Linn Lingo vs. Dr. Fuß or Square-Wave vs. Sine Wave Oscillator for Motor Control - Page 3 - diyAudio
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Old 9th July 2012, 07:25 PM   #21
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I've tuned the cap value on similar motors with a substitution box (be sure its rated for the voltage) and found a definite vibrational null at near the factory value, but not dead on. IMO, it's worth it to pad the value for minimum vibration, regardless of the voltages.
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Old 9th July 2012, 09:22 PM   #22
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Quote:
Originally Posted by fvale View Post
Thanks for advice. But what I need to know is what "theory" wants. I will not use a capacitor anymore. I've a DDS source that feeds two phase shifters followed by a TDA2050 based amp. Unfortunately I'm not able to find these informations anywhere.... even writing Hurts didn't help at all! I'm wondering if I've to look both for identical voltages through the two coils together with a 90° phase difference.
With a cap I got almost identical voltages with a 0.1uF cap, not 0.25, and much less vibration!
At record player turntable the "theory" wants only two things: lowest possible mechanical noise and highest possible constancy of speed
check out this - perhaps helpful:
http://www.anvikshikijournal.com/dow...4-d4ca158f1718
TT Motors
http://www.mclennan.co.uk/datasheets...nousmotors.pdf
http://umpir.ump.edu.my/27/1/CD3199.pdf

Quote:
Originally Posted by Conrad Hoffman View Post
IMO, it's worth it to pad the value for minimum vibration, regardless of the voltages.
I agree.

I don't understand this term:
Quote:
Originally Posted by Conrad Hoffman View Post
but not dead on.
Mean you, that a residual noise/hum remains ?

Last edited by tiefbassuebertr; 9th July 2012 at 09:44 PM.
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Old 10th July 2012, 01:06 AM   #23
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Assuming that the motor is well made and accurately balanced then the smoothest rotation can only come from equal voltage on each phase, otherwise the motor will be accelerating and decelerating several times within every rotation. There seems little point in adding another harmonic into the mix by having odd voltages and therefore unequal drive from each phase.

I'd vote for equal voltage, correctly trimmed phase and run the motor with a little applied drag to linearise it and swamp cogging effects. That was what gave the best results for me.
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Old 10th July 2012, 01:44 AM   #24
grufti is offline grufti  United States
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What did you do to get: "a little applied drag"?
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Old 10th July 2012, 09:29 AM   #25
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Originally Posted by sq225917 View Post
Assuming that the motor is well made and accurately balanced then the smoothest rotation can only come from equal voltage on each phase, otherwise the motor will be accelerating and decelerating several times within every rotation. There seems little point in adding another harmonic into the mix by having odd voltages and therefore unequal drive from each phase.

I'd vote for equal voltage, correctly trimmed phase and run the motor with a little applied drag to linearise it and swamp cogging effects. That was what gave the best results for me.

I've a very limited experience with these motors, only since a couple of weeks I'm playing with my dual phase generator, but I'd also vote for equal voltages.
Then there's another point. I've the suspect that super precise phase rotation setting isn't essential. I mean I can't feel changes with few degreeases changes. But I might be wrong... Thoughts on this?
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Old 10th July 2012, 09:58 AM   #26
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This is a very interesting part of a article on a link kindly posted above by Tiefbassuebertr.
It seems to be a very good idea to use lower voltages on coils to minimize vibrations, assuming you've enought torque expecially at platter start!



AC synchronous

The original motor in Linn Sondek (and most other TTs of that ilk) falls into this category. Because the rotor magnetisation is permanent, there is no slip induced between the rotor and the stator field so the rotor moves in synchrony with the stator field, hence the name. Generally the motors found in TTs are two phase multi-pole synchronous motors with iron stators. The run speed equals 120 x fsupply / pole number so a 24 pole motor gives 250RPM on a 50Hz supply and 300 RPM on a 60Hz supply. A synchronous motor must run at this speed until its pull out torque is reached where it abruptly stops, giving the "squared off" speed / torque characteristic shown in the graph. Loading techniques like eddy current brakes therefore have no effect. Changing the voltage supply only changes the pull out torque so cannot be used for speed control.

To vary speed an AC synchronous motor needs a variable frequency drive. These can be either digital or analogue, with digital being far more common as they are cheaper and easier to design to a given specification. Electronic supplies can be designed to give two output waves "in quadrature" - 90 degrees apart. On a single phase supply (like the mains) the motor uses a phasing capacitor to generate the cosine equivalent of the sine wave supply.

In theory the torque cogging produced by each of the sine and cosine phases cancels. This is because the torque produced by each phase is proportional to the square of the current and sin squared + cos squared = 1. In practice this only applies to the actual drive currents and then only if the two currents are exactly equal. In any real motor there will be excess currents due to the motor running at less than pull out torque and to winding resistance. The motor responds to torque requirement by changing the angle between the stator field and the rotor field. At zero load the two fields are aligned, at pull out torque they are at (90 degrees / pole pairs) apart. 90 degrees is the point of maximal torque conversion efficiency, so any greater torque than that which gives 90 degree separation will cause the motor to lose synchrony and stop.

Often the starting load is the highest load a motor will see, so in normal running the motor is at some fraction of its pull out torque. Lesser loads mean that the motor runs at a lower angle and thus a lower torque conversion efficiency. The lower efficiency lowers the inductance of the windings, changing the ratio of reactance (drive) to resistance (heating). The excess current therefore simply heats the motor windings and it appears that these heating currents do not follow the sine squared form of the drive currents so they create cogging torque, but I have been unable to confirm this. Lowering the drive voltage reduces the excess and thus the vibration but to eliminate it completely the voltage would have to be lowered to the point where the motor was at pull out torque at which point any disruption would stop the motor. An AC synchronous motor can usually be held in the stopped position for long periods without harm as long as the rated motor inout power is not exceeded.

There is also some cogging due to the attraction / repulsion between the rotor magnets and the stator pole pieces in iron stator motors. These motors are quite sensitive to their power supply which can be another source of noise.
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Old 10th July 2012, 04:54 PM   #27
Nanook is offline Nanook  Canada
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Default voltages and things...

Quote:
Originally Posted by fvale View Post
I've a question about Hurst motor installed in my old VPI 19 turntable. Could someone tell me how much important is to have a identical voltages on the two coils?
I ask this because with Hurst suggested 0.25uF cap, that supposedly should give a 90° phase rotation, tensions are different. I build a simple phase rotation system based on operational ICs, but, unexpectedly, when I regulate amplitude I also got phase variations! So I'd like to remove them....
I know the Linn/Airpax motors reportedly offer improved performance when the voltages used are between 72 VAC and 80 VAC (for 115/60Hz, and 220/50Hz wall power). The mechanical noise decreases to essentially "zero". I had contacted Lee Norton who kindly returned my email. He suggested using 2 variacs (1 for each circuit of the AC motor) to adjust both voltages independently. That way you could adjust each coil for the quietest operation, then modify the circuit to limit the voltage to what is needed for that particular motor.
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Old 10th July 2012, 06:59 PM   #28
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grufti, I use a Linn bearing and I replace their thin oil with Silicon fluid. The increase in speed stability was most pronounced. The Geddon clone + silicon fluid in bearing measures better than either a Lingo or Hercules.
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Old 10th July 2012, 08:58 PM   #29
volken is offline volken  Netherlands
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I have done some vibration measurements on the motor for the Thorens TD160 also a two phase motor with capacitor for the second coil.
First picture is the motor with the original cap.0.15UF and second the tuned cap from 0.22uF .
The voltage about the two coils is not the same a difference from about 20 Volt in a next measurement I tune this with a resistor .

Volken
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Old 10th July 2012, 09:49 PM   #30
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Quote:
Originally Posted by volken View Post
I have done some vibration measurements on the motor for the Thorens TD160 also a two phase motor with capacitor for the second coil.
First picture is the motor with the original cap.0.15UF and second the tuned cap from 0.22uF .
The voltage about the two coils is not the same a difference from about 20 Volt in a next measurement I tune this with a resistor .

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How did you measured the vibrations?
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