# Generator efficiency in a vacuum

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1. Mar 12, 2016

### siren314

If you had a generator in a vacuum with a magnetically levitated rotor and power was drawn from the system by induction, use your imagination on the design of the device, but my question is:

Where are the flaws in this concept? (Since perpetual machines are theoretically impossible). In other words, where are frictional forces found in this system and where does conservation of energy come into play? (eddy currents? Leakage?)

To further elaborate, if you gave this device an initial spin (think of it like a spinning-top) and no friction is involved then it would never stop spinning. So again, where's the friction/apposing force in this system?

2. Mar 12, 2016

### Staff: Mentor

You provides your own answer when you said "power was drawn from the system".

That power has to come from some place. Kinetic energy is a likely source.

Instead of fantasizing about perpetual motion, you should get used to using conservation of energy as a useful tool in understanding how things work.

By the way, perpetual motion is a forbidden topic on PF.

3. Mar 12, 2016

### siren314

I am using conservation of energy (and disregarded perpetual motion)...

I'm asking what force would be responsible for the slowing of the rotor? Since the rotors energy would be transferred through induction and no friction would be involved because of the vacuum and the maglev.

4. Mar 12, 2016

### Staff: Mentor

Through induction you said. That is power transfer. Where do you think the induction power comes from?

5. Mar 12, 2016

### siren314

permanent magnets on the rotor induce current in the coils(stator)

6. Mar 12, 2016

### Staff: Mentor

Right. Magnets apply a force!

There are real life magnetic bearing applications in both motors and generators (which are the same device). They bring the efficiency up a couple of percent: perhaps from 96-98%. Internal friction is a fairly insignificant source of energy loss in motors/generators.