Why Can't Self-Powered Motors Work Despite Their Appeal?

[Paddy]

I know this can't be done, else everyone would be doing it, this question is more of a "why can't it be done? and what are the difficulties that make this impossible?" rather than a "Why is no one doing it?".

We have a motor, naturally we would connect it to our mains and draw power, the shaft of this motor is connected to a load through a belt.

What would happen if we connect a generator of similar specifications to this belt therefore generating the power which would be used by the motor? This generator would be connected to a battery recharging this battery which in turn powers the motor, at a first glance, it would seem that this system is self powered and therefore could run indefinitely and without consuming power. Ideally we could manufacture self powered motors (in the same casing) so we would not have to worry about designing such systems and finding compatible motor/generator.

What I think would be the problem.
- I understand there would be losses in the system and power factor correction would be a very important concern here. Could this not easily be accounted for in the design?
- Maybe its got something to do with the fact that all kinetic energy is soaked up by the generator therefore making this system useless unless you want to power a fan or something light.

Me and my friend had considered the possibilities and we strongly feel like we are missing one of the most obvious and important points.

 

[sophiecentaur]

But the system would actually use up energy. Every energy transfer involves some loss. Your suggestion is yet another version of the 'perpetual motion' idea. People try to get around the problem by suggesting gears etc, to speed up the generator but that involves even more losses.
There have been countless mechanical systems proposed with weights moving in and out of rotating wheels - to compensate for the friction forces - but they all involve the problem that the energy you get out is less than the energy you put in - however you work it out.
It's fine as a talking point, over a few beers, but never - NEVER - put any actual money into such a scheme, however convincing the salesman may be.
I remember, in the late 1950s, my Grandad came up with the same suggestion but my Dad put him right. I always remembered that lesson.

 

[russ_watters]

Why would you expect a generator to generate more energy than the motor powering it creates?

 

[Mike_In_Plano]

I think that most people don't understand that generators require more than rotation, they require torque. A loaded generator becomes more and more difficult to turn as the power output increases. This in turn means that a driving motor requires more and more power to keep the generator turning.

Overall, the motor losses power doing its job, the generator loses power doing its job, and less power comes out than was put in.

Interestingly, in the 30's - 60's, there was a device out called a dynemotor that did the job described. It was commonly used to produce high voltage from car batteries so that transmitters could be operated portably. As I recall, the efficiency of these devices was less than 50%.

So, yes, some one thought of this long ago and even had a practical application.

- Mike

 

[chayced]

I think that most people don't understand that generators require more than rotation, they require torque. A loaded generator becomes more and more difficult to turn as the power output increases. This in turn means that a driving motor requires more and more power to keep the generator turning.

This also applies to motors. The more torque the motor is supplying the more power it needs. Countless 'free energy' demonstrations involve using a motor to prove power output. A 1kw motor powered by a 500 watt machine seems to be free energy, until you realize that the motor is rated at 1kw at full load, and unloaded it draws considerably less.

 

[sophiecentaur]

We all know Work (= Energy expended) = Force X Distance moved in the direction of the force. So:
Power = Torque X Angular Velocity.
Just the same as
Power = Force X Speed.

All you can achieve with gears or levers is to alter the ratio of the two quantities involved. Furthermore, any gear or lever will involve more power loss.

 

[Shinto]

The simple act of running your device will result in heating losses as well in the stator/rotors/wires etc.. Although they might be considered trivial the losses would add up over time.