# Electromotor for large platform

For a design project I am designing a plastic platform with a steel construction under it, which has a diameter of 4 meters which is able to support 15 persons. The platform should be able to rotate at slow speed (4 rotations per minute). Now I need to find out which electromotor is able to make this possible, does anyone have a suggestion?

• This isn't really an electrical engineering question until the mechanical engineering aspects are fleshed out. However, I'll put some ideas together below because it is a fun exercise. – Anindo Ghosh Oct 31 '12 at 10:14
• Not only is this off topic, but identical to the previous question electronics.stackexchange.com/q/44980/4512, which was also closed. – Olin Lathrop Oct 31 '12 at 13:29

## 1 Answer

There is no simple answer to your question. You need to at least put together the following data before any meaningful calculations can even be conceptualized:

• What is the total weight of the load-bearing platform with all supporting hardware?
• What is the mass distribution of this plate? This will affect the moment of inertia, hence the power required to spin up and spin down the platform.
• What is the suspension mechanism? Friction at the bearings alone could consume over half of the motive power of the motor. Add to this the friction at the edge stabilization mechanism, be it a circular rail and wheels, pure cantilever (it'll oscillate out of control pretty quick) or a load-bearing drive mechanism.
• What is the spin-up rate required? How quickly does the platform need to spin up to the full rotation speed - 30 seconds? A minute? An hour?
• How will the platform be brought to a stop? Brake shoes? Motor dynamic braking? Regenerative braking?
• What is your proposed drive mechanism for the motor? A belt-driven central shaft? Belt-drive around the outer circumference of the platform? A gear drive on the inner circumference of the platform? Multiple motors driving wheels distributed around the outer edge, running on a circular rail beneath the platform? All these mechanisms, and many others, have been used for designs similar to yours, and each mechanism requires very different motor sizing calculations.

Even with all this data, you're just making a start towards arriving at your answer.

Provided below are some broad assumptions and guidelines for your power sizing exercise.

Rotation Speed: The circumference of your platform is approximately 12.6 meters.
Assume a driving gear-head of diameter 5 cm on your motor, directly driving the outer edge of the platform: Gear head circumference is around 0.15 meters.
To traverse 12.6 x 4 meters a minute, the gear-head must run at around 335 RPM.

This will help in computing the gear drive ratio required, based on available motor RPMs. If you use a 4000 RPM motor, then a 10:1 gear reduction more or less serves, providing you leeway for rotation speed fine-tuning. That's just one step.

Motive Power: Assume average weight of a person to be 75 KGs (a mix of average weight male and female persons).
Thus the platform must carry 1,125 KGs of payload.

Assume that your platform itself weighs around 875 KGs - This is purely a number I picked for calculation convenience, you would need to compute the actual weight of the combination of plastic and steel construction of the load bearing platform plus central and stabilizing support mechanisms.

Therefore total mass to be rotated is 2000 KGs.

Now plug in the spin-up time required, and you have an approximation of productive power of drive mechanism.

Factor in friction losses to arrive at actual required motor power.

Double that number to account for sketchiness of computation ;-)

So finally you have it: A horsepower baseline and an RPM baseline for motor selection. Or, at least a vague starting point for product selection to begin.

• Thank you! The estimation certainly is a good starting point. The design project's focus is on a whole other point, so the motor is just a small detail which does not have to be elaborated too much. But with this information I should manage to find a suitable motor which is based upon a decent assumption. – iljoesjin Oct 31 '12 at 10:58
• Do you have any suggestions for a website where I can find a suitable motor? I am mainly finding very small electromotors :( – iljoesjin Oct 31 '12 at 11:17
• You might want to search for NEMA 143T, 144T ... up to 449T, in case one of those motor-frames meet your final requirements. Add in the keywords 2-pole and / or 3600 RPM as an exercise. – Anindo Ghosh Oct 31 '12 at 15:16
• As Electrical Engineering is based on Physics, you ought to apply 1st principles of rotational momentum to determine the acceleration, braking energy required and most important, safety. Balance, friction, rigidity, stability, vibration, momentum, jerk ( rate of change of acceleration), interial load, weight, are all key mechancial factors before the electrical motor drive requriements can determined and then the electrical motor. The biggest power surge may be for overcoming initial stiction & controlling the "jerk" forces as steady friction losses should be lower. – Tony Stewart Sunnyskyguy EE75 Oct 31 '12 at 15:53
• Automobile turntables may be suitable depending on budget. aluminum based with steel large diameter ring bearings – Tony Stewart Sunnyskyguy EE75 Oct 31 '12 at 16:55