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.
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.
AndrewT said:
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.
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,...).
synchronous motors do not slip.oshifis said:
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.
tade said:
Brainticket said:
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.
Mark Kelly said:
Don't deserve this! Well I've had nothing that will even hint at working out what i want ...So what have you given me above that will help me work what i want ? I'm looking for a formula that works..Pulley diameter...Belt thickness?? I don't know the pulley diameter this is what i'm trying to find out.
Belt is Flat Linn type, thickness is irrelivant. Motors Sync 250rpm..
I would appreciate someone actually coming up with hey Brainticket times this by this & this & there you go.
::
The cirumferance of the drive pulley should be (approx) 7.5 times smaller than the circumferance of the surface you intend to drive; i.e. the inner platter or the edge of the platter, which is why I asked you what the diameter of this component was - were you too busy stamping your feet to answer?
Brainticket said:
Umm, Like maybe the one I just gave you?
.Pulley diameter...Belt thickness?? I don't know the pulley diameter this is what i'm trying to find out.
Belt is Flat Linn type, thickness is irrelivant. Motors Sync 250rpm..
I would appreciate someone actually coming up with hey Brainticket times this by this & this & there you go.
I see you haven't understood my equation and yet you persist with the attitude.
Read the equation again, it has the information you seek.
P.S. Belt thickness is NOT irrelevant. You can't work out the pulley diameter without knowing the thickness and material of the belt. For best accuracy you also need to know the Young's modulus of the material.
Hi,
As stated if you want to be precise, belt creep or slip must be taken
into account but I've personally no idea of the exact numbers.
If you want a flat belt to stay on a pulley you will need a crowned pulley.
Another alternative is to start a thread asking people to measure the pulley
crown diameter and driven subplatter diameter on their 250rpm 50Hz turntables.
Taking the average for the answers of this ratio will give you what you want.
I suspect though the answer will be 7.5 for 33.3 and 5.6 for 45.
FWIW belt thickness does matter as previously described in the numbers.
Note that you are effectively adding 1/2 the belt thickness to the
radius of the pulley and platter - this reflects the belts velocity.
I suspect a lot of turntables have crowned pulley's with the 7.5
ratio and the end result is they run a little fast, around 1%.
/sreten.
As stated if you want to be precise, belt creep or slip must be taken
into account but I've personally no idea of the exact numbers.
If you want a flat belt to stay on a pulley you will need a crowned pulley.
Another alternative is to start a thread asking people to measure the pulley
crown diameter and driven subplatter diameter on their 250rpm 50Hz turntables.
Taking the average for the answers of this ratio will give you what you want.
I suspect though the answer will be 7.5 for 33.3 and 5.6 for 45.
FWIW belt thickness does matter as previously described in the numbers.
Note that you are effectively adding 1/2 the belt thickness to the
radius of the pulley and platter - this reflects the belts velocity.
I suspect a lot of turntables have crowned pulley's with the 7.5
ratio and the end result is they run a little fast, around 1%.
/sreten.The 1 / 7.5 comes from (platter speed / motor speed) in the equation I posted because 33.33 / 250 = 1 / 7.5.
The actual ratio is always slightly less than this because its (platter speed / motor speed) * (1 + creep rate). Assuming creep rate to be about .001 the ratio is about 7.4925. This difference is pretty trivial so you can ignore it.
The next bit is tricky - as sreten notes this ratio is not the ratio of pulley diameter to platter diameter. It is, as per my equation, the ratio between pulley diameter + belt thickness and platter diameter plus belt thickness. Put another way, pulley diameter = (ratio * (platter diameter + belt thickness) - belt thickness. SO here goes:
Hey Brainticket. Add the belt thickness to the platter diameter and times this by the ratio 1 / 7.5. Minus the belt thickness from the answer and there you go
Hopefully
It can't be. The 31504 is 24V type. It could be a 35104, that is 220V. Here are the data of both:Brainticket said:
http://www.premotec.com/pdf/synchronous/SY31.pdf
http://www.premotec.com/pdf/synchronous/SY35.pdf
From my experience the most difficult is to fix the pulley onto the shaft (apart from calculating its diameter
).oshifis said:
It can't be. The 31504 is 24V type. It could be a 35104, that is 220V. Here are the data of both:
http://www.premotec.com/pdf/synchronous/SY31.pdf
http://www.premotec.com/pdf/synchronous/SY35.pdf
From my experience the most difficult is to fix the pulley onto the shaft (apart from calculating its diameter
Your correct it's a A.C. 24V version..
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