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Old 16th February 2006, 11:14 AM   #101
Bobken is offline Bobken  United Kingdom
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Hi Pjotr,

Overnight I have been thinking some more about your question in post #91, about how much drag is caused by the stylus.

Regrettably, it is close to 50yrs since I was at school, and I have forgotten how to do calculus, for example in relation to calculating the acceleration due to gravity, which is a constantly changing situation, of course.

However, you seem to be quite clued up on theory and calculations, judging by your posts, so maybe the following thought might be possible.

If one could either time or perhaps count the number of revolutions for the TT to come to rest from normal speed, both with the stylus on the record, and without, this should indicate (in some way) how much drag is being caused by the stylus.

My problem here is that as the TT slows down, the stylus drag will decrease proportionately, and this is where I think some calculus might be needed, effectively to 'average out' or 'normalise' the results.

Then with your knowledge of the weight/inertia of the TT etc., could you then do this calculation to determine the overall stylus drag, perhaps?

Clearly in both cases (i.e. also with my earlier crude manual assessment methodology) it would be wise to make several 'passes' when carrying out any measuring/timing/counting revs., or whatever, and then averaging the results.
However, it would appear to be rather quicker and less fussy or subject to error, if one could simply calculate the required results, as opposed to many attempts at pulling strings whilst watching balances and so on.

Regards,
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Old 16th February 2006, 11:59 AM   #102
dice45 is offline dice45  Germany
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Default maxon REmax speed problems

Boris,

i have the same maxon motor as you. No longterm experience yet as i postponed the replacement of my TT's Pabst GS38/09.
But i did some preliminary experiments with it and i had no speed variation problems at all. Rock-solid stable speed with a standard LM317 circuit as reg, with leader tape as belt and a preliminary pulley with radially oriented set screw. Not even a collet chuck pulley.
My buddy Manfred (who designed the Teres motor controller unit und uses maxon since then) also has an REmax29 in use. Had he had problems, i would have gotten a warning from him.

I see 3 possible causes for your problem.

1)
The pulley has to run smoothly and without the maximum achievable lack of excentricity. Make sure that is the case.

Too much excentricity is not only bad for speed stability in itself, it also causes an oscillating radial load aging the shaft bearing prematurely (that word is a gross understatement )

2)
Your reg is performing some unwanted antics. Tame the thing.

3)
The maxon cannot stand excessive radial loads on the shaft. According to the data sheet the load maximum is 5.5 Newton for the slide bearing and 5.0 for the ball bearing. I presume you had chosen the slide bearing variant.
Now I remember that Manfred reported speed problems with one sample of the Teres maxon motor like you describe it above and he told me it turned out that the shaft bearings were gone. This also is a problems well-known to maxon; their experts warn from excessive radial loading.

So if the motor got speed problems, it is very likely that its shaft bearings are gone (one cause for that could be that the shaft is slightly bent due to too much axial forces when press-fitting the pulley). In this case, order a new sample.

Fitting of the pulley:
the pulley's center hole should slide smoothly and without radial play over the motor shaft. Use either glue or a set screw or a collet chuck mechanism to clamp the pulley on the shaft.
DON'T use axial press-fitting. You may use radial press-fitting (heat up the pulley, slide it over the shaft, let it cool.)
But: you will never get the pulley off the shaft undamaged.
As the pulley i am talking of is a precision component, hence expensive, and as pulley outlive approx. 5 maxon motrs, i do not recommend heat-fitting.

If you glue the pulley, make sure the glue joint can be destroyed at a temperature not yet damaging the pulley. Epoxy e.g. should give up at 160°C. So if you wnat to recycle your precision pulley later, heat it up, unmount it from the motor shaft and keep it at the temperature until you wiped the glue out of the the center hole. Then you can mount the pulley on another motor shaft.

My premium option would be a pulley with a collet chuck inside clamping it on the shaft.
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Bernhard
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Old 16th February 2006, 12:48 PM   #103
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Hi Bernhard,
actually I bought this motor-type, after Manfred Huber made some recommendations in the vinyl-lebt-newsgroup some years ago. And I got also the pulley from am member of this group.
Since the pulley is highly precise manufactured and I tested out a few different voltage-regulators, I can rule out the first two points of you.
The motor has slide bearings. When I ordered it, the sales-man of maxon warned me not to use slide-bearing with this low revolution speed and maybe this is why they put a prototype-sticker on the motor…
To avoid too much slipping of the tape, I gave it a more or less high tension. This leads to you third point: I can feel a slight radial clearance of the bearing. So it seems that the bearings are defective.
But I’m not disposed to buy the same type again. The lifespan is to short for an inexpensive motor like this one…
I wonder if anyone ever compared two similar motors whereas one has ball-bearing and the other one has slide-bearing. Is there really an audible difference??

Regards, Boris
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Old 16th February 2006, 05:15 PM   #104
pixpop is offline pixpop  United States
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Hi Boris.

It's very simple to check for record slip. Just put a small piece of tape on the edge of the record, and another right next to it on the platter. Play the record a couple of times, and the two pieces of tape should still be exactly aligned. I think it's worth doing this test, just to be sure. It would be silly to spend all this energy on the motor if the problem was slippage. It's always worth re-checking your previous assumptions.

You might also have slippage between the belt and the platter, or the belt and the pulley. I don't know how to test for this, but there's probably a simple way.

Quote:
Also I started looking for a small and stable frequency-generator circuit, which can drive the premotec-motor from my old Rega�
I've just acquired a Beuhler motor that uses an LM2817 to control the speed. It's very simple, and controls the speed quite well, though I haven't tried it in my TT as yet. All you need is some means of getting frequency pulses from the platter. This should not be too difficult. It doesn't matter what frequency the pulses are, as the LM2917 converts them to a voltage, and the feedback loop controls this voltage.

Neil
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Old 16th February 2006, 05:17 PM   #105
pixpop is offline pixpop  United States
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Oops.. it should be LM2917.
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Old 17th February 2006, 10:32 AM   #106
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Hi Neil,
although I still cannot believe that the record could slip, I will do this test. I have some slippage between belt and pulley during the start-up, so probably the slippage happens rather there, than between the record and the pulley.
But since Bernhards posting I’m sure that the motor is defective.

Regards, Boris
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Old 17th February 2006, 01:56 PM   #107
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Hi,

Don’t know if this is an issue but a LM317 regulator needs 100 nF ceramic caps close to its input AND output to gnd for stability reasons, holds for most IC regulators. A simple electrolytic cap of around 10uF – 100 uF at the output is not always sufficient and HF oscillations can occur impairing DC stability.

Just another 2 cents
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Old 1st March 2006, 08:46 PM   #108
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Hi folks,

A quick report back on my new motor trials to date.
I have resolved a regulator problem, now the voltage measured at the motor terminals remains constant. However, the speed of the platter consistantly increases slightly (as seen on strobe disc) toward the centre of a record. It seems therefore that the motor is just not load tolerant enough. This may be insufficient torque due to very low motor voltage and rpm (as has been suggested). However, given that the motor has enough torque to start the platter from standing, I would have thought torque was more than adequate (?). I think that instantaneous changes in drag due to groove modulations are smoothed out by the platter intertia and no open or closed loop control system could deal with that anyway, so I think it's a kind of "soft" load compensation system that is required.

If anybody is still interested comments are welcome!
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Old 1st March 2006, 09:50 PM   #109
pixpop is offline pixpop  United States
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It's hard to believe that the motor can see this tiny load variation.

However, this is the kind of variation that a feedback circuit would eliminate.

The original Burr Brown circuit you posted indicated that the voltage would need to change when the load changes, in order to maintain constant speed.

You will have to sense the speed somehow, if you want to compensate. You'll either need to sense the current through the motor, or look at some strobe pulses from the platter, using the 2917 device.

Presumably, what you'll find is that the current reduces when playing the inner tracks, because the load is less.

Have you actually tried the Burr Brown circuit?
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Old 1st March 2006, 10:17 PM   #110
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Quote:
Originally posted by Matt Rowland
Hello again!

Looking again at the maxon motor data (REMax226756), the speed constant is 185rpm/v which gives an operating voltage of 2.88v for 33.33rpm and the torque gradient is 33.1rpm/mNm which gives about 16mNm at this voltage. So the question is, is this sufficient torque to maintain constant speed?

Matt

Quote:
Originally posted by Bobken
Hi Matt,

Now you make it clear that the proposed motor is nominally 36v, quite frankly I would be astounded to learn that it would drive a TT adequately at a voltage somewhere in the region of merely 5% of its nominal rating.

I will certainly be amazed to hear that any TT is powered with a voltage as low as the 1.4 or 1.7 volts you mention, but I am always happy to learn something which is new to me.

Quote:
Originally posted by Pjotr
Hi,


The Maxon Remax motors require very low current at no load which is an indication that the mechanical motor losses are quite low. That 15W Remax motor has plenty torque available to keep the platter spinning at the correct speed I think, even when driven from a simple LM317 adjustable regulator.

Cheers
Pjotr is correct, you need to consider combination of back emf, drive voltage and torque when calculating the speed of the motor. For instance in Matt's calculation, the motor speed at the stated torque would be zero as all the voltage would go to driving the torque current across the winding resistance. I think the trick that you have missed in relation to DC supplies is that they are ideally not constant voltage supplies. The drive output impedance must also be included in the analysis of motor speed: The speed is given by

S = Ks. (V supply – (T load / Kt ) . (Rw + Rd ) )

so with constant voltage supply like a "perfect" regulator the motor has a constant negative speed torque slope given by

Delta S / Delta T load = Ks (Rw + Rd ) / Kt .

For the motor given this is 34.2 rpm / mNm. This is relatively good, but since load torque for a turntable is <B>not</B> constant it will still give problems with speed variation. The torque required from the motor is affected by stylus drag, the torque equivalent of which varies with stylus position. I calculate a speed variation of 1.2% if that motor is used without compensation in a standard belt drive set-up. Anyone who claims stable speed with a simple regulator fed DC motor is not measuring speed properly.

Compensation consists of using a transimpedance amplifier to reduce the output impedance of the power supply to below zero. If the sum of the output impedane of the supply and the motor's winding resistance is made close to zero, the speed torque slope becomes nearly zero so the problem disappears. At a combined drive / motor resistance of 1 ohm the speed torque slope for the motor above is 3.5 rpm /mNm, ten times better, but with a 226764 it can be brought down to 0.3 rpm / mNm at 1 ohm, which is a hundred times better. It is easy to achieve 1 ohm combined impedance, I used a compensation circuit giving less than this in the motor drive referred to by Vinyl Addict. I have just completed the PCB design for the fourth prototype, I will be publishing this in Bas Hornemann's on-line DIY mag.
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