DC vs AC motors - a newbie needs help

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G'day All,

I'm pretty new to the analog world (<6months) and have recently thought of heavily modifying my TT. A new motor would definitely be in the works, but

I did a search and the results went right over my head. I need help with understanding why DC motors are considered better than AC motors. I've got a few questions and have numbered them for ease-of-answering.

1. Doesn't AC have the advantage wrt ease of implementation of quality speed regulation? The Altmann DIY TT's motor setup is a model of simplicity to install.

2. Which motor emanates more spurious RMI/EMI? AC or DC?

3. What do I look out when I buy a DC motor? can anyone recommend any out there?

4. What do I look out when I buy an AC motor? can anyone recommend any out there?

5. Which motor type do most belt drive manufacturers use and why?

6. "All AC motors exhibit cogging, making them inferior to DC motors" - is this statement correct? What advantages do AC motors have then?

7. Weren't AC motors in fashion a few years ago? What precipitated the 'shift' to DC?

8. Which motors are more durable for a given price point? AC or DC?

Any help you could provide is much appreciated. Thanks all in advance! :)
 
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I've had a number of TT some with DC motors and some with AC. Bluntly I prefer ac operated motors whether line operated or using a power weinbridge oscillator like in my Thorens TD-125.

Most of the dc motor tables (dd and belt) I've owned had less than stellar speed stability.

Incidentally at the physical level there is no such thing as a dc motor, the commutation is handled either electronically or with brushes as found in a lot of dc servo motors by Panasonic. (Technics and the notorious Pink Triangle) The Technics TT could at least maintain a more or less constant speed across one side, the PT absolutely could not -even with a new motor and controller.. :headbash:

Also I believe all motors except for the possible exception of some slotless coreless types cog to some degree. I think this more of an issue with DD tables whether ac or dc servo types.
 
Hi,

some observations :

An AC motor needs an accurate pulley, a DC motor does not.
An AC motor needs the right size pulley for 50 / 60 Hz.
An AC motor pulley must match the diameter of what its driving.

Consequently a universal "upgrade" kit cannot use an AC motor.
The idea DC is chosen only because it is superior is marketing BS.

Anything will probably upgrade a tired / worn / unoiled motor.

Also some AC motors have recently gone out of production and
as DC is marketing flavour of the month, switches to DC motors
with inevitable claims of improved performance have been made.

:)/sreten.
 
kevinkr said:

Also I believe all motors except for the possible exception of some slotless coreless types cog to some degree. I think this more of an issue with DD tables whether ac or dc servo types.
There is at least one type of motor totally devoid of cogging: the eddy current motor. And if I remember correctly, Thorens had one TT based on this type.
LV
 
Hi
I have used dc and ac driven turntables.

Only AC give good long term speed control. DC drifts over a period.

However if you use ac and want the best performance you do need to either clean the mains or electronically derive the frequency. ( I currently do the latter )

If you use ac from a "clean" source you will be surprised by the difference that makes over conventional ac power supply. It provides much cleaner notes particularly in the base.

Don
 
While I'm no electrician I will add my 2 cents.

All electric motors offer errors, errors in speed accuracy and as such errors in wow and flutter etc. IMO, A.C synchronous or D.C. servo both can offer long term quality performance for audio playback use. The most important issue is making sure speed drift, speed accuracy and wow and flutter specs are all below the ability for most humans to detect. Once this is reached than anything below such is only guilding the lily. Poorer quality motors that were/are often used on cheap products may fail in reaching levels to where most humans could not detect their error. Better motors will obviously be engineered to surpass min. requirements for audio playback, regardless of A.C. or D.C. motor design.

I have heard turntables with A.C. belt drive, D.C. servo belt drives, and D.C. direct drives. Al offer their strengths and weaknesses. Neither is inherently better but only is better if the motor A.C. or D.C. was better designed and manufactured.

High end turntables use better motors because they pad the cost of development of these into their higher prices. Thankfully R& D costs of these generally filter down to lower priced motors that are used on more affordable turntables. But again the end result is the initial design and manufacture of any motor A.C. or D.C.

My current DIY turntable has a motor derived from an older Kenwood turntable (Sankyo is the motor maker) it is a D.C. servo motor and though far from high end it in my DIY design runs quite smoothly, with no real discernable inaccuracy to my ears or ears of others whom I have played the turntable for. I have other motors picked up from other thrift store tt buys that are D.C servo (Panasonic made) and A.C. synchronous motor (not sure what Japanese maker made it) and I have a Hitachi direct drive turntable from the mid-late 70s which I'm going to DIY into a better DIY direct drive. Each motor design will offer pluses or minuses and being used in future build will add to the sonic flavour of the design.

In the end IMO it's more important with how one uses a motor either A.C. or D.C. in a DIY design and how that DIY table is built. Plinth construction, tonearm, cartridge etc. will all add or detract more from the final sound than whether one uses an A.C. synchronous or a D.C. servo motors. The main point made earlier is that A.C. motors need an accurate pully if you want its speed to be more true. A D.C. servo can get away with an undersized or over sized pully as one just trims it out to get better speed accuracy.

Well that is my 2 cents :cool:
 
adhoc said:


1. Doesn't AC have the advantage wrt ease of implementation of quality speed regulation? The Altmann DIY TT's motor setup is a model of simplicity to install.

Yes, for a given accuracy AC is easier.

2. Which motor emanates more spurious RMI/EMI? AC or DC?

AC

3. What do I look out when I buy a DC motor? can anyone recommend any out there?

A low value of the ratio Ks / Kt where Ks is speed constant and Kt is torque constant. Maxon 110191 is my current favourite

4. What do I look out when I buy an AC motor? can anyone recommend any out there?

I'm not much help here unless you want to spend a lot of money on a three phase motor

5. Which motor type do most belt drive manufacturers use and why?

AC. see 1

6. "All AC motors exhibit cogging, making them inferior to DC motors" - is this statement correct? What advantages do AC motors have then?

All affordable AC motors cog. The rest is a matter for judgement. Again see 1

7. Weren't AC motors in fashion a few years ago? What precipitated the 'shift' to DC?

silence

8. Which motors are more durable for a given price point? AC or DC?

AC. DC motors must be considered as consumables due to brush wear. 10,000 hours is a good life.
 
Elvee said:

There is at least one type of motor totally devoid of cogging: the eddy current motor. And if I remember correctly, Thorens had one TT based on this type.
LV

Thank you all for the replies - they are indeed much appreciated. :)

I had had some teeth pulled out under GA recently, and was in no condition to reply - but I'm back now! ;)

I'll summarise my current situation:

- I recently came into possession of an early make LP12 in - for once - awesome condition. All the 'irreplaceable' parts (platter, shaft, bearing, etc) are in incredible condition considering the age of the TT - it has a serial number around 15,000, which places it sometime in the late 1970s.

- This is my first 'serious' TT, and while I understand it's basic design premise is arguably obsolete, I am in the process of modding the hell out of it. I have had wonderful experiences with modifying existing inexpensive DACs to get IMO wonderful performance out of them, and hope to replicate my experiences with this TT.

- I have some ideas so far for modification, and am in the process of putting them into place. Surprisingly, most of these 'modifications' were thought up independently by me, though you may recognise some of them as offerings from commercial entities. Like I said, I am very new to the analog world - a few months ago I would have not known a cartridge from a tonearm! :clown:

1. resonant top plate replaced with carbon fiber composite: stiff, light and from my experience with the material, not resonant. The current steel top plate rings like a bell when struck (surprisingly this is not so for the platters). I later found out that The Funk Firm (TFF) originally came up with this idea.

2. relocation of the motor to 7 o'clock position: to grossly oversimplify, a 'parallel' transfer of belt-borne motor-generated vibrations rather than a 'perpendicular' one - the former should be much more preferable to the latter, given the respective directions of travel and excitement in a stylus. I later found out that the now-defunct Pink Triangle originally came up with this idea.

3. stiffening of subchassis: the subchassis that came with my LP12 is a 'spot welded' version. I picked up a 'scrap' (seller's words) pre-Cirkus glued subchassis, and the former is definitely way more 'flexy' than the latter. I was thinking of bonding with epoxy some CF sheets to my subchassis. I later found out that Cetech, TFF and now Linn have adopted this idea, though implementation differs.

4. replacement of motor: I assumed (correctly) that an AC motor should be an EMI nightmare. I thought about going the DC route but then realised that the issue of 'hunting' and correct servo implementation was a serious setback, and difficult to get right. So I thought (admittedly rather simplistically) - hmm, why not keep a DC motor 'locked' onto a particular speed like how DACs are 'reclocked' with high-precision clocks? - but I lack the technical know-how (my speciality is medicine, not electronics!) and educational background necessary to translate my idea into reality.

So right now I'm torn between the effort needed to either:

A. take a higher-end AC motor, shield it and reduce vibration effectively.
B. take a higher-end DC motor and somehow figure how to implement a good servo.

Any suggestions? :bawling:
 
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Joined 2003
I'm afraid I have to disagree with Mark Kelly about which motor produces the most EMI. DC motors inevitably involve switching inductors off - whether by electronic commutation or mechanical. Switching inductors off always produces HF spikes, which is why you generally see small ceramic capacitors across the terminals of DC motors. Conversely, an AC motor fed from an oscillator has a pure 50Hz or 60Hz sine wave. Agreed, it's at a far greater amplitude than the spikes of DC motors, but because the wiring is so short compared to the wavelength (6000km), it's a very poor aerial and the main concern is capacitive effects. Screened mains lead solves the capacitive problem and is available from the electronics suppliers - I've always used it for turntables. The slight magnetic problem due to the motor current can be solved by twisting the wires together in the manner of valve heater wiring.
 
EC8010 said:
I'm afraid I have to disagree with Mark Kelly about which motor produces the most EMI. DC motors inevitably involve switching inductors off - whether by electronic commutation or mechanical. Switching inductors off always produces HF spikes, which is why you generally see small ceramic capacitors across the terminals of DC motors.

Fair enough point, although it should be aded that the spikes on a motor with PM brushes are pretty small. On the Maxon motors with which I am familiar the spikes are in the millivolts (helped by their internal capacitors which reduce the area of the shunt loop)
 
Hello all,

I'm really sorry to bother, but I had another idea - it's something of a variation of the typical motor-platter type 'mounting' found on all high-mass TT designs, only this time I have adapted it to my LP12.

The Linn plinth and design is supposed to be exceptional at filtering low-frequency vibration - exactly the sort that would emanate from a well made 3-phase AC motor would make. Quality rubber feet would also contribute towards eliminating low-frequency vibration even further.

So instead of mounting my motor to the top plate, why not take it a step further and mount it to the TABLE on which the TT is resting on?

In case the picture below is not clear - the motor enclosure is mounted WITHIN THE PLINTH, with it's base on the TABLE. The pulley would protrude through a hole in the top plate.

By keeping the motor enclosure extremely heavy, we not only damp it, but ensure that it remains stationary. Yet this would allow for easier adjustment of belt tension and positioning than an attached-to-the-top-plate motor. The enclosure will be made from a stainless steel film developing tank (like so) - which would significantly cut down on EMI.

Let me know what you all think.

Cheers,
Aaron

An externally hosted image should be here but it was not working when we last tested it.
 
Did any of you made experiments with 3 phase motors fed with sine waves ?

I found threads with floppy drive motors for example, what I believe is (like HDD motor) a 3 phase motor, BUT fed by switched supply with it's own control circuit. No smooth sine wave. I do not think that helps.

I saw on Mark's website the article about an EC motor (maxon 118890), but I do not see anything about how it sounds and I do not know what type of TT he drives with it (mass of platter etc.).

Mark can you also put some comments about what you found with EC motor ? Why not an EC-max (283860 for example) ?


A little background, but not important for the question above :

I have unpacked my turntable now what I have packed years ago for two reason :

- most of the discs I like are on CDs, I have very few good LPs

(o.k. it sounds I am an alien here, but I'm not that much. 9 years went by until I packed my TT. That time I had a Marchall Leach solid state amp and JPW sonata boxes. Now I enjoy a 2A3 PSE amp and AN E kit or like boxes, I'm more and more hungry now for a good source, but for sure can not pay a Voyd for example. You will lough, but I just listened to some of my old recording to hifi VHS tape, and I like it very much.)

- the motor was terrible noisy, the bearing is just weared out.

After I unpacked it, realized it is really bad. I can not enjoy it really.
Unfortunately I have just missed a group buy of the Mark's controller and motor etc., and I find very hard to get the motor from maxon (
110191). I still try to go that way, but I was wondering what about 3 phase motors or EC DC motors fed by 3 sinus.

Thanks a lot,

JG
 
I was thinking on doing some work on my turntable
its a scheu turntable and i would like to change my coupling method from the string method to idler wheel
like the http://www.teresaudio.com/verus-motor.html


I have a yaskawa motor 100watts DC servo brush type. I was thinking on getting a gecko drive(servo driver) to control the servo motor. 100watts would be good for the idler i think

Would you such a big motor pose any sonic issues?

I was thinking one the problems is the servo controller uses PWM and would cause switching noise. Switch frequency of the PWM is 20khz smack in the audio band

Torque would be rather high as this is an industrial motor.
 
Hi

I use an ac motor to drive a townshend rock turntable. The speed is controlled by an oscillator. It provides the most constant speed control that I have come across. This has translated into a very solid base performance.

I have been using this system for about 5 years and have not had to adjust anything in that time.

Don
 
another option :
have you guys considered to order a DC motor with encoder built on the other end ? The speed control would be very easy. For example the MEnc 13 built on a 110191 maxon gives 16 pulses / turn, to use it as a VCO of a PLL it would be easy with an MC4046 for example.

?

Regards,

JG
 
Hi,

For me this project was sleeping a bit, but a few weeks ago re started.

Recently I have tried several HDD motors, no success. Small stepper motors, no success.
I fed the HDD motors with 3 sinus wave 0-120-240 degrees, steppers with 0-90 degrees.

I thought it will run smooth with sinus wave. It does not. The hdd motors are difficult to spin up and has very small torq, the stepper has small torq and noise. With square wave the HDD motor has good torq, the stepper runs very smooth with a half step driver, almost silent I just hear the ball bearing.

An Airpax fed with 2 sinus, 0-90 degrees, has good torq, silent and has the minimum vibration of all above. - looks like it is designed for the job :)
The only one what I would consider as alternative, is a 3.5" FDD motor. It runs almost as smooth as the airpax.

In the other hand, all tests are just the motor alone, not record playback test.

Any suggestion ?

JG
 
Hello,
I have done an experiment with simple square wave switching with the FDD motor. I'we tried to have similar like what it's own driver makes, with no PWM. The result is surprising. Much less vibration, more torque, better start. Sinus drive seems to be a dead end for me.

What motor does the best TTs use ?

Thanks,

JG
 
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