-9
\$\begingroup\$

OK so here is something I have been curious about.

Let's say we connect a single electric motor (driven by e.g. an automobile battery) to 4 different generators (or alternators).

Would it be possible to get more electricity out of the system than what is put in to drive the motor? If not, what are the practical reasons why this would not be possible?

(Please no religious/philosophical references to the laws of thermodynamics or statements regarding perpetuum mobiles, unless you also provide the practical/pragmatic answer to the question.)

\$\endgroup\$
  • 1
    \$\begingroup\$ Conservation of Energy \$\endgroup\$ – Majenko Jan 11 '15 at 17:26
  • 14
    \$\begingroup\$ The laws of thermodynamics are neither philosophical nor religious, but simple facts. \$\endgroup\$ – Majenko Jan 11 '15 at 17:27
  • \$\begingroup\$ @Majenko, LOL, they also do not answer the question in a practical way. \$\endgroup\$ – coderworks Jan 11 '15 at 17:27
  • 7
    \$\begingroup\$ Yes they do, if you take the time to read and understand them. \$\endgroup\$ – Majenko Jan 11 '15 at 17:28
  • 1
    \$\begingroup\$ codeworks, If it would be possible, it would already be done. Energy can be converted, but not created or destroyed. The biggest source of energy is the sun. Trying to harness some of it would be more realistic. \$\endgroup\$ – sparky Al Jan 12 '15 at 0:27
13
\$\begingroup\$

Perhaps you're under the impression that generators don't put any load on the motor, so you can spin them for free. This is not true. Any current drawn from a generator will be converted into a mechanical load. A motor is a generator and a generator is a motor. The difference is in which direction the power's flowing.

\$\endgroup\$
8
\$\begingroup\$

The law of Conservation of Energy states:

Energy can be neither created nor destroyed, but can change form, for instance chemical energy can be converted to kinetic energy in the explosion of a stick of dynamite.

You put X amount of energy in, and you have to get X amount out in whatever form.

In your example, you put in X amount of electrical energy. That gets converted into a combination of kinetic energy (movement of the motor), heat energy (the friction of the rotor bearings), and noise. So the kinetic energy is obviously going to be less than the electrical energy you put in.

You then pass that kinetic energy into 4 generators. So it gets then converted into more heat energy, noise, and electrical energy. Again, that's a split.

In a completely perfect system, where there is no friction, and no sound, then you possibly get 100% of the electrical energy converted to kinetic, then 100% of the kinetic converted back to electrical (in 4 quarters, one for each generator), but that is currently not actually possible to achieve in the real world.

So you will never get anywhere near as much electrical energy out of the system as you put in.

Let's illustrate it more by adding some numbers.

Say each time you convert energy you get 5% lost as heat and 5% lost as noise.

You put in 100W. 10W of that goes, 90W gets passed to the generators.

10% of that is then lost by the generators, so 81W is what you'd get out.

In all you'd have generated 81W of electricity, 9.5W of heat, and 9.5W of noise.

The whole lot adds up to the 100W you put in.

\$\endgroup\$
5
\$\begingroup\$

Quite simply, because all of the devices mentioned run at less than 100% efficiency, and efficiencies are multiplicative.

For example, a system with a motor running at 99% efficiency driving a generator which runs at 99% efficiency will run at 98.01% efficiency from input to output. That means that for 100 watts out of the mains and into the system, the system will output 98.01 watts of electricity and 1.99 watts of heat.

If you add more generators it just gets worse because the motor has to overcome all of their losses and, instead of putting out extra electricity, the system puts out the extra losses as heat.

\$\endgroup\$
4
\$\begingroup\$

Each generator you add makes the motor consume more electricity (by making the shaft harder to turn, and thus 'loading' the motor), and that extra amount will always exceed the amount the added generator can produce.

\$\endgroup\$

protected by Dave Tweed Oct 7 '17 at 14:46

Thank you for your interest in this question. Because it has attracted low-quality or spam answers that had to be removed, posting an answer now requires 10 reputation on this site (the association bonus does not count).

Would you like to answer one of these unanswered questions instead?

Not the answer you're looking for? Browse other questions tagged or ask your own question.