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# Kinetic Generator

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#### WeirdWizard

##### New Member
Hi guys

I am currently experimenting with a project to generate a certain amount of Kinetic energy with a flywheel driven by a small motor to drive a 10KVA/8KW 2pole generator alternator. How do I calculate the flywheel size? For example, I want to build up momentum slowly with a 0.75KW or a 1.1KW 2-pole motor, then when the flywheel reached it's required RPM, I want to be able to run the alternator at 80% capacity at least. What I want to know is how do I calculate the size and weight of the flywheel?

Hi guys

I am currently experimenting with a project to generate a certain amount of Kinetic energy with a flywheel driven by a small motor to drive a 10KVA/8KW 2pole generator alternator. How do I calculate the flywheel size? For example, I want to build up momentum slowly with a 0.75KW or a 1.1KW 2-pole motor, then when the flywheel reached it's required RPM, I want to be able to run the alternator at 80% capacity at least. What I want to know is how do I calculate the size and weight of the flywheel?

Take the number of minutes you want it to generate and multiply it by a huge number to get the answer. Doesn't sound practical to me.

Mike.

The kinetic energy of a flywheel is 0.25 * mass * rim velocity * rim velocity, assuming the flywheel is the same thickness all over.

If you have the flywheel start at 3600 rpm, and is 1m diameter, weighing 100 kg, the rim velocity is 188.5 m/s2, so the energy is about 888 kJ. If you can run the generator down to half speed, you will get 8 kw out for about 80 seconds.

On a 1m diameter flywheel going that fast, the centripetal acceleration is about 7000g, so you will have a force of around 500 tonnes trying to split your flywheel. It would be a serious engineering project.

It all depends on how long you want to run the generator for. If it's for fractions of a second, it's not a problem.

Perhaps I'm misreading this?, but it sounds to me like you're somehow expecting the flywheel to 'magically' increase the amount of power you're putting in?.

In reality all you'll get out is less than you put in, so as soon as you connect a load to the output of your alternator the flywheel will start to slow down.

So, VERY crudely, if your 1KW motor takes ten minutes to get to speed, connecting a 10KW load will stop it in about 1 minute.

Perhaps I'm misreading this?, but it sounds to me like you're somehow expecting the flywheel to 'magically' increase the amount of power you're putting in?.

But that can't be! It's all over there at Youtube, and they don't lie!

On a 1m diameter flywheel going that fast, the centripetal acceleration is about 7000g, so you will have a force of around 500 tonnes trying to split your flywheel. It would be a serious engineering project.

But couldn't we harness the energy from the explosion of the flywheel?

I ate some beans yesterday...took about 3 minutes to eat and, subsequently, I produced at least 40 minutes of kinetic energy...is this phenomenon related to the question?

Look guys. No sarcasm is needed. I am not a Mechanical Engineer, Neither have I studied in these fields. I am merely experimenting. Thank you for being mature about your response.

You don´t need to be a mechanical engineer to understand basic points of high school physics, like conservation of energy, but nevertheless, can you explain a bit more about what you are trying to achieve?

so, let's crunch the numbers in the original question. your load is a 10 kVA alternator, and your input is about a 1 kVA motor, and you want to store energy in the flywheel. you didn't mention whether the drive motor remains connected to the flywheel after the alternator is engaged. in either case, the flywheel and alternator will spin down, so you will only get 8 kVA out of the alternator for a very short period of time, a little bit longer with the drive motor still connected, but it will still spin down. with the drive motor still connected, it will spin down to the point where 1 kVA in will deliver about 0.7 to 0.8 kVA from the alternator. with the motor disconnected, it will obviously run down and stop. you have several ways energy leaks out of the system, friction in the motor and alternator (bearings and such), aerodynamic losses in the motor, alternator, and especially the flywheel, and resistance losses in the motor and alternator windings.

while you can store a lot of kinetic energy in the flywheel, and the bigger the flywheel the better, the fact is that once you begin to use that energy, the only way of maintaining the flywheel's kinetic energy is for the input energy to exceed the energy delivered to the load.
Diver300 posted a formula that should be useful, as well as a valid warning that the flywheel material has to be able to withstand the g-forces at the rim. i have seen videos of flywheels failing, some of them almost explosively. there's also a Mythbusters episode where they test the failure of CD and DVD disks at high speeds, and there were at least a couple of explosive failures at something like 50,000 rpm

Look guys. No sarcasm is needed. I am not a Mechanical Engineer, Neither have I studied in these fields. I am merely experimenting. Thank you for being mature about your response.

A flywheel stores energy that is all it does. If you had a flywheel that could provide 8KW it would only last about 2 seconds unless you have a power source that can produce enough power to keep the flywheel turning at a continuous speed.

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