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Old 17th December 2005, 06:32 PM   #1
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Default AC motor frustration.....Help!

I am unfortunately an electronic moron and am having a lot of frustration trying to get a decent AC motor to power my recent turntable project. Prior to building the table I spent a lot of time researching different AC motor manufacturers. I managed to get a sample motor from a Saia-Burgess rep. locally but it is basically a toy motor, the motor shaft is 2mm and 5mm long.
I machined a pulley for it and put a motor pod together but it wouldn't hold speed. BTW, he gave me a 12V AC motor, I asked for a 120V so I had to use a transformer to make it work. Could that be the reason for the drop in speed?
Anyway, next I bought a Hurst motor SB-4005-001 which was a much more robust motor but it was very noisy, however when I used it with my table, the performance was quite good sonically.
After a few hours of operation the motor shaft started vibrating really badly so I had to return it. There was simply too much clearance between the shaft and upper bearing sleeve.
So back it went and I bought a different Hurst motor, AB-3005-001. This one was very quiet so I put everything together and tested it with my table. Complete bummer! THe sound coming out of my system was completely missing any low level detail, bass was gone and although the noise floor was excellent, the overall sound was boring.
Bottom line is that although I got a quieter motor with less vibration, I end up with boring, uninvolving performance. It holds speed very well BTW.
FWIW, if anyone can help me with the electronics end, I'd trade with my machining capabilities. I own and operate a machine shop so no problem building whatever you need.
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Old 17th December 2005, 07:38 PM   #2
Kees is offline Kees  Netherlands
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Just use this synchronous motor
http://rswww.com/cgi-bin/bv/initSes...tockNo=441-0423 which is a special low cogging motor.

Most people don't understand anything about turntable motors.
What you need a LOW power motor, that can just drive the platter. The torque that is needed to drive the platter shall be large in comparison to the torque that the motor can supply. This is the only way to ensure low speed variations.
Normal brushless motors CANNOT be used. A good synchronous turntable motor can supply just 1 Watt or so.

I trust this helps.
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Old 17th December 2005, 08:07 PM   #3
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Quote:
Originally posted by Kees
Just use this synchronous motor
http://rswww.com/cgi-bin/bv/initSes...tockNo=441-0423 which is a special low cogging motor.

Most people don't understand anything about turntable motors.
What you need a LOW power motor, that can just drive the platter. The torque that is needed to drive the platter shall be large in comparison to the torque that the motor can supply. This is the only way to ensure low speed variations.
Normal brushless motors CANNOT be used. A good synchronous turntable motor can supply just 1 Watt or so.

I trust this helps.
Thank you for your response. I understand the low power issue but I am confused with the torque necessary to keep the platter spinning without variation.
BTW, that Premotec motor looks like a stock Rega motor.
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Old 17th December 2005, 09:19 PM   #4
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I get a time out error useing the link but I am presuming it is a Premotec 31813. This is indeed the motor that Rega use in the latest P3, also the Linn LP12. I think it is a great motor for what it costs and is well proven and reliable with plenty of simple DIY power supplies available.

I think the best thing about it is that it is probably one of the few decent quality motors designed specificaly for turntables that can be bought over the counter, many other manufactures seem to specify OEM versions of stock products (lets not forget what a small market this is for motor manufactures).

For my Idler Drive project I would prefer a more powerfull motor and have been looking at This One , note it is available with a longer 4mm shaft which together with the larger bearing area gives it twice the axial load of the SY31 series. Of course it might run like a bag of nails, no way of telling without trying one although I am going to email Premotec in the new year and ask there advice about the matter.
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Old 17th December 2005, 09:28 PM   #5
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Quote:
Originally posted by Kees

What you need a LOW power motor, that can just drive the platter. The torque that is needed to drive the platter shall be large in comparison to the torque that the motor can supply.

So many ways to crack a nut!
Nottingham Analogue use a very low powered motor on there heavy platter decks, so low in fact that they cannot start the platter by itself. You give it a push start and then the motor just keeps the platters inertia "toped up"

DPS on the other hand use a lightweight platter with a "stiff" bearing and use a high tourque motor to keep the speed stable.

The NA method is the cassic KISS approach, but if were DIY'ing we dont always want to follow the sheep do we!

Note that the Linn Lingo power supply starts the motor with 110 volts and then drops this to around 85 once the platter is up to speed. The Valhalla PS does the same I think. Reducing voltage on a AC synchronous motor reduces torque and hence vibration.
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Old 18th December 2005, 05:10 AM   #6
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Quote:
Originally posted by Paul Dimaline



The Valhalla PS does the same I think.

It does not.
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Old 19th December 2005, 06:02 PM   #7
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Default Variac option

Quote:
Originally posted by Paul Dimaline
I get a time out error useing the link but I am presuming it is a Premotec 31813. This is indeed the motor that Rega use in the latest P3, also the Linn LP12. I think it is a great motor for what it costs and is well proven and reliable with plenty of simple DIY power supplies available.
I borrowed a variac over the weekend to see if I could affect the performance of the motor in a positive way but I failed to hear any noticable change at all.
My P3 Premotec motor suffers from speed variation, noticable during sustained piano notes. I'd rather avoid the Premotec motor but it may be my only option if I can't find anything else.
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Old 27th December 2005, 05:43 PM   #8
tubenut is offline tubenut  South Africa
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Quote:
The Valhalla PS does the same I think.

The supply on the Linn Axis drops voltage once it achieves motor lock. It is supposed to be superior to the Valhalla....

For DIY I think a geddon clone is easiest and as many still prefer the real geddon over the Lingo, ie it still is in the league, I would build with confidence.....

You could have a toggle switch that initially bypasses the voltage dropper resistor on the geddon for start up, or more elegantly, have a 110 and 85V winding on your geddon clone.

Regards
Guillaume
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Old 27th December 2005, 06:14 PM   #9
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Hi,

Dropping the voltage can be tricky on synchronous motors. The cogging then can become into play more with some and it can become noisier than at nominal voltage.

I am thinking of using a 2 phase small ac a-synchronous motor. These run very smooth with a constant speed (no cogging at all). These have also a high starting torque. But then there is a need for a pulse encoder on the shaft to control the speed. The drive electronics then put just enough energy in it to keep it spinning at the correct speed.

Cheers
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Old 23rd April 2007, 05:31 PM   #10
tubee is offline tubee  Netherlands
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Sorry for reviving this old thread.

I am busy to build a simple experimental TT. Not spring suspended as my previous Direct Drive TT (what a mess, DD) but a mass TT from MDF, bearing and platter from an old Swiss Lenco TT (this Lenco
had roller and AC motor)

The AC Lenco motor is big and could be used, but has a long axle with different conical diameters to alter speed via moving wheel along the cone. Now i want a belt drive system, and want a relative small motor.

For simplicity i will use a small AC motor from a very old 6-ties philips TT with cristal cartridge my father in law putted me in the garage to demolisch. This small AC motor runs very smooth, only in the wrong direction (had also wheel transmisson to platter)

I managed to remove the spidle and core of the motor, and mount it in the opposite way, now it runs clockwise!
But,not silent anymore, after a drop of new oil silent again.

Will lathe a bronze pulley for 45 and 33.3 cycles. I guess the motor runs 28** cycles/minute (@50Hz) The motor will be suspended in diy rubbers and placed in a separate housing next to the TT.

I think i will use a elastic cord for sewing for the belt, or order a 1X1 mm rubber belt. It will drive the outside of Zamac Lenco platter.

The arm: my diy arm, modded with carbon fibre tube,
Or a experimental "cantus" linear tracking arm with ball bearings from an old HD and on a hardened steel spindle from an old printer/scanner.
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