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Old 14th October 2008, 10:23 PM   #101
jkeny is offline jkeny  Ireland
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You guys are the experts here on Technics DD TTs & I have an OT question which I hope you will answer. I've looked at the engineering drawings of the SL1210 but can't figure out how it operates:

I'm guessing here but I figure the spindle has permanent magnets on it somewhere and this is rotated by the armature as per a BLDC? So there is no physical contact between spindle & armature.

Now if the platter & hence spindle was lifted off it's bearing (using a magnetic levitation), would the whole arrangement be stable & the spindle stay centered within the armature?
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Old 14th October 2008, 10:25 PM   #102
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Good call, though I have always used FF, even when it was called Firebird. Anyone not yet using it ought to heed your advise.
I'm proud to report I have never, ever, had *any* version of internet explorer!

It was just that the server was temporarily offline. It gave me the file today. Comments so far on it....


The mark 3 drive circuit has a voltage reserve of 64V dc pk-to-pk! Thats twice the supply voltage of earlier versions, although the actual drive voltage on the armature coils for normal running is around 3Vrms. That's not much more than the Mk2 drive voltage (which I think was around 1.9V).
So my drive circuit will not achieve the very fast run-up time of the factory Mk3. The L165 op-amps I'm using won't take 64v, so that part would have to be modified. Yet the circuit WILL run the motor if you can live with a slow start. Nothing is impossible, so if you really wanted the 64V drive, 6 extra power transistors and a suitable power supply component modification would double the output drive.
Even though the original SP10-2 drive system operated from 32V (or -16/+16 if you visualise it differently, I am only intending to use -9/+9 or similar. This too will make the run up time slower than originally intended, but the L165s will take -16/+16 if so required.
Lower voltages = lower heat generation = longer & better reliability.

With less drive reserve, the controller cannot compensate as fast for disturbances (torque load), but since the platter is so much heavier on the Mk3, this may not be a problem - small torque perturbations are more readily absorbed by the larger platter inertia.
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Old 14th October 2008, 10:39 PM   #103
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Quote:
Now if the platter & hence spindle was lifted off it's bearing (using a magnetic levitation), would the whole arrangement be stable & the spindle stay centered within the armature?
I think it will not work; the motor produces some degree of unbalanced lateral thrust.

The bronze bushes will have to stay, but if you just want to lift the platter slightly so it does not rest on the thrust plate (a major source of rumble I have read: precession), that will work fine - just dont lift it TOO much. Half a mm or so will not upset the motor.
Teac had a thing called the 'Magnefloat' bearing for their tape deck capstans.

The question is WHERE would such a magnetic assembly be located? Is there space to fit it.
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Old 14th October 2008, 10:54 PM   #104
jkeny is offline jkeny  Ireland
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Thanks Steerpike,
Yes the lateral bronze bushes, I have no problem with but as you say would there be enough space for platter magnetics & the opposing magnetics on the base?

Another question is would these magnets interfere with or be interfered with by the armature electromagnetism or fixed magnets on the base itself?
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Old 14th October 2008, 11:13 PM   #105
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Steerpike,

Since I am neither a DJ or a radio station the startup time/ peak torque is not of prime importance. I look forward to hearing your progress.


Colin
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Old 15th October 2008, 12:56 AM   #106
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Steerpike

The problem is slightly more complex. The ratio of the torque available and the Icm of the platter defines the mechanical slew rate which not only affects the acceleration from start but also the rate of response to any perturbation.

This slew rate thus defines the maximal rate of response of the servo loop in the "forward gain" direction. Standard feedback theory applies so this also sets the rate of response of the return loop. The rate of response of DD motor feedback loops has to be fast enough to overcome the cogging torque produced by pole saliency in the stator. In the Technics design this cogging torque iseems to be usually about 1% of the motor torque which would be very audible if the loop didn't cancel it properly.
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Old 15th October 2008, 09:44 PM   #107
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Quote:
but also the rate of response to any perturbation.
I think I said that:"With less drive reserve, the controller cannot compensate as fast for disturbances (torque load)"

Quote:
In the Technics design this cogging torque iseems to be usually about 1% of the motor torque which would be very audible if the loop didn't cancel it properly.
This is all true when the motor is under significant load, but once the platter is up to speed, the required torque is very low. The normal running drive voltage for the Mk3 is 3V rms. This is enough drive to counteract that cogging torque you mention.
Drag of the stylus spoils things slightly, but there is still a good deal of headroom before you start banging up against the saturation limit of the L165. I have no value for the current/torque spec of the mk3 motor, maybe you have it?

The inertia of the platter damps most of that cogging, to what degree I can't say without doing the maths. But if it were not so, you'd see a *large* correction signal on the motor drive voltages at the pole frequency of the motor, which isn't present.
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Old 15th October 2008, 10:06 PM   #108
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Originally posted by Steerpike



The inertia of the platter damps most of that cogging, to what degree I can't say without doing the maths. But if it were not so, you'd see a *large* correction signal on the motor drive voltages at the pole frequency of the motor, which isn't present.

Actually the drive waveform develops a torque dependent harmonic distortion component, which is present.

The inertia of the platter does not damp anything, there is no energy loss from the system. What the inertia does is slows the rate of response to pertubations (as we have both observed).
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Old 16th October 2008, 07:30 PM   #109
rich121 is offline rich121  United States
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I believe there is already a thread started on magnetic bearings you can post to.. as this one, is about upgrading SP-10 controllers and power supplies...

Thanks...

Rick

Thanks Steerpike,
Yes the lateral bronze bushes, I have no problem with but as you say would there be enough space for platter magnetics & the opposing magnetics on the base?

Another question is would these magnets interfere with or be interfered with by the armature electromagnetism or fixed magnets on the base itself?
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Old 16th October 2008, 07:46 PM   #110
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Quote:
Another question is would these magnets interfere with or be interfered with by the armature electromagnetism or fixed magnets on the base itself?
I think that is entirely dependant on what extra magnets you add, and where. it would be quite hard to upset the normal operation of the motor though, unless you has an EXTEMELY strong outside field.
I suspect the motor rotor would be likely to introduce wobble into a magnetic bearing, it was nearby.

Onto our SP10 controller though.. we need some MATHEMATICS soon, rather than all the speculation of what will or won't work. The Mk2 motor I can take measurements on, but the Mk3, one of its owners will need to do that. Most interesting would be the current/torque figure for that motor. And its moment of inertia.
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