# How does a flywheel generate electricity at a constant voltage?

I believe I am missing something simple here. My question concerns flywheel energy storage.

Say we have stored some amount of energy in a spinning flywheel. The flywheel is attached to a generator. So the flywheel (and therefore the generator) would be spinning rapidly at first, and then gradually slow down as the rotational energy is converted into electrical energy.

It is my understanding that the induced voltage decreases as the angular velocity of the generator coil decreases. So how is it that we can get electricity at a constant voltage from this setup? Am I misunderstanding how the generator works?

• When you say the generator, what do you mean? A specific instance of a generator? -located where? Or maybe specific type of generator? Have you read the wiki page? en.wikipedia.org/wiki/Electric_generator – hpekristiansen Jan 30 '14 at 22:05
• As background, you should understand what a synchronous machine is to begin with. Then understand that a flywheel generator is obviously not a synchronous machine. That means that other energy conversion equipment obviously has to be the go-between from the generator to the grid. Presumably, that's where things get complicated. However, a voltage regulator is a part of it all. It's like "gearing" of electrical voltage. Naturally, the AC phases make it more nasty, but those are the basics. – Alan Rominger Jan 31 '14 at 1:36

The problem with maintaining a constant voltage is not restricted to flywheels, but is important for all kinds of generators. Power plants have to generate a constant voltage independent of the momentary load on the grid, yet this is not how a plain electric generator works - it will spin slower under load, just as your cordless screwdriver does when you use it on dense materials.

What you need is some kind of voltage regulator, just as you need one in your car. How you would implement one in a flywheel electric energy storage will then be an engineering problem - you'd have to think about how to regulate the angular velocity so that the output voltage suits your needs.

Depending on the type of electrical device you want to power you may not have to pay that much attention to the voltage decline, e.g. those cheapo batteries that no longer power your digital camera will still work perfectly in your TV's remote.

Generators often have two sets of windings - one arranged on the stator (fixed) and the other on the rotor (spinning). One is chosen/designed to be the excitor (could be either the rotor or the stator); the excitor is fed a small amount of electrical power to maintain a magnetic field. The other winding generates the output as it moves through the field generated by the excitor (or the excitor's field moves around it). Varying the current in the excitor will vary the strength of its magnetic field and consequently the generated output voltage. So, as the flywheel slows down, the excitor current can be increased as needed to keep the output voltage constant. In this sort of arrangement, an electronic voltage regulator can be used to control the input to the excitor in response to output voltage.