Objective

To experimentally determine the basic properties (inertia and energy capacity) of a flywheel battery. When we give supply to a DC motor it is drawing power from the supply to accelerate and while decelerating, it is able to give power to another dc motor (kept on its side) by dynamic braking. One should be able to observe the first motor accelerate, attain standstill condition and when decelerated, the motor next to it starts rotating.

Background and Theory

A flywheel battery contains a rotating mass that is connected to the shaft of an electric motor/generator. Electrical energy is used to accelerate the rotating mass to a very high speed. Thus, the electrical energy is stored as kinetic energy at high speed. The kinetic energy of the mass can then be converted to electrical energy by using the motor as a generator, with dynamic braking, when the input supply is disconnected and the motor starts acting as generator supplying power to another dc motor.
In order to increase the efficiency, magnetic bearings are normally used to reduce the bearing friction. The entire rotating assembly will reside inside a vacuum chamber in order to eliminate air resistance. As a result, once a flywheel battery is set into motion, it will stay spinning with virtually no losses.

Flywheel Battery Calculations

A flywheel battery can spin at up to 100,000 rpm. The formula for the kinetic energy of a rotating mass is given by
E = ½ * I * ω2,
Where I is the moment of inertia and ω is the angular velocity.
For a thick walled cylinder, such as would be used in a flywheel battery,
I = ½ * m * (r12 + r22),
where
r12 is the inner radius of the cylinder, and
r22 is the outer radius of the cylinder.

The electrical energy stored in the flywheel represents the electrical energy put into the flywheel minus the conversion efficiency from electrical to mechanical energy. Upon converting the mechanical kinetic energy back to electrical energy, there is another conversion loss. Typical electrical to mechanical and mechanical to electrical conversion efficiencies will be between 85% to 95%. This experiment is a small-scale experiment which uses a small brushed DC motor. As a result, the mechanical to electrical efficiency is less – about 60%.

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