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 don’t 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
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 don’t 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
Pjotr said:
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.
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.
Cheers
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.
Cheers
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.
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.
sreten said:
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.
True, and for that you need a control loop with a speed sensor and then the idea is not so bad
Cheers
Pjotr said:
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.
Has anyone considered using feedback to operate an eddy current brake on the motor or platter. The Garrard 301/401 uses a permanent magnet who’s 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
Stuart
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.
Si.
Hi harwoodspark,
I think the motor you have is the 120-18 series as shown in my attachment. I have one branded premotec which I bought from McLennan Servos who supplied the same unit to Origin Live for their early conversion Kits. The idea was first published in an article in a HI-FI World magazine DIY supplement in whch I think a Rega deck was converted. I find this motor rather noisey and slightly under powered. That's why I am going to buy a better quality Maxon.
Matt
I think the motor you have is the 120-18 series as shown in my attachment. I have one branded premotec which I bought from McLennan Servos who supplied the same unit to Origin Live for their early conversion Kits. The idea was first published in an article in a HI-FI World magazine DIY supplement in whch I think a Rega deck was converted. I find this motor rather noisey and slightly under powered. That's why I am going to buy a better quality Maxon.
Matt
Hi,
Did look a bit deeper in it. When driving the motor with voltage source, the motor resistance works as a damper for the belt-motor inertia resonance. However with a real motor the motor resistance is of such a value that the motor is driven from a current source anyway concerning the dynamic behaviour. From that point there is not much difference between driving the motor from a voltage source or driving the motor from a current source.
To make this damping work effectively you need a much smaller resistance for the motor winding. For a 12V – 3W Premotec motor (120-18105) it should be aprox 1-2 ohms. With a lower than that motor resistance then motor damping becomes inactive again, but now for the belt-platter inertia resonance.
Cheers
Did look a bit deeper in it. When driving the motor with voltage source, the motor resistance works as a damper for the belt-motor inertia resonance. However with a real motor the motor resistance is of such a value that the motor is driven from a current source anyway concerning the dynamic behaviour. From that point there is not much difference between driving the motor from a voltage source or driving the motor from a current source.
To make this damping work effectively you need a much smaller resistance for the motor winding. For a 12V – 3W Premotec motor (120-18105) it should be aprox 1-2 ohms. With a lower than that motor resistance then motor damping becomes inactive again, but now for the belt-platter inertia resonance.
Cheers
For those interested in, attached the basic electro-mechanical model of a DC motor. The motor parameters are for a Premotec 120-18105 and were taken from the datasheet from www.premotec.nl
Cheers
Cheers
OL dc motor.
Hello Matt. Whilst I am still using the early Premotec motor (OL now supply a later unit, supposed to be quieter.I think it's the CL29/3) I've never really been troubled by noise.I admit it does make a sort of rustling noise if you get up real close but at normal distances it's ok.I only bought the OL kit because it was easy. They weren't as expensive then either! I wouldn't consider buying a complete kit again, maybe just the motor. The thing about the OL supplied motor is that it comes with the correct pulley. What will you do for your pulley? Can you buy or will you have to make? Also do you know the cost of the motor direct from Premotec? I'm making my next tt using a Linn subchassis and the platter, hub and bearing from and old Mission tt.I'll post some pics shortly. Si.
Hello Matt. Whilst I am still using the early Premotec motor (OL now supply a later unit, supposed to be quieter.I think it's the CL29/3) I've never really been troubled by noise.I admit it does make a sort of rustling noise if you get up real close but at normal distances it's ok.I only bought the OL kit because it was easy. They weren't as expensive then either! I wouldn't consider buying a complete kit again, maybe just the motor. The thing about the OL supplied motor is that it comes with the correct pulley. What will you do for your pulley? Can you buy or will you have to make? Also do you know the cost of the motor direct from Premotec? I'm making my next tt using a Linn subchassis and the platter, hub and bearing from and old Mission tt.I'll post some pics shortly. Si.
Hi Harwoodspark
I purchased my premotec motor from Mclennan Servo Supplies.
www.mclennan.co.uk it was about £30-40.
I am about to buy an REMax226756 maxon motor rated at 15watts, 36volts, quite pricey @ £97 inc. As for a pulley I have one about 10mm dia from an old deck, but its got a screw fixing so will need to be upgraded to a push fit, close tolerance type, have'nt found anywhere to get one yet. Alternatively I could revert to the Linn 20mm pulley with the motor running at about 1.4v.
Regards
Matt
I purchased my premotec motor from Mclennan Servo Supplies.
www.mclennan.co.uk it was about £30-40.
I am about to buy an REMax226756 maxon motor rated at 15watts, 36volts, quite pricey @ £97 inc. As for a pulley I have one about 10mm dia from an old deck, but its got a screw fixing so will need to be upgraded to a push fit, close tolerance type, have'nt found anywhere to get one yet. Alternatively I could revert to the Linn 20mm pulley with the motor running at about 1.4v.
Regards
Matt
Hi Matt, I think the pulley is going to be the hard part.It really does need to be accurate.That's why I bought the O/L motor because the pulley was already fitted. It definitely does need to be a good push fit onto the motor spindle.Half a thou out here is too much, so a screw fitting is out of the question. I don't know if it's such a good idea dropping the motor's voltage down so low so you can use the Linn pulley either.If the Linn pulley fits the spindle it would be better to machine it smaller so the motor can run faster. Are you modding a Linn turntable or something else?
Si.
Si.
Si is quite right here with all of these comments, and see again my post #12.
Once any pulley is firmly fixed on a shaft, it is possible to 'true up' its driving surface by the means I suggested, but this will be very hard work if it runs more than a thou. or two out.
If the shaft/spindle fit is already quite good, try Loctiting the parts together (instead of using the grub-screw) as this will not tend to offset the pulley, in my experience, as the layer of Loctite will try to form evenly around the annulus between the shaft and the pulley.
If using DC motors (where speed corrections can be made electronically), it is still worthwhile 'burnishing' the final arrangement with any pulley 'in situ.' so that the 'bearing surface' on which the belt runs is polished, and is absolutely concentric and true-running. This will always help with the ultimate sound, however well the parts are originally machined or attached to each other, and is something which DIYers can do which is not so practical in production.
Whatever 'tool' is used for this burnishing must be held firmly in a fixed support, so that it only contacts and consequently 'wears away' the high spots on the pulley, of course, until the entire circumference is perfectly polished. Softwood, well soaked in abrasive metal polish (Brasso) works well for this, and the wood can be readily formed to suit the contours of the pulley before commencement.
It won't harm the motor so long as you don't apply too much pressure which would slow the motor down a lot, and it is the only way of ensuring an absolutely true-running surface, as no parts are ever 'perfect' when made, and they always have some tolerance even though it may be very small. Not much pressure is needed here, anyway, but the wooden tool must be supported firmly, perhaps in a small vice or locking pliers to provide some mass to steady the job.
Although DC motors' speeds can be 'adjusted' by varying the voltage, in my experience it is best to keep to somewhere reasonably close to their nominal voltage for several reasons. If run too fast (higher V) too much heat can ensue, which could cause damage to the motor, and if run too slow (reduced V), the motor's torque will suffer. In the latter case, this is bad for speed stability, especially with highly modulated passages on records where the attendant stylus drag can be a problem, but you may also find that the TT will not 'self-start' at all, due to this much reduced torque!
I don't know anything about Premotec nominal voltages, but if it is near to the more usual 9-12 volt range, I am sure you will have problems running at the suggested 1.4v, unfortunately.
Once any pulley is firmly fixed on a shaft, it is possible to 'true up' its driving surface by the means I suggested, but this will be very hard work if it runs more than a thou. or two out.
If the shaft/spindle fit is already quite good, try Loctiting the parts together (instead of using the grub-screw) as this will not tend to offset the pulley, in my experience, as the layer of Loctite will try to form evenly around the annulus between the shaft and the pulley.
If using DC motors (where speed corrections can be made electronically), it is still worthwhile 'burnishing' the final arrangement with any pulley 'in situ.' so that the 'bearing surface' on which the belt runs is polished, and is absolutely concentric and true-running. This will always help with the ultimate sound, however well the parts are originally machined or attached to each other, and is something which DIYers can do which is not so practical in production.
Whatever 'tool' is used for this burnishing must be held firmly in a fixed support, so that it only contacts and consequently 'wears away' the high spots on the pulley, of course, until the entire circumference is perfectly polished. Softwood, well soaked in abrasive metal polish (Brasso) works well for this, and the wood can be readily formed to suit the contours of the pulley before commencement.
It won't harm the motor so long as you don't apply too much pressure which would slow the motor down a lot, and it is the only way of ensuring an absolutely true-running surface, as no parts are ever 'perfect' when made, and they always have some tolerance even though it may be very small. Not much pressure is needed here, anyway, but the wooden tool must be supported firmly, perhaps in a small vice or locking pliers to provide some mass to steady the job.
Although DC motors' speeds can be 'adjusted' by varying the voltage, in my experience it is best to keep to somewhere reasonably close to their nominal voltage for several reasons. If run too fast (higher V) too much heat can ensue, which could cause damage to the motor, and if run too slow (reduced V), the motor's torque will suffer. In the latter case, this is bad for speed stability, especially with highly modulated passages on records where the attendant stylus drag can be a problem, but you may also find that the TT will not 'self-start' at all, due to this much reduced torque!
I don't know anything about Premotec nominal voltages, but if it is near to the more usual 9-12 volt range, I am sure you will have problems running at the suggested 1.4v, unfortunately.
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