Turntable speed analysis part II
- Thread starter flavio81
- Start date
John Channing
fruit box forever
Reducing cogging torque appears to be quite an active research topic - a google search for any of:
brings back a number of papers.
As available computing power has increased it has become easier to test ideas with finite element analysis, certainly things have improved a lot since the 70/80s when most turntables were designed. Replacing the standard Airpax/Premotec motor with something that exploits some of the recent technical advances therefore would look like a good solution rather than trying to work round the problem.
- Skewing stator stack or magnets
- Using fractional slots per pole
- Modulating drive current waveform
- Optimizing the magnet pole arc or width
brings back a number of papers.
As available computing power has increased it has become easier to test ideas with finite element analysis, certainly things have improved a lot since the 70/80s when most turntables were designed. Replacing the standard Airpax/Premotec motor with something that exploits some of the recent technical advances therefore would look like a good solution rather than trying to work round the problem.
John Channing
fruit box forever
Still looking...
sq225917
Bit of this, bit of that
Jason, if I was you I'd expect a temp change with use of exactly SFA with a Linn bearing, maybe a fraction of a degree. The liquid used is stable over a hundred degree working range, so temp is no issue.
@JC Yeh, but sadly nothing has been made for under a couple of thousand for motor and controller that beats the humble Premotec with a bit of load applied. So all the development is for naught if it cannot be utilised due to cost and scarcity.
The 4 points you mention above have been known about for decades, the Lenco motors were all shaded pole with skewed stators.
@JC Yeh, but sadly nothing has been made for under a couple of thousand for motor and controller that beats the humble Premotec with a bit of load applied. So all the development is for naught if it cannot be utilised due to cost and scarcity.
The 4 points you mention above have been known about for decades, the Lenco motors were all shaded pole with skewed stators.
YNWOAN
100% Analogue
I've experimented with the Maxon motor and the very latest in load compensated DC motors, they all replace one issue with another (or more).
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It's true that motor technology has developed, but it has been largely developed with an eye to producing motors with different requirements than those of a TT.
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The Thingap motors look intersting though.
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It's true that motor technology has developed, but it has been largely developed with an eye to producing motors with different requirements than those of a TT.
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The Thingap motors look intersting though.
John Channing
fruit box forever
Of course you can spend a couple of grand on the Linn Radikal solution, but I cannot see any reason why an AC synchronous motor driven by a simple power supply should not be as good - if the motor design is correctly optimised. Obviously, no one is designing motors specifically for turntables any more but there is clearly still a lot of interest in minimising cogging for other applications, so there may be a better affordable solution out there. Specifically, I would be interested to see the torque cogging/ripple curve for the Premotec motor. If torque varies by >20% over a 360 degree rotation, it should be possible to find (a lot) better. If you can measure that torque ripple decreases as a function of load, even better. I don't think this data has been published anywhere.
Well, electromagnetic theory was essentially complete in about 1880, but that does not mean we knew everything about applying it. ;-)
sq225917
Bit of this, bit of that
Experience says the torque doesn't vary like anything like that on the Premotec, you can read the torque curve direct from one of the samples. See those twelve bumps every rev, there's your torque variation, repeated at each pole crossing. I figure it's in the low single digits %.
Now look again at mine with the added drag, see how there's next to no apparent variance any more- there's your published data. The speed data is the torque data, give or take belt compliance.
Just swamp the inherent torque variance with added load and the job is done.
Now look again at mine with the added drag, see how there's next to no apparent variance any more- there's your published data. The speed data is the torque data, give or take belt compliance.
Just swamp the inherent torque variance with added load and the job is done.
martin clark
pinko bodger
= sub-critical damping.
Just pick your target Q and there will be several ways to approach it.
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Just pick your target Q and there will be several ways to approach it.
No; fortunately it's dead linear, else generators would not work as they do. Fleming's right-hand rule etc.
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SQ, what oil would you recommend? I would like to see if this makes a difference to the PL-71..
YNWOAN
100% Analogue
OK, I will do some reading.
I wonder how strong the magnetic field will have to be to achieve the level of drag I envisage as appropriate.
Paul R
pfm Member
John C's LP12,
The centre frequency seems to be 3166Hz, so the turntable is running a little fast. An ideal candidate for a little added drag...
The spectrum between 1 and 20Hz (the eccentricity was dominating at 0.55),
And, for future reference, the spectrum between 45 and 55Hz, this might relate to the intrinsic cogging, there's a smaller spike at 100Hz.
The polar,
If John could make another recording using the Lingo's 45rpm function then we can do an interesting comparison to see if the 50Hz spike really is drive frequency cogging.
Paul
The centre frequency seems to be 3166Hz, so the turntable is running a little fast. An ideal candidate for a little added drag...
The spectrum between 1 and 20Hz (the eccentricity was dominating at 0.55),
And, for future reference, the spectrum between 45 and 55Hz, this might relate to the intrinsic cogging, there's a smaller spike at 100Hz.
The polar,
If John could make another recording using the Lingo's 45rpm function then we can do an interesting comparison to see if the 50Hz spike really is drive frequency cogging.
Paul
sq225917
Bit of this, bit of that
That's a decent reading for a standard (motor wise) LP12
A word of warning re increasing drag, the Hercules didn't like it at all, it made speed stability worse, I guess it's not up to the task from a current delivery POV. Certainly with a Geddon clone it's none issue. I'd investigate cautiously with the Lingo.
A word of warning re increasing drag, the Hercules didn't like it at all, it made speed stability worse, I guess it's not up to the task from a current delivery POV. Certainly with a Geddon clone it's none issue. I'd investigate cautiously with the Lingo.