Thorens 124 motor variable freq. drive - diyAudio
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Old 15th April 2014, 08:03 PM   #1
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Default Thorens 124 motor variable freq. drive

I'd like to build a power supply with variable frequency to drive the motor in my thorens 124. This would replace the halls effect speed control in the original design. I don't know how to do this, and would like to work with others to develop the project and improve the sound floor & general operation of this legendary turntable.
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Old 15th April 2014, 08:19 PM   #2
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But it has a synchronous motor and 50 or 60HZ mains is as accurate as you can get!
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Old 16th April 2014, 12:53 AM   #3
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I believe the pitch control on the original is +/- 2%. So an adjustable frequency of 2Hz either side of 50 or 60 hertz should be a good target.
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Old 16th April 2014, 02:14 AM   #4
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Actually it is neither synchronous nor uses hall effect. Check out the thread in analogue source on the TD-124.
The speed control mechanism is the design's real achilles heal, it can be very difficult to get the table to both run on speed and do so quietly.

Restoring and Improving A Thorens TD-124 MKII

I will move this thread to the analogue source forum as it is more relevant there...
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Old 16th April 2014, 02:39 AM   #5
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The TD-124 uses a Thorens E50 which is a shaded pole induction motor, it is sensitive to both line voltage and load in addition to being partially locked to the line frequency. Normal operation is pole slipping so that the motor actually runs a little slower than a synchronous motor and the eddy current brake located in the intermediate (stepped) pulley provides a variable load on the motor - properly adjusted (frankly hopeless) the eddy current brake provides a speed adjustment range of +/-3% according to Thorens.

There is a later replacement motor, the Papst Aussenlaufer which was manufactured in 1977 for replacement of worn out E50 motors once Thorens ran out of replacement stock, this is actually a 12 pole delta connected 3 phase motor. Normally run using the steinmetz (capacitor phase shift) connection on single phase power its performance is generally considered inferior to the original E50 dues to noise issues and limited torque.

Where does VFD come into the discussion? I am currently putting together a Mark Kelly VFD thanks to the generosity of a friend here who gave me his unbuilt board set, in addition I have on hand a Papst Aussenlaufer which I will install in one of my two TD-124 once I get the drive finished.

I have a MKII which given half a chance runs too fast, I now run it off of a small bucking transformer that reduces the supply voltage to either 110V or 105V to reduce the motor torque sufficiently to keep it from completely overcoming the eddy current brake.

Older E50 motors produce very slightly less torque IMLE. (I have a MKI as well which runs directly on my nominally 117V mains.)
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Old 16th April 2014, 02:45 AM   #6
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Quote:
Originally Posted by anystereo View Post
I believe the pitch control on the original is +/- 2%. So an adjustable frequency of 2Hz either side of 50 or 60 hertz should be a good target.
+/-2Hz is a reasonable compromise, and since all of these tables can run on either 50Hz or 60Hz I would figure out if there was some noise advantage to one over the other.

Consider using an inexpensive DDS module from eBay with a binary counter IC and single D type flip-flop at the output to divide a couple of MHz down to 50 or 60Hz.
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Old 17th April 2014, 02:20 AM   #7
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I am about a week away from having ten kits suitable to drive shaded pole motors such as Garrard, Lenco and this Thorens. I will be selling them at cost: these are leftover parts from the development work on the V/T based motor controllers plus a modified version of a commercially available power module which uses the IRS 2092 amplifier.

Check the classifieds on Audio Asylum.
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Old 17th April 2014, 02:44 AM   #8
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Quote:
Originally Posted by JonSnell Electronic View Post
But it has a synchronous motor and 50 or 60HZ mains is as accurate as you can get!
Not true, the mains has an Allan deviation of around 10^-4 at the periods of interest.See here: Accuracy and stability of the 50 Hz mains frequency

Quartz oscillators easily better this, I use a chip that comes in around 10^-5. With a bit of effort I can get down below 10^-12 (but this is completely unnecessary for this application).
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Old 17th April 2014, 11:05 AM   #9
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Let me put out a few basic questions:

Does the E50 put out a constant torque and have a constant velocity when drive by a pure sine wave AC source?.

I ask this because of the nature of the shaded pole configuration seems asymmetric at best, and probably doest result in a constant torque/rate throughout a single cycle. I am just guessing though.

Is it possible to shape the AC source wave to compliment the shaded pole load and make the torque and velocity constant?

Mark, how is the AC signal generated in your proposed kit and can it's shape be 'tuned'?
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Old 17th April 2014, 12:02 PM   #10
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1. No

2. It is the nature of the beast with any form of induction motor that the rotor must slip behind the stator field and the degree of slip must increase with increasing load, that's how the rotor magnetisation is induced (hence the name). I'm not sure what asymmetric means in this context.

3. See 2.

4. I use a hybrid circuit which works like a DAC with fixed waveshape and variable clock frequency, this avoids the difficulties associated with filtering a square wave and with analogue signal generation.

PS: possessive "its" doesn't have an apostrophe.
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