but is reduced the platter/pulley ratio).

Even if the platter remains at constant speed under load, the temporary change in the relationship between rotor and magnetic field can create a temporary change in pitch of the recovered signal analogous to phase modulation vs frequency modulation in wireless communications.
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Old 6th November 2021, 05:19 PM   #33
wiseoldtech is offline wiseoldtech  United States
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Quote:
Originally Posted by Pyramid View Post
The phase angle between the rotating magnetic field in the stator coils and the rotor is how the motor produces torque; it is a necessary component if the motor is going to do any work.

Any change in load whether from friction, groove modulation or other outside forces will necessitate a change in torque requirement (higher current draw) from the motor which will manifest itself as a change in phase between rotor and stator fields if the platter is to remain at a constant speed.

Therefore Arch Stanton's analogy is correct: There is a compliant element in the drive train even with a direct drive motor (this effect occurs with belt drive and idler drive motors as well, but is reduced the platter/pulley ratio).

Even if the platter remains at constant speed under load, the temporary change in the relationship between rotor and magnetic field can create a temporary change in pitch of the recovered signal analogous to phase modulation vs frequency modulation in wireless communications.

What you're describing is of course the feedback/compensation designed into such circuitry, necessary for stability of platter rotation and loading inconsistancies.
And this can happen at extremely fast rates but depends on platter mass and other mechanical time constants.


With the system designed into my own turntable, it's not even a worry, in fact inaudible to any variables of platter loading.
The stylus at 1.5 grams is a trivial factor in such things.
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Old 7th November 2021, 04:46 PM   #34
rickmcinnis is offline rickmcinnis  United States
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I found while using PYRAMID's 3 phase motor and controller with my Lenco that with the stock platter the motor would keep close to perfect speed consistency.

But it sounded terrible.

When I would use all of my extra copper mats to add weight the sound was dramatically improved but it took some time to get the motor and the heavy platter to work well together.

With the PYRAMID controller where one can change the frequency the time it took for the platter to respond to changes was too long for some sort of servo to do anything more than compensate for gross changes from the motor heating up. The changes would be instantaeous wiht the stock platter and took many revolutions to settle down with the heavy platter.

I will probably add some kind of simple servo like the ARDUINO based one but only for that purpose. I could see that being fed new frequency instructions would only make for a tremendous amount of confusion.

Of course, this is idler drive but I would think the same thing would happen with a belt.

Getting all adjusted properly with the PYRAMID motor made for very good speed consistency but one does need to make adjustments to the frequency between start up and after the motor has been running for an half hour or so. One can attempt to assume that leaving the frequency where it was and not minding the incorrect speed until the motor settles is not a sure thing if you want to get as close to 33.333 rpm as possible. in my case the final frequency is always slightly different with each listening session.

Anyone working with turntables needs a ROAD RUNNER to really know what is happening with the platter speed. Luckily these are available from SOTA.
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Old 7th November 2021, 09:36 PM   #35
DNic is offline DNic
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Did not know the Roadrunner was still available, so Sota must have bought out or licensed ''Pyramids'' original design. may get one to try with my belt driven L75.

Cheers
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