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/download.ashx?Type=PAPER&PCode=c2888847-fd7c-434b-b924-d4ca158f1718
TT Motors
http://www.mclennan.co.uk/datasheets/european/synchronousdata/acsynchronousmotors.pdf
http://umpir.ump.edu.my/27/1/CD3199.pdf
I agree.
I don't understand this term:
Mean you, that a residual noise/hum remains ?
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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.
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?
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.
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.
voltages and things...
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.
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.
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
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
The article you quote was written by me some years ago. I have since learnt more about these motors and a couple of the assertions in the article need to be modified.
To the topic being discussed: in theory exact quadrature and equal phase voltages are ideal.
In practice, the ideal angle and voltage ratio for quietest operation will vary from motor to motor. As stated previously in this thread, the quietest drive I have been able to build had adjustable angle, phase voltage and third harmonic distortion.
With a B@k accellerometer and a HP FFT Signal Analyzer unfortunately the horizontal span where different so its difficult too see the difference.
Next two current spectrum measurements are from the same motor both tuned with capacitor and resistor to correct the voltage about the two coils.
First on the mains supply 230 Volt the other on a clean powersupply made for idler turntables .
The mains has a lot pollution over here a clean supply gives a large sound improvement for this kind off turntables.
Volken
Next two current spectrum measurements are from the same motor both tuned with capacitor and resistor to correct the voltage about the two coils.
First on the mains supply 230 Volt the other on a clean powersupply made for idler turntables .
The mains has a lot pollution over here a clean supply gives a large sound improvement for this kind off turntables.
Volken
Question to Mark Kelly- does the 3rd harmonic 'flatten' the top/peak of the main wave form? It looked like it should on paper, and an Audacity recording of an Audacity track(60+180HZ at .1 the 60HZ level)seemed to confirm this(the leading edge was higher than the trailing, no idea why, but the peak was flattened). If so, assume this would produce smoother operation.
Third harmonic can be in phase - tending to accentuate the peak of the primary wave, or antiphase - tending to flatten the peak. IIRC most motors responded best to a moderate amount of antiphase 3HD at low applied load.
My take on this is that the antiphase 3HD reduced the drive torque when the rotor was pulling in to the higher reluctance spots in the stator field and increased it as the rotor passed beyond this, thus effectively reducing the largest source of cogging in a synchronous motor (the variable reluctance of the stator field).
I'm currently focsussing on the way this effect changes as the angle between the rotor and the stator field changes.
My take on this is that the antiphase 3HD reduced the drive torque when the rotor was pulling in to the higher reluctance spots in the stator field and increased it as the rotor passed beyond this, thus effectively reducing the largest source of cogging in a synchronous motor (the variable reluctance of the stator field).
I'm currently focsussing on the way this effect changes as the angle between the rotor and the stator field changes.
Thanks-was referring to anti-phase, the 180 negative and 60 positive lining up, and visa versa, if they start from a common zero point. Since pm synch motors operate in what Edgar Villchur called "a series of jerks"( Audio magazine article, Sept&Oct 1962, on the new AR turntable), moderating the "jerks" makes intuitive sense.
Intend to find a program which allows intro of 3rd HD, like Audacity, combined with precise phase control, like Cognaxon(currently using)-or,one that allows a waveform to be 'drawn' to suit. I have two spare cheap Hurst motors I can torment.
I assume these tricks will also help with hysteresis motors(have some ampex motors, Ashlands, to try) though not as dramatically;as far as I can tell, Villchur went with the then new pm motors on account of cost, as an undersized hysteresis motor could actually slow under load (whereas the pm would break synch), so a large, expensive motor would be required. He relied on the belt to filter out the cogging.
Intend to find a program which allows intro of 3rd HD, like Audacity, combined with precise phase control, like Cognaxon(currently using)-or,one that allows a waveform to be 'drawn' to suit. I have two spare cheap Hurst motors I can torment.
I assume these tricks will also help with hysteresis motors(have some ampex motors, Ashlands, to try) though not as dramatically;as far as I can tell, Villchur went with the then new pm motors on account of cost, as an undersized hysteresis motor could actually slow under load (whereas the pm would break synch), so a large, expensive motor would be required. He relied on the belt to filter out the cogging.
Firstly, the "series of jerks" idea is totally misleading. If the stator field was perfectly uniform and the motor operates at its design torque there would be no torque ripple at all - both fields are sinusoidal, the torque is their vector multiple and is thus proportional to sine squared. The other coil is the cosine, sin squared plus cos squared = 1.
No, hysteresis motors are a totally different kettle of fish. No stator reluctance, torque spring effect quite prominent.
Thorens TD124.2 versus Linn LP12 Lingo Speed measurement
interesting comparison:
http://www.diyaudio.com/forums/anal...versus-linn-lp12-lingo-speed-measurement.html
more URLs regarded Lingo from Linn
Linn Forums - Valhalla with electronic 45 rpm?
?????? :: ???? - Linn's PSU Lingo
LINN Lingo - neuer Schalter - Phono - Restaurierung und Selbstbau - Analog-Forum
Lingo fault finding
http://www.diyaudio.com/forums/anal...versus-linn-lp12-lingo-speed-measurement.html
Netzteil Dr. Fuß - Phono - allgemein - Analog-Forum
interesting comparison:
http://www.diyaudio.com/forums/anal...versus-linn-lp12-lingo-speed-measurement.html
more URLs regarded Lingo from Linn
Linn Forums - Valhalla with electronic 45 rpm?
?????? :: ???? - Linn's PSU Lingo
LINN Lingo - neuer Schalter - Phono - Restaurierung und Selbstbau - Analog-Forum
Lingo fault finding
http://www.diyaudio.com/forums/anal...versus-linn-lp12-lingo-speed-measurement.html
Netzteil Dr. Fuß - Phono - allgemein - Analog-Forum