Motor calculation for a rotating platform

[Mrhohoho]

Hi! My team and i have been stuck in this school project for awhile. Been reading up a lot but can't find the answer.

We have been designing and rotating platform that is able to rotate a load of 2000kg. So the rotating platform would something similar to those car turntables where there would be caster wheels below it. We have made a few calculations but not so sure are we correct. The calculations are to determine what type of motor should we use. The diameter of the rotating platform is around 2m.

The time taken to turn one round is 30s and we calculated our angular velocity and angular acceleration using the formula (2 pi / T) and (w / t.) But based on my understanding, we have to calculate the moment of inertia which is 1/2mr^2 because we are assuming to be rotating a solid cylinder. From the moment of inertia, we calculate the torque required which is T = I * angular acceleration. Then Power = Torque * angular velocity. The power and torque required is quite low and we find it weird. I know there is rolling resistance but not so sure where to apply it.

However, we are not so sure whether that during constant speed, is there any calculation need to be done?

Hope the people here could help us as our project deadline is approaching...

 

[anorlunda]

I hope the forum can help you with your project.

The power and torque required is quite low and we find it weird. I know there is rolling resistance but not so sure where to apply it.

However, we are not so sure whether that during constant speed, is there any calculation need to be done?

Yes indeed, the key factor is friction. Once the object has finished accelerating and moves at a constant rotation rate, all of the motor power goes toward overcoming friction (and air drag which is a kind of friction). Friction is very difficult to calculate. It is usually determined by experiment.

So you need a motor powerful enough to overcome friction. Then you need some extra power to accelerate it from a stop. That is were moment of inertia comes in. But another big factor is how fast must it accelerate from a stop to the final speed?

Different motor types have different relationships between starting torque and torque at rated speed.

How is your motor connected to the platform? Does it drive the central shaft, or does it contact the outside perimeter of the platform?

Motors are relatively inexpensive. That suggests it is not so bad to add a motor several times bigger than you think you need. Is it a quality measure of the project to find a motor exactly matched to the minimum needed?

 

[Mrhohoho]

Thank you for the quick reply!

The motor shaft would be directly connected in the center. I believe the formula for friction would be Fr = coefficient x mg(?). The coefficient of friction i see for rolling friction should be around 0.001?

So safe to say that during constant speed, the force to overcome would be friction and air drag? My team would probably state negligible air drag and give a safety factor of 3?

 

[Mrhohoho]

And also is my calculation for the first part somehow correct? Like am i in the right direction? I believe all my formulas during acceleration would be under uniform circular motion.

 

[Lnewqban]

...
The motor shaft would be directly connected in the center.

Direct connection of an electrical motor to a to-be-rotated-from-repose 2-ton mass seems to me not to be a great idea regarding the longevity of that motor, even if VFD is used.

 

[anorlunda]

You may want to consider a capstan wheel turntable. For example, vinly record turntables use them as in the picture. The advantage is that the electric motor can run at much higher RPM than the turntable. Motors are more efficient at higher RPM. Also, startup from a complete stop is easier with a capstan.

The capstan wheel on the motor shaft, can either touch the turntable as in this picture.

Or the motor shaft can drive a belt that goes around the turntable as in this picture.

The coefficient of friction is highly dependent on materials and on arrangements. For example, it is easier to slide on ice than on concrete. So, I would be skeptical that the first value you found, 0.001 was the correct value.
What about bearings? Might you have ball bearings, or thrust bearings to hold the turntable up?

 

[Mrhohoho]

Hi Thanks for the reply! Would be using caster wheels to support the structure and hole the turntable up!

 

[anorlunda]

Hi Thanks for the reply! Would be using caster wheels to support the structure and hole the turntable up!

That's good.

Another question, Do you have a local advisor to help you? We can help a little bit here, but that is no substitute for an advisor in the room with you to help with the many small details. For example, with castors, How many? How are they arranged?

Ask the organizers to assign advisors to the teams.