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Old 9th January 2006, 08:48 AM   #21
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Hi,

So far so good. But when you connect a rubber belt to the motor to connect it to the platter things becomes more complicated. Now you are adding a spring (the rubber belt) to the motor and the motor system becomes a higher order system. In the electric model in my previous post you are connecting an inductor (the spring action of the belt) between the capacitor of the motor (its inertia) and another capacitor (the inertia of the platter).

But that holds for any motor driving a TT platter, whether it is a DC motor or a synchronous motor. Such a system needs to be well damped, either mechanical or electrical. Mechanical damping is tricky and difficult and can be clumsy. Electrical damping is much easier to accomplish and to tune, but for that you need a control loop anyway. You donít have that possibility with a simple voltage drive of a DC motor (whether or not with a negative resistance of de voltage drive). The current drive with a speed sensor is the best way to go IMO.

A stiff motor will make the resonance of the belt-platter combo (which is a mass spring system by itself) higher. A weak motor dampens that resonance.

Maybe this is going a bit too far for the moment. The first goal is to keep the noise coming out of the motor itself as small as possible and for that you need a good quality motor in the 1st place.

Cheers
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Old 9th January 2006, 10:15 PM   #22
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Quote:
Originally posted by Pjotr


A stiff motor will make the resonance of the belt-platter combo (which is a mass spring system by itself) higher. A weak motor dampens that resonance.

Maybe this is going a bit too far for the moment. The first goal is to keep the noise coming out of the motor itself as small as possible and for that you need a good quality motor in the 1st place.

Cheers
Hello again Pjotr, the OL motor I have been using is made by Premotec and is reckoned to be a good quality motor.It is quite small, not very powerful and can be made to run reasonably quietly.The later motors are considerably better in the noise department than the earlier units.I used this motor initially because it was easily available, came with a convex pulley already fitted and the electronics were in the kit with it. I have found another small 12v motor in my spares box today.I think it came out of a good quality cassette deck.It also sports a convex pulley.Also quite small and not very powerful.I'll post some details tomorrow as I've left it at work.If a small motor is needed then this may fit the bill.I may try to put some photos on but I'm not vey PC orientated.I feel I've done well to get this far!
I'm off to bed now, Si.
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Old 10th January 2006, 08:42 AM   #23
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Hi Si,

I am drawing an electro-mechanical model of the motor, belt and platter combo and see if I can do some simulations this week to see what it is all about. What Premotec motors are you using exactly (type no, and voltage) and what is the pulley size? Can look up the motor constants then to use.

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Old 10th January 2006, 11:55 AM   #24
sreten is offline sreten  United Kingdom
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Hi all,

I think P is getting his electrical equivalents mixed up.

mass <=> inductance
spring <=> capacitor
force <=> voltage
speed <=> current

So the motors inertia (inductance) is linked by the belt
compliance (capacitor) to the platters inertia (inductance).

And the simplest way to drive a DC motor is voltage drive.
Current drive in a simple form is a very bad idea, as its
ignoring the fundamental principle of the motors operation.

To a first order approximation:
the speed of DC motor is dependent on the applied voltage only, the speed
of the armature gives its back emf and this equals the applied voltage.

/sreten.
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Old 10th January 2006, 12:19 PM   #25
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Quote:
Originally posted by sreten
Hi all,

I think P is getting his electrical equivalents mixed up.

mass <=> inductance
spring <=> capacitor
force <=> voltage
speed <=> current

So the motors inertia (inductance) is linked by the belt
compliance (capacitor) to the platters inertia (inductance).
Hi sreten,

You can model it either way depending on if you consider toque as a voltage or as a current.

A capacitor is an integrator for current with voltage as output.
An inductor is an integrator for voltage with current as output.


Quote:
Current drive in a simple form is a very bad idea
True, and for that you need a control loop with a speed sensor and then the idea is not so bad

Cheers
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Old 10th January 2006, 12:41 PM   #26
sreten is offline sreten  United Kingdom
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Quote:
Originally posted by Pjotr

True, and for that you need a control loop with a speed sensor and then the idea is not so bad
Cheers
Hi Pjotr,

its still not good, using feedback to control a current that
effectively ends up setting the right voltage across the motor.
Its a lot more sensible for the control loop to vary output voltage.

/sreten.
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Old 10th January 2006, 03:19 PM   #27
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Hi Sreten,

Can you explain that more in depth for me? I donít see your point.

Cheers
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Old 10th January 2006, 03:22 PM   #28
405man is offline 405man  Scotland
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Has anyone considered using feedback to operate an eddy current brake on the motor or platter. The Garrard 301/401 uses a permanent magnet whoís position is varied to provide increasing drag to slow down the motor this provides very fine control of the load on the motor by using an electromagnet the braking effect would be proportional to the current and this could be used to control the motor speed as a sort of mechanical shunt regulator. Philips used eddy current brake servos in the N1500 video recorders so precise phase control is possible.

Stuart
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Old 10th January 2006, 07:26 PM   #29
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Hello Pjotr, I'll have to remove the baseboard from my turntable to see which Premotec motor is fitted.I know it's a 12v model and it's the usual unit that OL supply for the LP12. The other motor I have and am thinking of using on my home built turntable is made by MPM Singapore. It appears to have some kind of circuit board built in so may need modifying.I will post some pics shortly.
Si.
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Old 10th January 2006, 07:54 PM   #30
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Hello Si,

It's no problem, I will pick a 12v - 3W or 5W Premotec motor from the catalogue. OL is not very specific about the exact Premotec motor as far as I can see on the website. That simulation is more to see what it is all about.

Cheers
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