# Momentum of Flywheel

## Main Question or Discussion Point

I am considering using a flywheel to power a machine for an indefinite period of time. I intend to bring the flywheel up to operating speed by manual means. Once the flywheel is rotating at operating speed, a motor will be used to keep the flywheel rotating. In order to maintain the operating speed of the flywheel for an indefinite period of time, will the motor be required to supply as much energy as it took to get the flywheel up to speed, or will the energy requirement placed upon the motor be reduced somewhat by the momentum of the already rotating flywheel? Thanks!

## Answers and Replies

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The motor needs to supply only energy lost due to flywheel bearing friction and air drag losses. Big flywheels sometimes use special bearings and vacuum to minimize losses. If you extract power from the flywheel while the motor is driving it, the motor will also need to supply that power.

russ_watters
Mentor
The motor needs to supply only energy lost due to flywheel bearing friction and air drag losses. Big flywheels sometimes use special bearings and vacuum to minimize losses. If you extract power from the flywheel while the motor is driving it, the motor will also need to supply that power.
In other words, unless the load is intermittent, you gain nothing by adding a flywheel to the equation.

Redbelly98
Staff Emeritus
Science Advisor
Homework Helper
Welcome to PF grumpyoldman

The motor must supply as much power as is being lost due to friction, air resistance, and any load attached to the flywheel.

EDIT: I should refresh my screen more often before responding!

So, once the wheel is in motion, the motor needs only to overcome the effects of friction, air resistance and whatever constant load is placed upon the flywheel? So if the flywheel is driving a generator supplying enough power for the motor, the system should run indefinitely?

Redbelly98
Staff Emeritus
Science Advisor
Homework Helper
No. The generator would represent a load on the flywheel. So an equivalent amount of power would have to get delivered to the flywheel to keep it running.

EDIT:
Moreover, if that power is not supplied to the flywheel, it will slow down and eventually stop.

EDIT:
This statement is correct:
grumpyoldman said:
So, once the wheel is in motion, the motor needs only to overcome the effects of friction, air resistance and whatever constant load is placed upon the flywheel?

russ_watters
Mentor
So, once the wheel is in motion, the motor needs only to overcome the effects of friction, air resistance and whatever constant load is placed upon the flywheel? So if the flywheel is driving a generator supplying enough power for the motor, the system should run indefinitely?
The generator is one of those "constant loads".... So the generator provides almost enough power to power the motor...

Once the flywheel was manually brought up to operating speed, if a 1 HP, 12V, 80 amp DC motor could keep the flywheel running fast enough to spin a 300 amp alternator in order to charge the 12V battery supplying power to the motor, wouldn't the system run indefinitely?

http://en.wikipedia.org/wiki/History_of_perpetual_motion_machines" [Broken]
In 1917, Garabed T. K. Giragossian is claimed, reportedly fraudulently, to have developed a free energy machine. Supposedly involved in a conspiracy, Woodrow Wilson signed a resolution offering him protection. The device was a giant flywheel that was charged up with energy slowly and put out a lot of energy for just a second.

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russ_watters
Mentor
Once the flywheel was manually brought up to operating speed, if a 1 HP, 12V, 80 amp DC motor could keep the flywheel running fast enough to spin a 300 amp alternator in order to charge the 12V battery supplying power to the motor, wouldn't the system run indefinitely?
If? If my mother was a man, she'd be my father.

1 HP is 746 watts. A 12V, 80A motor is 960 watts. A 300A, 12V alternator generates 3600 watts. Obviously, since these numbers are not equal, there is a violation of conservation of energy there. A 1 hp motor cannot power a 3600 watt alternator, it can only power a 746 watt alternator.

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No wonder I'm so grumpy . . .