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Old 3rd January 2006, 04:27 PM   #1
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Question dc motor speed controller

Hi folks,

I have read lots of discussion on speed control for TT dc motors. I've decided to try the OPA548 using back EMF sensing as suggested by one contributer. My question is how can I temperature compensate the circuit to avoid long term drift? I could make an "oven" but do not favour this solution. Ideas and circuits welcome!

Matt
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Old 4th January 2006, 04:23 AM   #2
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You can use either NTC or PTC thermisters. They'll give you reasonable correction over small ranges. But there is more than copper coil temperature that requires compensation. You also have to take into consideration bearing lubricant temperature.

As things get hot, I believe the bearing speeds up (less friction) and the motor slows down (higher resistance).

Anyway, you'd have to attach the temp sensor right to the motor.

jh
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Old 4th January 2006, 04:52 AM   #3
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If u go the tachogenerator way like explained in LM2917, u will get better speed control.

Gajanan Phadte
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Old 4th January 2006, 02:54 PM   #4
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Thanks for the suggestions. I'm trying to avoid the tacho route if possible. I understand the principle of using a thermistor but am unsure of the best way to incorporate it into the circuit or how to choose component values to make it operate correctly. I've attached the proposed circuit in hope of suggestions!

Cheers

Matt
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File Type: pdf dc motor speed control 1.pdf (51.9 KB, 704 views)
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Old 4th January 2006, 03:23 PM   #5
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Take a look at a circuit by Rod Elliot ...

Versatile DC Motor PWM Speed Controller

Cheers
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Old 4th January 2006, 03:38 PM   #6
sreten is offline sreten  United Kingdom
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Quote:
Originally posted by argofanatic
Take a look at a circuit by Rod Elliot ...
Versatile DC Motor PWM Speed Controller
Cheers
This circuit belongs nowhere near a turntable, /sreten.
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Old 4th January 2006, 04:11 PM   #7
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LMAO

So that's what TT stands for.
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Old 4th January 2006, 04:24 PM   #8
Bobken is offline Bobken  United Kingdom
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Default TT P.S.

Hi Matt,

I tried that particular circuit out a couple of years ago using the recommended OPA547 (low current version) with a Michell DC motor on a Gyrodec.

Regrettably, after many months of extremely determined experimentation and using the highest quality (very low tempco)components available, I was forced to abandon the idea as the speed was just not stable enough.

It was very disappointing, but using an extremely accurate (and expensive!) LED-based independant tachometer, it showed that the speed was continually 'hunting', and this was the cause of the audible problem. It was clearly a low level 'cyclical' problem, which appears to be inherent in this circuit design, and the author does accept that it is not up to the best tacho designs in its operation. Possibly, for larger and less sensitive motors in less critical situations, it may well be a good design, though.

Over the years I have built 4 different supplies, some for AC and this one for DC motors, and a really good-sounding TT supply is more critical than perhaps one might imagine. In my set-up, doing without the feedback arrangement, which was provided by this chip, improved the sound considerably, and I don't have any problems with speed stability now, without any tachometer feedback or temp. compensation for the motor windings.

Interestingly, I spoke to Michell at that time, and it was more or less admitted that their own tacho supply was not considered a noticeable improvement over their similar non-tacho version, but for marketing reasons, they produce the far more expensive tacho version.

I hope this helps.
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Bob
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Old 4th January 2006, 04:56 PM   #9
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Quote:
I've attached the proposed circuit
You don't need such a fancy circuit unless you need to vary speed over a wide range. If you're just going to use 33 & 45 rpm, then there is no need to worry about back EMF compensation. The circuit you present is to improve the linearity relationship between voltage and speed. You're only working at one or two speeds.

Best to remove the complex feedback paths. Try just a straight linear regulator.

jh
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Old 4th January 2006, 06:21 PM   #10
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Good information folks!

I have tried the no feedback straight voltage reg option with a premotec motor (I think the original type Origin Live used). I have found it not speed stable and possibly a bit under powered (as well as mechanicaly noisey).
Listening to the experience of everyone else, going dc seems quite a tricky route. Feedback systems are inherently fraught with timing issues- overshoot, hunting etc. Without feedback there are issues with temperature and setting the speed accurately given the platter intertia.
Origin Live use back EMF compensation on their top PSU and it seems to work according to reports! Teres use a sophisticated "soft" tacho sytem, also supposedly good. The Teres is a bit complex to build and tweek for my liking. I would be interested in seeing the Origin Live circuit. In the meantime I am going to buy a better motor to experiment with, probably the Maxon 110189.

Cheers for now

Matt

PS anyone know where to get pulleys accurately machined?
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