Russia asks: How was recording speed set on 1950's lathe motors?

[Steve E.]

I'm posting this on behalf of Serg, who is having some trouble navigating the site while translating to and from Russian.


My question is the following:

In the Soviet Union in the 30s
there was a popular magazine "Радиофронт", which described various designs
of radio amateurs and enthusiasts, including sound recorders. Based on
articles from these magazines, I made a sound recorder, but I ran into one
very significant problem-during recording, the disk rotation speed drops
from 78 to 74-75 rpm. In the quality of the drive, I used various
asynchronous electric motors that were produced in the 50-70s in the Soviet
Union, because we can't find another one. What was the situation with the
drawdown of the disk rotation speed on Presto recording machines and the
like? What electric motors were used there and how was the exact recording
speed set?

I apologize again, I hope at least something will be clear from
this message.

Serg, I wonder if you are saying that the cutting stylus is causing the speed to drag? Does the speed slow only when cutting?

[Fela Borbone]

I think that the motor should be synchronous, not asynchronous. The speed in an asychronous allways decrease as it developes torque.
Synchronous motors are rare to find...
Hope this helps.

[Serg]

Thank you so much for your help!
Yes, the speed slows down only when cutting.

In the Soviet Union in the 30's and 60's the subject of home recording was very popular, so there were several books on making your own recording machine (in our country the state did not produce such machines for home use)

In one of such books the author recommended to use the electric motor from Melody MG-56 tape recorder. And he regulated the frequency of rotation of the disk by changing the pressure of the intermediate roller, thereby slowing down or speeding up the electric motor.

Of course, this approach is not good... So I started making a low-power frequency converter to adjust the speed of the motor during recording. The recording speed will have to be adjusted by the stroboscopic disc right during the recording...

I enclose photos of my "recorders" and photo of electric motor from Melody MG-56 tape recorder

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melodija56_7.jpg

[Serg]

I think that the motor should be synchronous, not asynchronous. The speed in an asychronous allways decrease as it developes torque.
Synchronous motors are rare to find...
Hope this helps.

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Such synchronous electric motors are widespread in our country. But they are not suitable for use in a recording device. They have high noise and vibrations, and their rotation speed is too high (3000 rpm)

[Dogtemple]

Interesting, that green motor is the same in my lathe but branded papst. Mine has a similar issue though, it slows down too easily with some drag

[Fela Borbone]

Serg:
"In one of such books the author recommended to use the electric motor from Melody MG-56 tape recorder. And he regulated the frequency of rotation of the disk by changing the pressure of the intermediate roller, thereby slowing down or speeding up the electric motor."

... I wonder if that may be achieved with an electromagnetic brake... and why not a signal from a tachometer controlling it? That closing a feedback loop...

[Serg]

I don't quite understand what you mean by the term "electromagnetic brake". Do you mean slowing down an asynchronous motor by applying direct current to the windings as in the circuit I attached to this message?

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[Fela Borbone]

I mean, outside the motor, some electromagnetic device that mechanically can impede the free turning of the plate, maybe atracting metal moving parts, or best without physical contact, just magnetic attraction, or eddie current action... It does the job of the intermediate roller you mentioned above, but controlled electronically.

[Serg]

Understood, thanks for the explanation. An interesting thought.
In one of the Soviet tape recorders (the scheme was probably copied from some foreign device) there was a scheme for stabilizing the engine speed. A metal disk with slots and an ordinary magnetic head from a tape recorder were used as a tachometer. The pulses from the tachometer were converted into a voltage varying depending on the pulse frequency, which was used to control an asynchronous electric motor.

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[Fela Borbone]

Thanks for posting the schematics, it's an interessant way of controlling speed...
And your recording machines look fantastic!

[Serg]

Or here is another design, described in the magazine "Radio" in 1941. It uses an asynchronous motor from a fan with a capacity of 50 watts. (But 50 watts is the power consumption, useful power on the motor shaft there is no more than 7 watts).

The cone pulley on the motor shaft is used to regulate the rotation speed. Interesting design...

I hope the pictures make it clear what I'm talking about.

While I made a frequency converter, it allows you to adjust the speed of the disk in the range of 72-90 rpm.

But what's inconvenient is that while I'm recording I have to keep an eye on the evenness of the filament winding and on the stability of the engine rpm. I have to adjust the speed during the whole recording process.

How to make the feedback in my circuit I have not thought up yet...

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[markrob]

Hi,

Some time ago, I hacked a modified sine car inverter to make a poor man's VFD. If worked well with a Presto turntable I picked up that used asynchronous AC induction motor. I could vary both the frequency and voltage to the motor to manually to adjust the speed. I think an approach like that or some of the other electronic solutions shown by the other posts would be way better than a mechanical break. I'm not sure if current car inverters use the same or similar circuits these days. I can post the schematics if you are interested.

One other option. Pickup up one of the low cost class D audio power amp modules by companies like Sure Electronics and drive it with a variable oscillator to drive the motor directly. You will probably need a matching transformer to get the correct voltage to the motor. You could use an extra audio output from a DAW to generate the sine wave of the needed frequency. Here is one example:

https://store.sure-electronics.com/product/AA-AB31316

From the spec sheet it will drive about 20 Vrms into 1.5 ohms at about 250 Watts at 1% distortion with a 36 Vdc power supply. Find the closest power transformer with a 20 Vac secondary and 120 or 220 primary (depending on the motor voltage) with the correct VA rating and run backwards.

Mark

[Serg]

Hello! Thanks for the hints. At the moment I use such a simple 12-220 inverter circuit, in it I made a frequency adjustment. The output signal is far from a sine, but the electric motor, due to the large inductance of the windings, works well

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[markrob]

Hi,

That is what we call a modified sine inverter here in the US. That is basically what I did with the inverter I used, but it was a different design.

You are correct, the motor inductance smooths out the current waveform. The motor might run a bit hotter than if it were run with a pure sine wave, but it should not cause a problem.