# Variable-inertia flywheel

1. Apr 3, 2015

### jimgram

If you model a variable-inertia flywheel connected to a load inertia you have fundamentally a single spinning mass with the ability to vary the mass moment of inertia. The total energy of this spinning mass is constant, so when you vary the inertia, the velocity will change in order to satisfy a constant momentum. In so doing, the energy will change. The change will exactly equal the work required to change the position of the variable position mass (flyweight). Therefore, any energy change in the load inertia will be a direct result of the work input. In other words, if you want to transfer 'X' joules from the flywheel to the load, you will need to input 'X' joules to re-position the flyweights. Is this correct?

2. Apr 3, 2015

### Staff: Mentor

Is it constant or does it change?

Overall energy is conserved, I just don't understand where and how you want to transfer energy.

3. Apr 4, 2015

### jimgram

The fundamental question is: Can we use a variable-inertia flywheel as a power source (e.g. to accelerate a vehicle). I phrased the question poorly. The answer to your question is: I want to transfer energy from the VIF (variable-inertia flywheel) to the load inertia. I understand that the total energy in the system described cannot change without an input of work and that input is required to re-position the flyweights of the VIF. But is there any way that the energy change to the load can exceed the work input?

4. Apr 4, 2015

### Staff: Mentor

You can, but I don't see the point.
You can have masses moving outwards (slowing the rotation), driving something that extracts energy from that motion.
Which work input? I'm not a clairvoyant.

5. Apr 4, 2015

### jimgram

Work is required to re-position flyweights in a variable-inertia flywheel. Positive work (I.E. work in) when inertia is decreased and negative work (I.E. work out) when the moment of inertia is increased. I guess I should have made that clearer.

Flywheels can be used to capture and store kinetic energy in a regenerative braking system for automobiles. Using that energy to re-accelerate the vehicle is not easily accomplished.

6. Apr 8, 2015

### timthereaper

I'm not seeing the connection from the speed of the flywheel to powering a device. Variably changing the moment of inertia will change the speed, but the energy stored will be the same. You'll have to do work on the flywheel to move the weights in toward the axis and the flywheel energy could be used to move the weights back out away from the axis, but you still haven't done any useful work there.

About getting the energy back out, there are a few ways to get it back out, both mechanical and electrical. I don't agree it's "not easily accomplished".