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Legacy Personal Blogs Vintage Turntable repair: Can I fix a Perpetuum Ebner from 1958 - part 2 - Calculating the Motor Speed
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  • Author Author: Jan Cumps
  • Date Created: 12 Feb 2015 7:04 PM Date Created
  • Views 1021 views
  • Likes 1 like
  • Comments 10 comments
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Vintage Turntable repair: Can I fix a Perpetuum Ebner from 1958 - part 2 - Calculating the Motor Speed

Jan Cumps
Jan Cumps
12 Feb 2015

From the previous post:

 

For almost a year I have a vintage turntable from Perpetuum Ebner at home.

It's not mine. It belongs to someone that asked me to fix it. And it turned out that fixing the motor would cost too much.

I asked the owner to collect the tt. That hasn't happened yet and the machine is collecting dust at my home.

 

So I'll take the freedom to attempt a non-intrusive repair with modern components. I'm also thinking about making it an Enchanted Objects Design Challenge.

 

In this post I'm measuring up the different gears, pulleys and wheels, and I calculate the speed of the original motor.

 

image

 

To measure the original motor's speed, I need to know the size of the gears that play a role in power transfer.

Because the desired platter speed is known (16 - 33 1/3, 45 and 78 RPM), I can  trace back the original motor's speed when I know the reduction ratios.

 

Three measures play a role here:

  • The motor pulley,
  • The step pulley
  • The outer rim of the table platter.

 

image

The motor speed is transferred to the step pulley via a belt. So the first factor is the size of both pulleys.

The speed of that step pulley is then transferred to the platter. The idler takes care that the spinning of the step pulley is transmitted.

That idler can move up and down to select a particular position of the step pulley.

That is how the right speed is selected: by putting the idler on the part with the correct diameter for the given speed.

image

So two factors play a role in deducting the speed:

  • the fixed ratio between motor pulley and step pulley. It doesn't change when the speed selector is operated, hence a fixed ratio.
  • the variable ratio between the steps on the step pulley and the outer rim of the platter (those are the two parts that are connected via the idler wheel)

 

The size of the belt and the diameter of the idler don't come into play. They just transfer the power between two rotating parts.

 

image

 

That gives me the following measurements and results:

 

image

 

The formula to derive the motor speed from the platter speed is:

 

[platter speed] * ( [platter diameter] / [stepper speed diameter] ) * ( [stepper outer rim diameter] / [motor pulley diameter] )

 

examples:

45 RPM: 45 * (177.2 / 14.7) * (29.7 / 5.5) = 2940.3 RPM

33 RPM: 33.3333 * (177.2 / 10.9) * (29.7 / 5.5) = 2934.0 RPM

 

 

Platter Speed
(rpm)
Motor Pulley
diameter
(mm)
Outer rim
diameter of
stepped pulley
(mm)
RatioSpeed dependent
diameter of
stepped pulley
(mm)
Platter outer
rim diameter

 

(mm)
Ratio

Calculated motor

speed

(rpm)

165.529.7

5.4

5.5177.232.22782.1
16 2/35.529.75.45.5177.232.22898
33 1/35.529.75.410.9177.216.32934.0
455.529.75.414.7177.212.12940.3
785.529.75.425.4177.27.02948.4

 

 

All 4 calculations should result in the same motor speed number. My measures are not precise, and that shows in the results.

The 3 highest speeds are close. 16rpm is off compared to them. I must have made a mistake when measuring that part on the step pulley - also its bigger ratio makes that a mismeasure effects the calculation most.

 

The above table and explanation is edited after a comment from RPLaJeunesse. He explained that 16 RPM is not exactly 16 RPM, but the half of 33 1/3 RPM. The 16 RPM figure is now in line with the 3 other speeds.

 

Related posts
Vintage Turntable repair: Can I fix a Perpetuum Ebner from 1958 - part 1
Vintage Turntable repair: Can I fix a Perpetuum Ebner from 1958 - part 3 - Infineon Motor Driver shield
Vintage Turntable repair: Can I fix a Perpetuum Ebner from 1958 - part 4 - Hercules LaunchPad Enhanced PWM try-out
Vintage Turntable repair: Can I fix a Perpetuum Ebner from 1958 - part 5 - Yes I Can
Vintage Turntable repair: Can I fix a Perpetuum Ebner from 1958 - part 6 - Speed Adjustment with Variable Duty Cycle
Vintage Turntable repair: Can I fix a Perpetuum Ebner from 1958 - part 7 - Make Speed Sensor from Scrap Parts
Vintage Turntable repair: Can I fix a Perpetuum Ebner from 1958 - part 8 - Sample the Motor Speed with Microcontroller
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Top Comments

  • RPLaJeunesse
    RPLaJeunesse over 8 years ago +2
    Jan, you might want to redo your spreadsheet using 16-2/3 RPM as the target speed, not 16 exact. IIRC that speed was intended to be exactly half the 33-1/3 speed, which made cutting the low speed master…
  • ipv1
    ipv1 over 8 years ago +1
    Very interesting build. You should consider making it a haunted object like with a small projector for the ghost and spooky old songs and stuff. It could work via twitter or gmail notifications... Imagine…
  • D_Hersey
    D_Hersey over 8 years ago +1
    I saw your earlier discussion of this device, late afternoon, don't have the will budget to go look it up at the moment, tried a little google fu the other day. Can you confirm that the motor is "asynchronous…
  • Jan Cumps
    Jan Cumps over 8 years ago in reply to RPLaJeunesse

    You are correct, RPLaJeunesse. When I make a more precise calculation using (100 / 3 / 2) for 16 RPM, I get 2898.

    That's well in line with the expected motor speed.

    Thanks!

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  • RPLaJeunesse
    RPLaJeunesse over 8 years ago

    Jan, you might want to redo your spreadsheet using 16-2/3 RPM as the target speed, not 16 exact. IIRC that speed was intended to be exactly half the 33-1/3 speed, which made cutting the low speed master easier.  A 16-2/3 master could be done on a record lathe at 33-1/3 by running the master tape at twice the original speed. At 16-2/3 the motor speed calculates closer to 2900 RPM. The 2940-ish speed you calculate seems about right for a 50Hz motor, as the no-slip speed for a 2-pole motor would be 3000 RPM. But any non-synchronous motor has some small amount of slip, and thus runs a bit slower. In the 60Hz world motors are 1800 and 3600 RPM before slip, with 1750 RPM being the number I remember for a 4-pole motor with slip.

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  • mcb1
    mcb1 over 8 years ago in reply to Jan Cumps

    I stand corrected ...

    3000rpm looks very much like it fits your motor.

     

    Do you have any pictures of the actual motor.

     

    Does it run or just sit there quivering.

    If you spin it does it run.?

     

    Mark

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  • Jan Cumps
    Jan Cumps over 8 years ago in reply to mcb1

    I was coming to 3000 based on the required resulting speed of the platter, and the ratios of the gears. If the rpm of the motor is 1500, I must have made a mistake in the measurement and calculations (big enough to be off by 100%).

     

    The motor has no visible damage, the coils are not interrupted, give a similar resistance and don't smell burned. I've fully revised it  mechanically and by hand it spins like a new one.

     

    I have one particular idea of what may be wrong in the original TT, but I'm too big of a chicken to try it out.

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  • mcb1
    mcb1 over 8 years ago in reply to Jan Cumps

    So motor speed is bound to be a multiple of 50. Most likely 3000 rpm.

    More likely to be 1500.

     

    I have some motor theory notes about how to check the number of poles, which then tells you the speed.

     

    I'm presumming the damage is much worse as normally a dismantle and clean of the bushes and re-oil of the bushes is all thats required.

    Thet tend to be self centering so a tap as they run brings them back into line.

     

     

     

    Mark

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