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AC speed control and tachometer

Wed Sep 15, 2021 1:34 am

I´m building my own turntable - based on an old Lenco L75.
This turntable is powered by an AC motor, and the speed adjustment is mechanical. This is ok, but as I have a little bit of experience with Raspberry Pi, I thought maybe it would be possible to do something fun with old and new tech.

So I wonder: Is it possible to control the speed of an AC motor with a Raspberry Pi? Guess I need some extra parts, and do some soldering perhaps.
The plan is to somehow control the input (voltage/frequency?) to the motor with 2 buttons. One for 33.3 RPM, and one for 45 RPM. (Guess this must be a custom program)
It may be too complicated for me, but it would be great if I could do it.
I think I should adjust for the faster speed (45) as standard, and use the raspberry for slowing down. So in that case maybe one button is enough.

Also (with or without speed adjust) I would like to have a tachometer that shows current RPM for the platter. Just a small LCD to show RPM

It´s been many years since I did RPi work, so a guide would be nice.

Looking forward to see if there are any doable solutions

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Re: AC speed control and tachometer

Wed Sep 15, 2021 7:36 am

Generally it would have to be the frequency that will control the speed of an AC motor.

First things first - is it a mains voltage motor? If so, you will be working with dangerous voltages, so it is not really advisable unless you have lots of experience working with such circuits.
If it is a low voltage motor, probably supplied via a transformer, then that is better it is much less likely to kill you and burn you house down. ;)

Either way you are probably going to have to convert the power to the motor to DC to get rid of the existing frequency, then convert it back to a (near) sine wave AC at the desired frequency. Quite a job.

Anyway, the first thing you should be looking at is the specifications of the motor - Voltage, Current and Frequency.

The software side should be fairly easy - the Pi can generate a low voltage sine wave output for it's Audio jack using PWM, for example.
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Re: AC speed control and tachometer

Wed Sep 15, 2021 8:52 pm

usually for turntables its a shaded pole motor....

you could try using something like this to "manually" regulate the speed of the shaded pole motor
AC motor speed controller.jpg
AC motor speed controller.jpg (45.68 KiB) Viewed 451 times
then later replace the potentiometer with a potentiometer IC that can be controlled digitally by RPi

but its better to use a VFD instead (though it might be bigger/bulkier)
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Re: AC speed control and tachometer

Wed Sep 15, 2021 11:45 pm

I'm guessing its the early L75 with variable speed control, I have what I think is an an L70 that I greatly modified many years ago. The later L75/200 had electronic speed control but it might have been a combination of mechanical and electronic.

Instead of doing this electronically have you considered using a stepper motor on the speed control and if you wish more precision add a sensor to measure the rpm and correct the speed.

I have a feeling if you change the motor speed by much you might hit some funny resonances, although these can be damped with blue-tack in the turntable.

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Re: AC speed control and tachometer

Sat Sep 18, 2021 12:59 am

Hmmm. A lot to consider :-) Thanks for all answers so far.

#"Pidd" Stepper motor intrigued me a bit. Is that a kind of motor that can switch between for instance two points A <---> B ? That would be an easy solution, and elegant to
Can you point me in the right direction of how to achieve this?

#"LTolledo" Speed Controller could also work, but I dont´t feel the need for digital remote. A speed controler, is that almost the same as Stepper motor?

#"rpdom" Thanks for the warnings

I think maybe the stepper motor could be interesting. Any known usage for this motor where i can see it in action?

I did a quick drawing of the Lenco motor and how it works. Quick explained Idler wheel turns the platter. Speed is adjustet by moving the idler wheel further or closer to motor, as the spindle is increasing in size - runs faster.
Now I have to move the slider manually, but if I could have a motor/flip that could do this by pressing one button that would be nice. Standard position 33.33 - when button pushed once 45 rpm - when pushed again 33.33 rpm.
Motor must of course stop at given point so it´s not spinning

May be a bad idea - no engineer here :D
See attached drawing.
Stepper motor.jpg
Stepper motor.jpg (68.89 KiB) Viewed 359 times
Many thanks :-)

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Re: AC speed control and tachometer

Sat Sep 18, 2021 3:23 am

Perhaps a linear stepper would be better. It gets a little complicated that steppers don't know their position when initially switched on so you need at least one switch/sensor to calibrate its position. Sometimes this is done with an endstop that stalls the stepper when you tell it to step many times in that direction, but this is a noisy method and not recommend for linear steppers with plastic gears.

Unfortunately there appears to be a supply problem with the cheaper linear steppers on ebay at the moment. ... fbd482f886

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