# B 100kg flywheel torque or HP?

1. May 1, 2017

### kashif ali

Dear friends,
i want to know if flywheel (30inch) 96kg weight running on 1400rpm then how much power can save in hp or tarque? and how much power required to drive this wheel?
regards
kashif

2. May 1, 2017

### Staff: Mentor

The power required to keep a flywheel spinning at a constant speed depends on the amount of friction in the system, not on the size or weight of the flywheel.

3. May 1, 2017

### XZ923

You would have to define the magnetic properties of your flywheel. For example, it's going to require significantly more energy to drive the flywheel if it's made with alnico magnets vs. N-52 grade neodymium magnets.

4. May 1, 2017

### nasu

Why do you think that magnetism has anything to do with the question in OP?

5. May 1, 2017

### RonL

I got the impression that XZ923 was trying to be a little bit snarky, Nugatory's answer was correct but a little dismissive, but then the OP ask a question that did not have much substance.
I think it would surprise a few people as to how little power it would take to sustain the flywheel @ 1400 rpm compared to how much power it would take to increase the speed to 1500 rpm in some short time frame. but we need a little more from Kashif ali ?

6. May 1, 2017

### litup

Maybe his question was incomplete, like 'how much energy or horsepower does it take to go from 0 RPM to 1400 RPM of the 30 inch 96Kg wheel. My guess is that is wrong also, he specifies 'Running on 1400 RPM' so he probably does not realize the power needed to keep it at 1400 relates to the friction of the system.

For instance, if the wheel was spinning in a total vacuum (of course impossible even if it was halfway to Andromeda galaxy) would theoretically be zero energy, it would rotate till the end of the universe with no further power input needed.

7. May 1, 2017

### Baluncore

Welcome to physics and PF.
Energy can be stored in a flywheel. Energy is measured in joules.
Power is the rate of flow of energy. Power is measured in watt, kW or HP.
Torque is a force. Power = torque * angular velocity.

https://en.wikipedia.org/wiki/Kinetic_energy#Rotating_bodies
So how much kinetic energy can be stored in the flywheel; assuming it holds together at 1400 RPM?
KE = ½ * I * ω2. Where I is the moment of inertia and ω is the angular velocity.

“Flywheel (30inch) 96kg weight running on 1400rpm”.
Assume that the 30” is the flywheel diameter. Radius = 30”/2 = 15” = 0.381 m
Angular velocity; ω = 1400 RPM = 23.333 rev/sec = 146.6 rad/sec

Assume all mass is in the rim; I = m * r2.
Then moment of inertia, I = 96 * 0.3812 = 13.935

KE = ½ * 13.935 * 146.622 = 149742. joules.

8. May 2, 2017

### RonL

Thanks Baluncore for the post.
I would like to ask about the energy to increase speed to 1500 rpm, as I understand the KE would increase exponentially and at 1500 rpm would the joules used to increase the speed be equal to that stored energy ?

9. May 2, 2017

### Baluncore

Yes, you are on the right track. You can always count on energy.
The energy needed is not strictly “exponential”, it is a square law as shown by the ω2 in the KE equation. So the energy needed to accelerate from 1400 RPM to 1500 RPM will be more than that needed to accelerate from zero to 100 RPM.

Conservation of Energy is the principle that says the amount of energy you must provide to accelerate between two states will be the difference in energy between the two states. You can get that energy out again while it is slowing down.
If you put 1500 RPM into the same equations you can work out the total KE in the faster state. Then you can work out the difference in joules by subtraction.
The time it will take to accelerate the flywheel will depend on the rate you can deliver that energy. That is where the power in watts of the motor comes into the game.

To understand this you must be a mathematician or play with the numbers. It would probably be a good idea now to use a spread sheet or your favorite computer program to tabulate the values of KE at different speeds.

10. May 3, 2017

### RonL

I'm one of those people that spent a lifetime stooped over picking up pennies and then after retirement stood up and realized there have always been big bills swirling around at head level.
Never mastered much math but learned a bit in the mechanical area, cheating energy is not possible, but I think there is still a few ways to manipulate the carriers.
I'm a believer in the compressed air use, but think much of the research takes it to an unsafe extreme. I have ideas of that large diameter flywheel operating a little like a sliding vane air motor.
Baluncore, your post have help more than you might think
To say more, I think a new thread might be required.