60Hz oscillator for turntables (also produces three phase)

[Morgan Jones]

For my one hundredth post, I thought this might be useful. Running from a +/-15V supply, the state variable oscillator produces 60Hz sine and cosine waveforms of about 24V peak to peak, with amplitude set by VR1. This large amplitude means that the noise of the (TLO74) op-amp is negligible. If you need a smaller amplitude, attenuate the output. RV2 gives a fine speed control. U2 generates the three phases from sine and cosine. If you don't need three phases, lose U2 and use the cosine output. The BZX55C Zeners should have a little foam house built round them to protect them from draughts and also underneath the board where they are; 2mm EVA foam will do nicely. The circuit takes a few seconds for oscillation to build up.

To drive a motor, you need power. Ready-built TDA2030 amplifier modules are readily available and will drive a small motor.

Some turntables have 24V motors (Thorens TD125) with a capacitor to produce the required 90 degree phase shift. The oscillator produces an accurate pair of phases 90 degrees apart, but a TDA2030 can't produce enough voltage - you'll need something that can swing more voltage.

[Paul Barker]

Thank you Morgan. Im not sure I’ll build it for a while though. The uTracer 6 kit has arrived and Im to say the least overwhelm’d by the sight of that so far. Slowly slowly catchy monkey. I was going to build the ETF 6 eL36 but sod it I have a great sounding EL38 amplifier of my own design. Evry time I go up and listen to it, I don't feel the need to build anything else.

But I think its really great that youve given us this circuit.

[Paul Barker]

Not trying to be clever but I have a Harrison vertical Milling Machine that is three phase which Im tricking with a capacitor to start a false phase, and boosed the voltage with a giant transformer wound with 3mm diameter winding wire but though it works its never as good a true three phaseI I could now generate three phase wource with youre circuit I can increase the power but that might need a big piece of mosfet amplification to power the Mill.

Or could I use 3 833a’s? I have enough. Then I can power my Harrison L6 Laithe from three phase too. Its not very happy on its single phase motor, the one it has anyway, which isnt the original.

[shane]

Now that could be really useful. Thanks Morgan!

[Morgan Jones]

A lathe or a mill requires a lot of power. Even a Class B power amplifier is only 78% efficient (maximum) from its HT power. And the idea of three push-pull 833 amplifiers to power a machine tool is mindboggling, but would give serious bragging rights. I looked up Harrison L6. Nice. I use a Transwave static converter on my Colchester Bantam and it seems fine. I've never looked to see how accurately it sets its three phases because I get very nervous around that sort of power.

[Nick]

TDA7293 will be fine for 24v if needed.

[Morgan Jones]

I forgot to include the phase tweak...

[Paul Barker]

I beg you’re pardon my laithe is only an L5. At the moment though it needs a better motor. The problem is that most motors available run twice the speed an old laithe like this was designed for. I guess if I google the issue I’ll find my answer. I suppose I could always half the speed with a different dimention drive pulley.

It has a beautiful big four jaw chuck and the three jaw. The four jaw with a dial gauge set up to centre it does great work. Taking longer to set up but centres perfectly, and stays perfect. The laithe leaves a great finish on youre work.

The Mill has motor problems also. I have to use it on low gear. But thats because it is a 3 phase motor run through a transformer to up the voltage and a capacitor to make a false phase. This does not get the best torque from the motor. But I suspect making a powerful three phase supply from youre cirquit is a bit too difficult for me.

Darren and I used to talk about these things a lot. Darron went for the rotary style inverter. I think you run another three phase motor with the transformer to increase voltage and phase shifting capacitor, this motor just rotates, and the electrical power for the tool drive motor comes from this motor. I suspect its not optimal but provides a bit of a flywheel effect. I guess you need this motor to be more powerful than the tool motor.

Aonther thought is to spin a three phase motor mechanically with Petrol or diezel Generator as a three phase generator and power youre tool motor with that. Cheaper in the long run from paying for three phase supply, not to mention the cost of installig the cabling for three phase.

So nothing I have is fed enough power as it stands, and it sits redundant mostly.

When we had a model engineering club in Scarborough we met in the then technical collage and used their bridgeport Laithe and their colchester laithe and their gillotine, and there was no need for my inadequate machine tools. Maybe the best option is to run them on mechanical motors with big flywheels.

My laithe came from my uncle for £500 and same price for the mill from a school.

We used to have in town a group of three or four retired enginners one was a chief guy at a company that made power supplies and wound components. Another was Billy Sweeting who started up Sweetings, an engineering company still operating. Sadly they have all passed away. Billy had a set up in his basement and the three of them restored for selling engineering equipment until they passed away.

You always take for granted these local characters, but suddenly theyre gone and there is noone with those skills to fill the void.

[Paul Barker]

I think making a three phase turntable has more chance of success than fixing my machine tools problems.

My uncle has bigger better more automated machine tools. But they wont come to me. He has step children who will inherit, and you will see them on the Yorkshire Auction House TV program. Oh yes his younger wife has already told me shes had Angus round. He was drooling at the mouth the greedy get.

[Morgan Jones]

I bought a very informative book "Electric Motors" from ARCEuro (£7.75, I think). I thought like you that a rotary converter was essentially a motor/generator set, but apparently it isn't and the motor can be smaller than the load motor or motors. Sounds like it would be worthwhile for you to get a copy of that book. I took the easy way out with the Transwave converter and have had no problems. Yes, I'm sure dealers do very well buying workshops from the uninformed relatives of deceased mechanical engineers. It's probably compounded by said engineers not letting on how much money they've spent on tools!

[Nick]

Three phases from Morgans circuit + one of these

https://www.behringer.com/product.html? ... e=0105-ABE

And three step up autoformers to match the motor voltage should do the job.

[Paul Barker]

Not bad price and probably perfect power wise better than inverter.

[Paul Barker]

Three phases from Morgans circuit + one of these

https://www.behringer.com/product.html? ... e=0105-ABE

And three step up autoformers to match the motor voltage should do the job.

Ref step up transformers any idea what outlet voltage of behringer for audio might go to?

[Nick]

The link says "4 x 1600 Watts @ 2 Ohms; 2 x 3000 Watts @ 4 Ohms"

So that's either 4 * 56v or 2 * 109v

[Paul Barker]

So I have to step up 56 to 400. Simplest route is a three phase 400v autoformer if there is such a thing. Its looking out of my price range.