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Old 18th July 2007, 11:04 AM   #1
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Default So How's it done?

Hi Comrades

So whats the mathmatic formula for deciding the motor pulley diameter to sub platter diameter for AC 50Hz cycle?

Thanks in advance..
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Old 18th July 2007, 12:03 PM   #2
AndrewT is offline AndrewT  Scotland
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Hi,
it will depend on what slip occurs between the belt and each pulley.

But [motor rpm] / [platter rpm] gets you close.

Do you realise that mains frequency has a high tolerance around the nominal 50Hz.
That in part is due to the enormous flywheel energy in all the generator/motor assemblies that are "locked" together throughout the British Isles.
As the power demand goes up so the generators slow down releasing some of that energy rather than only using fuel to meet transient demand. When demand reduces the fuel builds the speed back up to above nominal waiting for the surge (TV tea breaks).
Effectively, allowing a wide tolerance reduces the cost of energy supply.
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Old 18th July 2007, 12:50 PM   #3
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Quote:
Originally posted by AndrewT
Hi,
it will depend on what slip occurs between the belt and each pulley.

But [motor rpm] / [platter rpm] gets you close.

Do you realise that mains frequency has a high tolerance around the nominal 50Hz.
That in part is due to the enormous flywheel energy in all the generator/motor assemblies that are "locked" together throughout the British Isles.
As the power demand goes up so the generators slow down releasing some of that energy rather than only using fuel to meet transient demand. When demand reduces the fuel builds the speed back up to above nominal waiting for the surge (TV tea breaks).
Effectively, allowing a wide tolerance reduces the cost of energy supply.
Hi Andrew
! I'm sure i can sort everything out from that thanks




Ok in English, i have an extremely well made platter,sub platter & bearing i wish to use but need to work out a pulley to suit my Premotec AC motor.I'm in the UK on a 50Hz cycle..Not intrested in 45rpm only 33.3..All i need are the aid to the mathmatics, then i can take it from there.
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Old 18th July 2007, 01:12 PM   #4
AndrewT is offline AndrewT  Scotland
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How many poles are used in the synchronous motor?
A two pole motor turns at 1500rpm.
Most belt drive motors are much slower, 8pole/12pole/16pole are fairly common.
Can you measure or estimate the motor speed?
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Old 18th July 2007, 01:21 PM   #5
oshifis is offline oshifis  Hungary
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It depends on the revolution per minute of the motor. For the most frequent 1500 rpm the diameter ratio would be 1:45. In reality a bit larger pulley diameter is needed because of the motor slip. Slip is called the few percent slowdown of the asynchronous motor related to the rotating magnetic field (and not the belt "slipping"). Once I have made a pulley like this by machined to about 5 % larger and then gradually reduced it with sandpaper while rotating (belt removed, diameter adjusted, belt up, speed check,...).
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Old 18th July 2007, 01:30 PM   #6
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Its a Premotec 9904 111 31504
50Hz 250rpm. Synchronized Turntable motor..That im wanting to used to turn the platter at 33.3 rpm..
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Old 18th July 2007, 01:45 PM   #7
tade is offline tade  United States
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33.3/250= factor
factor* platter diameter= pulley diameter.
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Old 18th July 2007, 02:16 PM   #8
AndrewT is offline AndrewT  Scotland
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Quote:
Originally posted by oshifis
In reality a bit larger pulley diameter is needed because of the motor slip. Slip is called the few percent slowdown of the asynchronous motor related to the rotating magnetic field (and not the belt "slipping").
synchronous motors do not slip.
Induction motors slip. As the torque imposed on the motor increases the slip increases and the current increases to the point that the motor is turning slowly and current is sufficient to cause overheating. The next stage the motor stalls.

In a synchronous motor the phase lag of the motor increases as the torque increases but the speed stays locked to the incoming frequency. As the torgue varies the motor oscillates up and down the phase lag. Again increasing the torgue too far and the motor drops out of lock and stalls. The belt stretch and platter inertia hide/attenuate the speed variations due to torgue variation.

But the belt does slip as it comes out of contact with both the drive pulley and the driven pulley.
The overall effect is that the drive pulley should be slightly larger than calculated.
But what is the correct calculated diameter?
The drive pulley is not cylindrical, it is barrel shaped to keep the belt central on the pulley. I suspect the narrowest part of the barrel should be close to the calculated diameter. The middle of the barrel will be significantly more than the calculated diameter.
I have no idea what the ratio of diameters for the barrel should be to achieve good belt centreing.

BTW,
250rpm tells you it is a 12pole synchronous motor.
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Old 18th July 2007, 09:51 PM   #9
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Ok right so i'm back to square one & obviously no one knows...

DIY Audio forum!!! Seems not.

Thanks anyway..
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Old 18th July 2007, 11:42 PM   #10
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Quote:
Originally posted by tade
33.3/250= factor
factor* platter diameter= pulley diameter.

Quote:
Originally posted by Brainticket
Ok right so i'm back to square one & obviously no one knows...

DIY Audio forum!!! Seems not.

Thanks anyway..
You don't deserve this but here goes:

The correct answer depends on the type of belt used. If you are using a standard rubber belt the required pulley diameter is given by

Pulley diameter + belt thickness = (platter diameter + belt thickness) * (platter speed / motor speed) * (1 + creep rate)

This refers to the crown of the pulley.

Since creep rate depends on pulley diameter this is a recursive calculation, start with an estimated creep rate of 0.1%. Creep rate is not slip and it cannot be cured, although it can be ignored as most designers do.

If you use any material other than rubber the equation changes and you must use (belt thickness * 2 * Poisson's ratio) where I have belt thickness. Rubber has a Poisson's ratio of 0.5 hence the simplification.
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