For the longest time I have always wondered why they say self contained generation isn't possible.

Has to do with perpetual motion I guess but I still don't understand it.

It seems to me that in theory you could connect the output shaft of a motor to the input shaft of a generator, connect the output wires from the gen. to the input of the motor, give it a good healthy spin and it would go on forever feeding itself.

Why is this impossible?

glenn35

I'm not too sure on this myself and often wondered about it. But I think the fact nothing works at 100% efficency all the time over a period of time the system would fail.
If you loose .1% eff each revolution its only a matter of time but you reach a stall condition.

Yes, it's the fact that the conversion of electrical to mechanical energy and back again is not 100% efficient. Some of the power supplied to the motor, for example, is dissipated as heat in the resistance of the windings. More power is lost in overcoming the inertia and friction of the bearings in the motor and generator.