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Determination of Moment of Inertia & Radius of Gyration of Flywheel (Part – I) 

Spoked flywheel

Simple flywheel in motion.

A flywheel is a heavy rotating disk used as a storage device for kinetic energy. They come as an alternative energy storage device. Flywheels resist changes in their rotational speed, which helps steady the rotation of the shaft when an uneven torque is exerted on it by its power source such as a piston-based, (reciprocating) engine, or when the load placed on it is intermittent (such as a piston-based pump). Flywheels can also be used by small motors to store up energy over a long period of time and then release it over a shorter period of time, temporarily magnifying its power output for that brief period. Recently, flywheels have become the subject of extensive research as power storage devices; see flywheel energy storage.

A momentum wheel is a type of flywheel useful in satellite pointing operations, in which the flywheels are used to point the satellite's instruments in the correct directions without the use of thrusters.

The kinetic energy of a rotating flywheel is

where I is the moment of inertia of the mass about the center of rotation and ω (omega) is the angular velocity in radian units. A flywheel is more effective when its inertia is larger, as when its mass is located farther from the center of rotation either due to a more massive rim or due to a larger diameter. Note the similarity of the above formula to the kinetic energy formula E = mv2/2, where linear velocity v is comparable to the rotational velocity, and the mass is comparable to the rotational inertia.

The flywheel has been used since ancient times, the most common traditional example being the potter's wheel. In the Industrial Revolution, James Watt contributed to the development of the flywheel in the steam engine, and his contemporary James Pickard used a flywheel combined with a crank to transform reciprocating into rotary motion.

NASA G2 Flywheel

Flywheel Energy Storage (FES) works by accelerating a rotor to a very high speed and maintaining the energy in the system as inertial energy. Commercially available FES systems are used for small uninterruptible power systems. The rotors normally operate at 4000 RPM or less and are made of metal. Advanced flywheels are made of high strength carbon-composite filaments that spin at speeds from 20,000-100,000 RPM in a vacuum enclosure. Magnetic bearings are necessary since friction in conventional mechanical bearings are directly proportional to speed and at such speeds, it would be high enough to cause such a great energy loss as to defeat its purpose of energy storage. Quick charging is done in less than 15 minutes. Long lifetimes of most flywheels, plus high energy densities (~ 130 Wh/kg) and large maximum power outputs are positive attributes. The energy efficiency (ratio of energy out per energy in) of flywheels can be as high as 90%. Since FES can store and release power quickly, they have found a niche providing pulsed power.

Compiled by Samar Das