# Limit on mechanical & electric energy conversion efficiency

1. Oct 14, 2015

### Le Jergens

I regret to say that I know little of engineering, but, to make a long story short, I'm nagged by two questions about energy conversion efficiency. There is a thread i made titled "giant railguns recycling their own energy in space" in the science fiction & fantasy forum here that explains my curiousity further. Still, I can sum it up in two questions:

1) is there a specific limit on the efficiency of converting mechanical energy into electric energy, and if so,, what is it?

2) Is there also a limit on how much electrical energy can be converted to mechanical energy, and if so, what is it?

2. Oct 15, 2015

### JBA

Theoretically there can be a 100% conversion either way. Finding an actual device that will perform a conversion without any losses is the problem. Any many engineering applications the basic conversion equation is for the idealized conversion and then for its final applications an efficiency factor based upon the observed efficiency of that machine, process etc or similar machines, processes, etc is added to that equation.

3. Oct 15, 2015

### Le Jergens

So there are no *fundamental* physical laws against 100 percent conversion, but great practical barriers to it, correct? Ok, just a little more I wanna know: what is the most energy efficient mechanical-to-electrical engine? And what is the most efficient electrical to mechanical engine? I mean that has ever been physically invented and demonstrated to work in real life. And if, in theory, it could have been made even more efficient, what were the barriers to doing so?

4. Oct 15, 2015

### JBA

That is a question best answered by an electrical engineer with more knowledge of the various methods of generating electricity and electric motors; however, for an example of the basics we can use a standard rotatory electric motor and we will start by placing it in a vacuum to eliminate issues of aerodynamic drag of the spinning parts and make it a permanent magnet induction motor so there are commuter brush friction and contact conduction issues.
With both of those issues resolved then the next issue is the resistance of the wire carrying the current to power the motor and providing the coils that are cut by the spinning permanent magnets on the armature. We should be able to solve that issue by using a super conductor material for that wire (if we are in space we may be able to do that by shielding the motor from any significant sources of radiant heat, i.e. the sun).
Next there is the issue of the armature bearings which need to be frictionless. This might be achieved with magnetic repulsion bearings but I am not knowledge enough about this type of bearing to know if there are any magnetic field drag losses associated with this type of bearing (as far as I know, if any such drag as this exists it very low is their normal applications, but for our application, the drag must be zero).
OK, at this point, the last issue for our 100% efficient motor is actual efficiency of the magnet/spinning current generation or if there is a feedback from the coil current that can generate heat in our magnets and deteriorate their field strength (you maybe able to get some information by googling about this). if no actual issue exists for our magnets then it appears within the limits of my knowledge that we may have a 100% efficient motor. However, we are now faced with providing a 100% efficient power generating supply to run our motor. Since a motor can also be utilized as generator, lets say we use a duplicate of our motor for the needed generator.
Unfortunately that bring us to the need for 100% efficient power source to run our motor. now it may be that by coupling our motor to our generator and using some outside temporary power input we may be able to have a circuit that is self sustaining, i.e. the sought perpetual motion machine. Actually, assuming everything in our system is 100% efficient that is not as ridiculous as oit might sound based upon the fact that perpetual current in small super conductor wire loops has already been demonstrated in laboratories.
But now we have a problem with this arrangement in that we cannot connect anything to our motor shaft to drive some application because that will require extracting energy from the system that is driving its generator and if we do that our system will abruptly come to a full halt without another external source that puts exactly the same energy into the our system as we want to get out.
So, in the end we are now left with developing some other 100% efficient power source to drive our 100% efficient generator to provide power to run our 100% efficient motor. In the end, we might as well just eliminate our whole generator and motor system and use that other 100% efficient other power source to run the equipment we wanted to operate in the first place.
The issue at its heart is that generator/motor systems are not actually energy sources they are only a method for transferring the energy from connected outside resource location to a remotely located energy requirement location.

Last edited: Oct 15, 2015
5. Oct 15, 2015

### Staff: Mentor

Electric motors and generators are basically the same thing and operate up to about 97% efficiency.
Cost. With superconductors and more exotic materials, you can probably get above 98%.

6. Oct 15, 2015

### Staff: Mentor

What is your educational background, so we can better tailor our responses to your questions...

7. Oct 15, 2015

### Staff: Mentor

Are you asking about perpetual motion machines (PMMs)? There are debunking links in the PF rules under "Info" at the top of the page.

8. Oct 16, 2015

### Le Jergens

Ok, quick response before i get back later on. I am possessed of only a high school education (and not a very good one at that), so I know little of science, or any other academic matters. I will say thanks, to everyone who responded, I very much appreciated your input. And just to clarify, I'm really interested in knowing as close as possible the "theoretical upper limit", so to speak, on how efficiency any engine could get under the laws of physics without actually violating them.

I've got more to ask, but am a bit busy now; hopefully I can say the rest within the next day or so.

9. Oct 16, 2015

### Staff: Mentor

The context of Le Jargens question is the idea of a space based magnetic launch system for travel. In an old book they read about a speculative design in which one magnetic launcher accelerates a spaceship and another catches it to decelerate. The supposed advantage of this system being that the second launcher recovers 99% of the energy the former used to accelerate the ship. Thus making a highly efficient transport system that takes littl energy to maintain after the initial energy investment has gone in.

10. Oct 16, 2015

### Staff: Mentor

Ah, thanks Ryan. I don't get over to the SciFi forum much. Thanks for the clarification

11. Oct 27, 2015

### CWatters

Did they say anything about conservation of momentum?

To convert a high percentage of the energy back to electricity I think the "catcher" has to have a lot of mass relative to the spacecraft. That mass also has to be launched?