Frequency pitch power supply for motor

[Stefanoo]

Hi everybody,

i would like to make PSU that would control the amplitude and frequency of the main 230V/50Hz (or 110/50Hz for who live in the US), something that would supply a stable 50Hz frequency and amplitude even though we know that tolerances on the electric system should be within 10% of the nominal specifications?
I would actually like to make something that controls the frequency that i can for instance vary it and therefore fine adjusting the speed of the platter.

I would use it to power up the AC motor of my turntable giving it then a better and more stable speed regulation.

Do you guys have any suggestion?

thank you very much for your attention.

Best,
Stefano.

[davemar73]

Sounds like you need an inverter being powered from a mains power supply. So you convert your unknown input AC supply to a nice fixed DC voltage, then power an inverter to generate a cleaner mains.

I wonder whether cannibalising an uninterruptable power supply would be a way to go. Having worked on the design of them many many years ago I know some of them have very good output stages that could be tweaked to change frequency and voltage. You could remove the battery and charging parts and use the rest somehow.

I imagine the platter motor isn't particularly power hungry, so you shouldn't need anything too massive; but the output transformer will still be the bulk of the weight. You might find a SMPS for the AC->DC stage.

[Stefanoo]

sorry for my ignorance i will try to recall whether i understoond correctly or not.

I would need a SMPS which is a AC-DC converter (is it not a normal power supply with transformer rectifier and capacitors plus eventual regulation?) then i need a DC to AC converter which i don't know what it is.
The only feature i would like to have is the variable frequency for proper adjustments.

The motor is not hungry of current, but of course, i don't know how much current it sources.
I guess i could just connect the meter in series with the main and verify it.

Would you be so kind to point out some schematic or something that you think would offer a refined regulation?


Thank you very much for the attention.


Best,
Stefano.

[davemar73]

You wouldn't need an SMPS for the AC->DC converter part, a conventional linear (transformer, rectifier, caps, regulator) would do the trick, it depends upon size and complexity. You might have an old SMPS supply from some kit you could use to save you building that part. Something that generates 12v DC is fine (12v is a handy voltage to aim at as a lot of inverters are designed to run off car batteries).

I couldn't find a circuit diagram for an inverter from a quick google, but you basically want a 50Hz (or whatever frequency you want) sinewave generator feeding a PWM generator feeding some switching transistors (typically a MOSFET H-bridge) which connect to a mains transformer with the secondary used as the primary.

You can generate the PWMed sinewave using a microcontroller if you're feeling digitally minded.

[eds65gto]

Hi:
A long time ago and far away , the way this was done was to have a low distortion sine wave generator at the right frequency , 50 to 60 cycles , and feed a 30 to 40 watt audio amp with a 70 volt output transformer secondary to feed the turntable motor. You could even use an isolation transformer and get the right impedance and voltage match with the motor. Some motors run better with lower voltages.
You then have a stable , low distortion , variable frequency turntable motor supply.
Linn does it this way if memory serves.
Now you can run the euro turntable at 50 cycles to lower hum and not have to change the motor , or you can correct for slow or fast speed or vary the turntable for 45 rpm if desired.
Hope this helps
Ed

[Stefanoo]

good idea.
i will do this way.
I have noticed that the motor works at lower voltage.
I tried to power it up at 110V instead of 220V. I didn't have a way to check if the speed was correct, but it looked normal to me.

I have a schematic for a low distortion sine wave.
Is a little bit complicated, but i am sure it will work.

I actually don't know how much current i need (i guess i can measure it) and what is the impedance of the motor (can i measure it somehow?) and what is the best voltage for it.
I mean: how can i drive the motor with the signal generator that probably has a few mA output source capability? (i have never used isolation transformer)?

[star882]

Why not just replace the AC motor with a precision DC servo motor?

[Stefanoo]

good idea, but it would imply to many changes: motor and plus a dc servo.
I think that the system platter+motor+belt has to be well calculated to obtain good performances.
I don't think i wanna follow this path, at least for now.
I just want to build a "simple" frequency pitch control system".

thank you anyways for your advice, i might consider it on the future.

Best,
Stefano.


P.S. is that on the left side yor picture or what

[star882]

Originally posted by Stefanoo
good idea, but it would imply to many changes: motor and plus a dc servo.
I think that the system platter+motor+belt has to be well calculated to obtain good performances.
I don't think i wanna follow this path, at least for now.
I just want to build a "simple" frequency pitch control system".

thank you anyways for your advice, i might consider it on the future.

Best,
Stefano.


P.S. is that on the left side yor picture or what

It would be simple compared to a VFD.

And yes, that is my picture. I'm one of the few pretty EEs.

[Stefanoo]

No, i am not really convinced it would be more practicle but i am sure that it could be a good idea too.
Since i was planning already on having a low distortion sine wave generator, this is the perfect occasion to build one i guess.

I also liked your idea of the Avatar and decided to put one too.


BTW, yes, you are one of the few female EE and also rare, very pretty!

Just for curiosity: Is your friend on the Background of your pic an EE too?

P.S. i have sent you a private msg for some infos.