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Old 3rd March 2010, 01:08 AM   #1
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Default maxon motors for TT use

Hi Folks,

Various people have asked about this so here is something I wrote for the maxon AU web site. It is by no means an exhaustive list, although the suggested model families are a good choice there are many variations within those families, primarily nominal voltage, which as explained below can prove critical in the selection process. Also $AUD one off prices are given in the pdf..they are for comparison only. Call maxon for an exact quote as the exchange rate varies. I do not claim to be the foremost expert on this topic, I respect others, such as Mark Kelly for one, have done much work on this.

Maxon Brushed motors for Hi-Fi turntables

Much has been written about this subject and I appreciate that others have gone into considerable detail. The purpose of this document is to put together suggested motor choices available from maxon in one place, and give an idea of the relative pricing. Note these are guide prices only as for example, the exchange rate fluctuates. Also note, we have ‘cherry picked’ some likely motors here, but this list is in no way exclusive. Other motors in these series and other series may be suitable. If you are not sure please call maxon.

Before we start we should outline the basic requirements

1) Sufficient power, ie speed x torque
2) Low noise, electrical and mechanical
3) Minimal torque ripple
4) Reasonable cost

For most people, particularly novices, a brushed motor is a good choice. The control electronics are comparatively simple and as they can be linear, low noise and the cost is lower. The wiring is also simpler, just two wires to the motor. The drive system is inherently smooth and quiet as long as we are aware of some basic points.

Motor Selection

For a retrofit situation we often have little control over the mechanics, the motor speed is much as determined by the original design. In the case of TT’s employing synchronous type motors this may be quite low, much less than 1000 rpm.

This requirement is usually at odds with the brushed motors available, which tend to be high revving when run at their design voltage, typically 4000 to 6000rpm.

Fortunately the solution is at hand. By using a large diameter motor (which provides good torque, and more power than is ever likely to be required) we can under run it from a lower voltage than its nominal design voltage. This will reduce the speed whilst torque remains purely proportional to the supply current. From a datasheet perspective this means selecting motors with a low rpm/volt speed constant, line 13 in the maxon data sheet tables.

Taking the A-max 26 110191 as an example, a nominal 48V motor, this has a speed constant of 127 rpm/V. So we can see that in a typical turntable application (~600rpm) the supply voltage will be only approximately 4-5V. This is perfectly fine as long as the motor has sufficient torque. From the datasheet the motor makes 15mNm torque at max current.

As the turntable belt drive multiplies the torque by 20X approximately this is quite substantial. We could go into more details as regards start up times if we know the moments of inertia, frictions etc etc, but unless we are designing for broadcasting applications this is probably not required here. In most real world situations belt slip will limit the torque in any case.

So we need to look at some large diameter brushed motors, large in this context is ~>25mm diameter, with low volt/rpm..

What else is important? We want the lowest noise, mechanical and electrical. This means minimal brush noise and low torque ripple. So the choice is now towards precious metal brushes (PMBs), for minimal drag, noise and best efficiency, a long life capacitor (CLL) module to prevent brush sparking and as many poles as possible for smooth torque delivery.

Some suggestions, in order of cost, lowest first.

A-max 26 series

A-max 26 7W 13 pole 110191 an old favourite and still a good choice

RE-max 29 series

RE-max 29 9W 13 pole 226779 (bit more power has extended shaft)
RE-max 29 15W 13 pole 226761 (more power for heavier platters, faster start etc)

The RE-max 29 and A-max 26 support both DC tacho (as a special) and encoder options.

RE 25 series

RE 25 10W 11 pole 118748 (with extended shaft, more power smaller size, tacho option)
Can have DC Tacho 118909 factory fitted for closed loop control

The RE 25 comes with the option of a factory fitted DC Tacho 118909, and so if required a true ‘closed loop’ control can be closed around the motor and the speed controller. It is also the most powerful motor for its diameter, which may help in some conversions, although it is also the longest. See table for approximate dimensions.

Speed control.

The LSC 30/2 4Q linear controller 250521 could be used for TTs. It supports I x R mode. This technique provides motor speed control by a form of feed forward. A brushed motor can be modelled as a source of voltage (the back EMF) in series with a resistance (the winding and brush resistance) . The voltage is proportion to the motor speed and opposes the supply voltage. The current is proportional to the motor torque. As motor torque and current increases, and the motor slows, the current increase is measured and used to estimate an increase in motor supply voltage in an attempt to stabilise the speed.

As there is no true feedback control, the accuracy of this arrangement is limited. The advantage is that no feedback device is required, so simple, low cost, and there is no NFB loop to stabilise.

The more accurate option is to use a motor with the Tacho option. Coupled with the LSC30/2 or other DC servo amplifier this would give absolute control of the motor speed. However be warned the combination of a high mass turntable, compliant drive belt system and low motor inertia will make the loop more difficult or impossible to compensate.

Further hints and tips

Check the drawing of the motors carefully in the datasheet to get the exact dimensions; those above are a guide only.

Be careful not to damage the motor when fitting the drive pulley. In particular the shaft of the motor must not be subject to excessive axial loads which might damage the bearings. If press fitting chose a motor with an extended ‘through’ shaft, then the press can be applied carefully directly to the shaft.

Never press on the motor body to press fit a pulley.

Details of all motors are on our web site:
www.maxonmotor.com.au
Attached Files
File Type: pdf TTs.xls.pdf (2.8 KB, 337 views)
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Old 6th March 2010, 11:44 PM   #2
3GGG is offline 3GGG  Australia
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Wow, great info. I am building a TT and looking for a motor, all the info needed.
Thanks alot for the thread.
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Old 8th March 2010, 11:37 PM   #3
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Hi, thanks 3GGG

Since I wrote this I have received some feedback regarding the 110191 motor in this application.

Because the motor is mounted vertically, has sleeve bearings...and is run at low speed I understand the top bearing can get worn after about 12 months of continuous use.

The sleeve bearing is chosen for low noise (and low cost), but hydrodynamically the lubrication will not be optimal at low rpm and there is no proper bearing to support the motor shaft in the vertical axis. Therefore I would recommend that the top bearing is a ball bearing. This adds around $30AUD to the cost. The bearing will be slightly more noisy, but the life will be much extended. This is a factory option, as is two ball bearings.

Another alternative would be to use a sintered iron sleeve bearing, which has better wear chacteristics but provide an end cap where the motor shaft rests on a single ball (as in a traditional platter bearing). This now becomes a custom, so incurring some engineering cost, time etc. It is really the sort of mod that might be offered to a volume manufacturer, but should provide the lowest noise. A brushed motor will always be more noisy than an EC motor, so it may not be worth the trouble.
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Old 9th March 2010, 09:09 PM   #4
3GGG is offline 3GGG  Australia
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Thanks Audiopip, i am looking at building a 75mm thick platter and was thinking of using the 226761, would this need the ball bearing upgrade?

thanks
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Old 16th March 2010, 03:00 AM   #5
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Hi 3GGG,

The 226761 has sleeve bearings as standard, so my recommendation would be to go for a ball bearing, at least on the output shaft side. However I personally have no idea how this will effect the mechanical noise level. There will be a different mechanical signature.

Perhaps order a sleeve bearing motor for lowest cost/shorter leadtime prototype use and see how you go. You will get at least 12 months use out of this and probably much much longer.

BTW just so you know maxon do not support sale or return unless the product is obviously faulty and these motors are not able to be retrofitted with a ball bearing as the end caps are different. Thats the official position on bearings anyway!! A keen machinist might not share that view...
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Old 21st March 2010, 09:52 AM   #6
3GGG is offline 3GGG  Australia
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Ok, thanks audio pip.
I was advise against using the ball bearing upgrade from premotec due to increased noise.
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Old 1st April 2010, 01:01 AM   #7
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The bearing noise dominates in an EC brushless DC motor, as it is the only mechanical noise I guess this makes sense. In a brushed motor you obviously have some brush noise, but using the PMBs this should be very low.

So its a bit of a moot point if ball bearings are more noisy than the brushgear.

Unfortunately how the motor is suspended/decoupled will have an effect as will the drive belt and a myriad of other mechanical nasties.

So I would take the low cost option and go for the sleeve bearings, as discussed.

Cheers!
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Old 5th June 2010, 08:27 PM   #8
jev is offline jev  Netherlands
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Thanks Audiopip, one question. When I would like 33 and 45 rpm is that possible with a LSC 30/2? Maybe put a resistor in series for 33 rpm with a switch for 45 rpm?
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Old 6th June 2010, 01:01 AM   #9
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Hi Jev,

The LSC30/2 can be configured to output a motor control voltage which is proportional to a DC input. By adjusting the DC input you can change the speed. So a simple resistive divider chain, from the LSC30/2 reference to ground, with a switch to switch an additional resistor in or out across the bottom resistor, is in theory all that it required. This can all be low powered circuitry. In reality you would make the bottom two reistors trim pots so you can set up the speeds exactly.

However, I personally think the LSC30/2 is probably an overkill for TT use, it has a much larger current capability than required, it is also 4 quadrant, for forward reverse speed/braking control. For a TT you can get similar results using an adjustable 3T regulator, or one of the specialist TT controllers, such as those designed by Mark Kelly.

Unfortunatley I speak only from theory, I do have a TT at the moment, but it is not mine to experiement with!

Best regards
Jon Pippard
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Old 6th June 2010, 11:54 AM   #10
jev is offline jev  Netherlands
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Hi Jon, thanks for your answers! I am not sure what 4 quadrant means (four controllers?) but anyway I think I will build my own supply. There is a very good thread in the pinkfishmedia dot net DIY section about a simple dc motor controller. I think I will build that one in combination with a 226761 motor. I will not choose the ball bearing option, I wil see how it goes...
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