Single Phase Power Source (AC Regenerator) for Induction Motor Powered Turntables - diyAudio
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Old 16th September 2014, 01:57 AM   #1
kevinkr is offline kevinkr  United States
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Default Single Phase Power Source (AC Regenerator) for Induction Motor Powered Turntables

This is somewhat of a different area of endeavor for me; usually I tend to concentrate on tube based signal paths.

Some months ago I decided that I would work on a simple low distortion AC source for use with the plethora of cheap chip amps available on eBay. My thoughts were I would use LM3886 to drive an EI step up transformer to provide clean sine wave power at 100V - 120V or 220V - 240V at 50 or 60Hz to power a couple of turntables I have. More specifically I was interested in having some fun and running a 50Hz platter equipped Garrard 401. Figured rather than just do a 50Hz platter strobe, why not do a whole supply, given the claims made by those using fancy expensive supplies on their turntables.

This is a work in progress.. My initial efforts with an off the shelf PIC based 50Hz source disappointed me in that the frequency was 0.06% low. I felt I could do better than this.

The original design comprised regulators and a sixth order Sallen-Key based VCVS to convert the square wave from the source into a clean sine wave.

Subsequently I designed my own square wave source which had the added benefit of costing less and supporting the generation of both 50Hz and 60Hz with an order of magnitude better frequency precision.

All of the parts are cheap and readily available.

I laid out the original project in Altium, but in a story too long to share here no longer have access to a license. The additional board was hand built p2p on vector board.

I will post the schematics and more detail in the next post, this one is getting a bit long...
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Old 16th September 2014, 02:04 AM   #2
kevinkr is offline kevinkr  United States
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Currently what I have is built on a number of PCB I laid out and a hand built prototype of the oscillator/source.

I have yet to cross the bridge of marrying the oscillator and LPF board with any sort of power amp and step up transformer.

I have a chassis box on the way from China, and am waiting for Antek to have more stock on their 100VA 15V toroid for the raw supply. The step up I will be using is a universal type from Triad UPL10-5000 which is a 10V/5A type with dual primaries which I can wire in series for 240V output if desired.

Others who have blazed this trail cautioned against toroids due to the possibility of saturation and the effect on the power amplifier if that happens. I plan to use a fairly aggressive zobel across the output of the amp on the order of a a couple of uF in series with a 2.7 - 4 ohm power resistor, this should help with inductive loads..

I caution that I have not tried to make high voltage AC yet, and may run into some very odd issues, and including exploding LM3886TF..

Oscillator:
Anyway referring to the oscillator schematic (KiCad), the binary counter used, a 74HC4060 incorporates a competent crystal oscillator. I chose to use a 2.4576MHz crystal to reduce radiation of harmonics resulting from the use of a higher frequency crystal, and because this frequency subjected to binary division results in a value friendly to simple further division by a 74HC4017Johnson counter. I chose a divisor of 4096 which results in an output frequency of 600Hz, a further division by 5 or 6 results in 120Hz or 100Hz respectively, and a final division by 2 via a D flip-flop gives you a duty cycle of 50% and 60Hz or 50Hz with a single crystal, three cheap chips, a few caps, and a couple of headers for configuration are it.. Not elegant, but functional. I probably should not have used 74HC series parts due an amplitude asymmetry that results in DC offset at the output of the filter. (It can be nulled.) I have not yet evaluated 4000 series CMOS equivalents, but they may work better, the 74HC74 would have to be replaced with a 4013 requiring some minor changes, but the 4060 and 4017 slot right in for the 74HC series parts or would had I had sockets on hand.. LOL

LPF Filter:
This was also surprisingly cheap compared to through hole switched capacitor filters and performs more than adequately, note that it was really designed for 50Hz, but I have not retuned it for 60Hz (there is such a version) as this would degrade the 50Hz performance substantially.. The filter is about -3dB at 60Hz compared to 50Hz a compromise I am happy to live with, and the current filter is down more than 50dB at 150Hz relative to 50Hz, and is even better at 180Hz when outputting 60Hz.

The filter is two cascaded 3rd order moderately high Q LPF filters, Q is ~2.2, no attempt has been made to be clever or to optimize anything other than stop band attenuation - it's not necessary for this application.

Bipolar power is used in order to try and minimize power up LF transients that might trip the amplifier protection circuit when driving a transformer. (Transformer behavior when confronted with DC) The jury is out on how effective that might be. Long mute period may be needed, or even ramping up the drive signal to the amplifier.. To be seen I guess.
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File Type: png 50_60Hz Source.PNG (62.4 KB, 564 views)
File Type: png revised LPF.png (73.6 KB, 546 views)
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Old 16th September 2014, 02:32 AM   #3
kevinkr is offline kevinkr  United States
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Some basic measurements..

For frequency related measurements refer to the HP counter as it is more accurate than the Amber frequency counter.

THD measurements are done with the 30kHz LPF engaged. (No significant difference without it) Measurement set is an Amber 5500 with the high performance pack.. (LOL)
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File Type: jpg 50Hz.jpg (41.2 KB, 537 views)
File Type: jpg 60Hz.jpg (39.1 KB, 519 views)
File Type: jpg 50Hz thd.jpg (232.9 KB, 138 views)
File Type: jpg 60Hz thd.jpg (85.2 KB, 123 views)
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Old 16th September 2014, 02:46 AM   #4
kevinkr is offline kevinkr  United States
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Note this could power other things depending on your willingness to invest in bigger power amps and transformers. (200W linear amps are available on eBay)
Class D might be an option as well.
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Old 16th September 2014, 03:02 AM   #5
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How do these motors work exactly? Is it like a PSC with a toothed rotor or something? Maybe a driver amp capable of good low supply voltage behavior would allow a soft start and shutdown implemented on the output supply level (safeguard) in addition to some gain or volume control tracking supply voltage to keep the drive sinusoidal and play nice with any transformer, through the ramp periods and even line loss. Not trying to be distracting. Just some thoughts.
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Old 16th September 2014, 03:15 AM   #6
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Specifically targeting shaded pole induction motors like those used in TD-124, and Garrard 301/401. There are countless others..

I don't expect the transformer/amplifier problems to be insurmountable.

What surprises me is that the circuitry is not complex nor expensive, I guess it is the specialization that makes the few commercial (and nameless) options so ridiculously expensive, admittedly they are probably also a lot more sophisticated than this one.
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Old 16th September 2014, 03:37 AM   #7
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A plain induction motor with squirrel cage rotor would develop some minimum slip just with unloaded rotor losses. That would mean available line frequency and voltage would almost never be exactly what is needed to establish a particular platter speed. I guess some AC synchronous motors employ permanent magnets. For the lack of cogging I can see why tweaking the drive to an induction motor would be worthwhile, even if complicated.

Last edited by Andrew Eckhardt; 16th September 2014 at 03:40 AM.
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Old 16th September 2014, 01:53 PM   #8
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The induction motors used in turntables have a pretty predictable velocity/torque curve based on line frequency, voltage and load. Because they pole slip but don't loose lock with the line varying the load on them deliberately allows them to run over a range of speeds - this is how the speed on a TD-124 or Garrard 301/401 can be adjusted using an eddy current brake. Speed is very consistent if voltage and load are as expected.

I have seen a few very inexpensive synchronous motors that used ring magnets attached to the rotor. Over time they usually fall off and since they are not marked for balance once this happens it is a devil of a job to get the motor to run smoothly again. Garrard as one example made this type of synchronous motor after inexpensive Japanese turntables appeared on the scene. (I believe BSR may have too, but most of their inexpensive changers seemed to have had two pole induction motors)
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Old 16th September 2014, 02:44 PM   #9
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So I guess since any sort of meshed transmission is out for noise and vibration, no matter what kind of motor, slip at some point is inevitable, and all systems except direct drive employ "overdrive" in the ratio (for tables with speed adjust), since faster than synchronous speed is impossible? My question then is specific motor frequency to the nines a little unimportant so long as it is quite stable?

Last edited by Andrew Eckhardt; 16th September 2014 at 02:51 PM.
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Old 16th September 2014, 04:50 PM   #10
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I think that such a system could benefit from a fine-tuning vernier: it could be used to compensate for various sources of inaccuracies in the slip percentage and mechanical transmission for example.

I have already examined the possibility of such a vernier on a digital generation chain (for other purposes but the principles remain identical).

Basically, I see two promising paths, both based on the offset-PLL principle: one rather conventional, except the frequency subtractor and phase comparator would be digital, and the other using a V/F converter to servo the difference frequency to an analogue voltage.
Both options would give a convenient, deterministic and accurate means of varying the output frequency, by say +/-0.5% full range.

I will try to come up with such an add-on circuit when I find some time (and inspiration)
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