Circuit ideas for Xtal locked AC synch turntable motor drive

[djsb]

Hi,
I have a turntable (record deck) here at home and would like to explore ideas for driving it's motor electronically.
The deck uses an AC synchronous motor that is currently reliant on the AC mains frequency. That's fine and it works OK.
I always wondered  as a design excercise how it could be driven from a more accurate frequency source like a quartz crystal.
I know I would need a crystal oscillator but how would the motor be driven? The AC waveform (50Hz Sine wave) would have to be stepped up to around 240v (or 110v as the motor is rated at if fed through phase capacitors). Maybe a circuit could drive a conventional step down transformer but in reverse?
Does anyone have any suggestions or ideas?
Thanks.

David.

[joelby]

Would it be safer to replace the motor with something that works at lower voltages?

There are some dsPIC chips designed for motor control that may be appropriate, though maintaining a constant speed shouldn't be a very big challenge.

[djsb]

Hi,
Yes that would be something I could look at. There are DC motor conversion kits available for the turntable but they are expensive (classed as high end audio kits like this (which uses the original motor) http://www.cabezon.eu/product_info.php?products_id=126). I'm not worried about the voltage being high. The current motor is cheap and easy to mount. I'd have to do a lot of mechanical mods to fit a DC motor.

David.

[jimmc]

Remember that, except for direct drive, the actual speed of the turntable is dependent on the diameter of
the motor spindle (idler) or pulley (belt).
Using a quartz drive will not reduce any errors from this source.

Jim

[joelby]

If you attached a rotary encoder sensor to the turntable, you could probably make it pretty accurate.

[rossmoffett]

You could just run the motor off of a transformer or a variac.

The mains could be rectified with a 1:1 transformer and then a custom-designed inverter with a tunable frequency used to adjust the speed.

As was said though, it's not going to make it noticeably better.  Do you know what the deviance of line frequency is?  Not. Much.

[Zero999]

You could just run the motor off of a transformer or a variac.

The mains could be rectified with a 1:1 transformer and then a custom-designed inverter with a tunable frequency used to adjust the speed.

As was said though, it's not going to make it noticeably better.  Do you know what the deviance of line frequency is?  Not. Much.

Sorry, it's going to make it noticeably worse because the mains frequency is much more reliable than the voltage.

I can think of two ways to make a crystal controlled motor:

Use a stepper motor with the driver driven from a crystal oscillator.

Use a standard PM motor connected to a standard PWM controller, with a tachometer on the shaft, use a PLL and divide by n counter to synchronise the tachometer to the oscillator. I'll post a block diagram if you like.

I don't see any point in this because the mains frequency is normally very accurate. I suppose if this is going to be used in a developing country with a poor power supply or run off a generator then it might be a good idea, but if it's not, don't worry about it.

[djsb]

Hi,
Thanks for the replies.
I'm just doing a kind of thought experiment and thinking out loud. I might build a prototype at some stage who knows.
Maybe I could use a MCU (PIC24) generating a 50Hz sine wave using a PWM output and low pass filter? This would feed a power amplifier stage or driver. The output of this would drive a 110volt transformer used in reverse (as a step up rather than step down primary voltage TBD).
I wonder if the output of the MCU would be more stable than the mains frequency.
Of course it would all need to be run off mains power anyway.
Looks like it would be more trouble than it's worth when I think about it.


David.

[Zero999]

You could use an inverter but it would be very inefficient (<50%) unless the amplifier is operating in class D, not class AB.

[Simon]

your 50 Hz mains supply is very stable and controlling the frequency on your current motor may not be the way.

My dad has a quad deck was a very pricy peice of kit in it's day, it has an opto sensor which consisds of and IR led and a photodiode made up into one part with a space between them. the dec turn table has a tab that passes through the sensor on each turn. if you grab the turn table to slow it down the arm will lift and it will take a few seconds to resync and the arm will go down again.

I can only assume that the circuit times the delay between passes and ajusts a PWM signal until the timing is right. I don't think it is a stepper drive system I think it is a standard motor driven either by PWM or voltage controlled.

beware of your control circuit interfering with the audio pickup, with just 5 mV coming out of the dec you could easily get yourself some hum or high pitch sound, might be worth running it at 30+ KHz

[rossmoffett]

Building a motor driver isn't as simple as building an inverter, either.  There is inductive kickback and transient voltage to deal with also.  You'll have to use some schottky diodes as protection for your switching circuit, maybe a .1 uf capacitor across the motor leads also.

[Simon]

well protecting from inductive side effects should be common place for anyone deling with power control,