# I Flywheel inertia

1. Feb 2, 2017

### Emna

Hi,
I couldn't find how to calculate this flywheel inertia , any help please?! I know that I=1/2(mass*radius²) but it seams more complex then that!

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2. Feb 2, 2017

### Staff: Mentor

Hi Emna.

Is this a homework question? If so, then I'm sure you were provided with more details than you have shown here.

If not a homework question, then how has it arisen?

3. Feb 2, 2017

### Emna

I'm trying to work on a project, a system that helps minimize the electricity consumption . for example using a 1kw motor , few belt pulleys, flywheel and a generator you can get a 4 to 5KW of electricity for the daily consumption.

4. Feb 2, 2017

### Nidum

Not going to work . You can't get more power out of a system than you are putting in . Generally you get less useful power out than you put in .

5. Feb 2, 2017

### malemdk

To find the Inertia of this flywheel model it in 3D CAD software (I think you have already done)where from you get easily what you want ie Inertia
You cannot get more power from than the input power, it's fundamental law of nature , law of energy conservation.
Efficiency of any machine always less than 100%,so the output from machine will be less than your input

6. Feb 2, 2017

### Khashishi

Well now, that depends on exactly what the Emna means to do. If you only need 5kW for a short time, you can use the 1kW to spin up the flywheel, then turn on the generator only when you need the burst of power. You can certainly get more power out than you put in; just not continuously.

I=1/2(mass*radius²) is correct for a point mass or ring. For a 3D solid object, you can think of it as being built out of a bunch of point masses. Sum or integrate over each point that makes up the object.
$I = \frac{1}{2} \int \rho r^2 dV$ where $\rho$ is the density (as a function of position).

7. Feb 2, 2017

### Staff: Mentor

That equation only works for a solid, uniform wheel. For a more complicated shape you may be able to take it in simple pieces and calculate the moment of initial of them individually.

8. Feb 2, 2017

### Emna

you can increase the angular speed by adding belt pulley and the flywheel will increase the torque as much as you add mass to it!!

9. Feb 2, 2017

### Emna

well i'm going to recycle a flywheel of a bicycle, it's made of some kind of plastic an add it some mass like it shown on the picture, i don't know how to find the density of the existed flywheel!!

10. Feb 2, 2017

### Emna

I need to do calculate it as a proof for the jury members, and also i need it so i see how much the torque became and depending on the torque needed for the generator i ill know how much mass i should add to the flywheel so the system works perfectly

11. Feb 2, 2017

### Emna

if any one have a pdf support that i can read and that might help in my research would you please send it on this email < amaniguefrech@gmail.com > thank you.

12. Feb 2, 2017

### Staff: Mentor

Adding mass to a flywheel does not increase the torque on a constant speed system. A flywheel doesn't have any effect on a constant speed system besides adding friction and loss.

13. Feb 2, 2017

### Staff: Mentor

By jury, you mean people grading you for a school project? Please stop and pick another project because if you present this one with a wrong conclusion you will likely get a bad grade.

14. Feb 2, 2017

### Emna

I'm looking for another opportunity too I'm not liking it at all, i just got an answer from an industry today to work with them on a project , I have an interview on Saturday, hope it's an interesting one. Pray for me :) thank you xD

Last edited: Feb 2, 2017
15. Feb 2, 2017

### CWatters

Can you explain how the system is intended to work?

16. Feb 2, 2017

### Emna

the motor has an angular speed of 1500rpm and 1kw power, the belt pulley will make the generator turn 4 times the motor and the flywheel is supposed to increase the torque as much as you add it mass. i need to study the system to make the generator provides 4kw of power

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17. Feb 2, 2017

### Khashishi

Is the 4kW just for a short burst? You haven't said anything about that, which makes me worried that you are ignoring the results you don't like.

18. Feb 2, 2017

### Emna

as long as the system is plugged in it must provide 4kw , if the flywheel helps stocking enough energy for at least 15min so the motor be shut down for that long it would be better.

19. Feb 2, 2017

### Khashishi

15 minutes of 4kW is a LOT of energy. I think you need to reconsider the feasibility of the project. There's no way a bicycle wheel will store that.

20. Feb 2, 2017

### Emna

the belt pulley will do all the work , and it's ok if the flywheel don't stock that energy , it at least ill smooth the energy variation , all i need id how to calculate it's inertia while it has a complicated shape and i got that i should calculate each part of it alone , there's no other way.