# Motor calculation for a rotating platform

• Mrhohoho
In summary: Do you think that during constant speed, the force to overcome would be friction and air drag?Yes, friction and air drag will be the main forces during constant speed.
Mrhohoho
Thread moved from the technical forums to the schoolwork forums
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...

Mrhohoho said:
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?

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?

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.

Mrhohoho said:
...
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.

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?

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

Mrhohoho said:
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?

## 1. How do I calculate the required motor power for a rotating platform?

The required motor power for a rotating platform can be calculated by considering the weight of the platform, the desired rotational speed, and the torque required to overcome friction and acceleration. This calculation can be done using the formula P = (M x ω)/η, where P is the power, M is the total mass of the platform and its load, ω is the desired rotational speed in radians per second, and η is the efficiency of the motor.

## 2. What factors should be considered when selecting a motor for a rotating platform?

When selecting a motor for a rotating platform, factors such as the weight and size of the platform, the desired speed and torque, the available power supply, and the operating environment should be taken into account. It is also important to consider the type and control of the motor, as well as its efficiency and cost.

## 3. How do I determine the gear ratio for a motor and a rotating platform?

The gear ratio for a motor and a rotating platform can be determined by dividing the desired rotational speed of the platform by the rotational speed of the motor. This will give you the number of rotations of the motor needed to achieve one rotation of the platform. The gear ratio can then be calculated by dividing the number of teeth on the motor gear by the number of teeth on the platform gear.

## 4. What is the maximum weight that a motor can handle for a rotating platform?

The maximum weight that a motor can handle for a rotating platform depends on the torque and power of the motor, as well as the efficiency and operating conditions. It is important to carefully consider these factors and select a motor that can comfortably handle the weight of the platform and its load without overheating or causing damage.

## 5. Can I use a stepper motor for a rotating platform?

Yes, a stepper motor can be used for a rotating platform. However, it is important to carefully consider the torque and speed requirements of the platform and select a stepper motor with appropriate specifications. Additionally, the control and programming of a stepper motor may be more complex compared to other types of motors, so this should also be taken into consideration.

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