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Very slow speed


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#1 kwarkdwift

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Posted 17 May 2006 - 11:07 AM

Greetings from France !
I am trying to play very old audio media like Edison Wax records and shellac records and want to make my own professional turntable for this, direct drive, with existing products. I already made a device, belt driven with a capstan motor from a Swiss made tape recorder, which had a small commercial success (visible at www.archeophone.org) but I am not fully satisfied by rotation.

Do you think that a small Variable Frequency Drive with a AC motor would do it? I assume 1/4HP would be enough, if not more than necessary. I need any slow speeds between 16 and 250 rpm, variable with a simple potentiometer, and do not want gear but true direct drive, like on marketed direct drive turntables or record players, as the best gears I have seen remain too noisy for audio purposes.

Chosen speeds should be constant, one have to avoid so called "wow" and "flutter", i.e. variation speed in time) and should not vary within 0,1 percent. I think I do not require a close loop feedback system, but it is an open option.

Should I look for stepper motors driven super smoothly by an appropriate signal, or should I prefer DC brushless and the appropriate electronics?

Specifications on all what I can see on the market up to now (looking closely for the last months, DC or AC) never quote such slow speeds, and commercial are sometims embarrassed in their answers.

Thank you in advance for your help and answer, and thank you already for such a great forum, which as you can imagine has no equivalent in France !
Best regards,
Henri

#2 marke

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Posted 17 May 2006 - 07:24 PM

Hello Henri

Welcome to the forum.
Long live the vinyl!!!

For your application, you require minimum torque stepping as this will affect the sound, so I would eliminate the stepping motor which will have a high torque ripple, even under the best of conditions.

You can not effectively control the speed of shaded pole motors with any repeatability, if you reduce the voltage, they will slip more but not necessarily at the same speed as last time. Shaded pole motors tend to be two pole, so you will need to reduce the sped a long way. If you use an oscillator and amplifier to provide a variable voltage variable frequency to the shaded pole motor, you will be able to reduce the speed, but not low enough for a direct drive unless you get a multipole shaded pole motor and I have not seen any of those around.

My leaning would be towards a brushless DC with many poles, but this is not without problems either.
The standard brushless DC has a permanent magnet rotor and the windings are fixed in the stator. This means that at a given point away from the motor, you will see a changing magnetic field as the motor rotates. This will influenct the pickup cartridge unless the positioning and screening are well done.

Sounds like a challenging project and it is going to be difficult if you are constrained to using standard components.

Best regards,




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