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Feed Back Generator 

A Feedback Generator CIrcuitA common example of a perpetual motion machine that violates the first law of thermodynamics is a circular feedback generator.

An example is a battery driving an electric motor which then drives a generator that charges the battery. It is very easy to construct such a machine from readily available components and they seem to work. It is often reported that they have run for days or even weeks. The typical arrangement is shown here. For more explanations of energy and power click here
 
This machine cannot be a perpetual motion machine because of the first law of thermodynamics. It does not have a renewable external source of energy and because of the second law it will eventually stop. So how is it that these machines run for a long time?
 
Initially the machine is supplied with a good store of energy in the battery, and motors and generators can be quite efficient with relatively low losses.
 
This initial energy was obtained from somewhere else and is the equivalent to the initial push for the unbalanced wheel described in the article about them.
At the outset there is almost no load on the generator because the battery is fully charged. So according to the second law the only energy being lost is to the friction of the bearings in the machines. This can be quite small for a well constructed machine and therefore the amount of energy being taken from the battery is also quite small. If you think of this loss as being equivalent to the energy taken by a small warning light in your car then you can see how a large car battery could keep that light on, or the motor turning for many days.
 
Gradually the battery will be discharged and soon the load on the generator will increase as it tries to recharge the battery. This in turn will increase the load on the motor which will discharge the battery even faster as it needs more power from the battery. As the motor and generator combination is unlikely to be more than about 80% efficient this means that 20% of the energy being taken from the battery is being lost continuously. Clearly this cannot go on for very long and soon the system will come to a halt. This is an illustration mainly of the second law of thermodynamics.
 
If we put an additional load on the motor and try and get it to do useful work as well, for example to drive a pump, then the drain on the battery will be even higher and it will stop all the sooner.
 
The whole system can only keep going as long as there is some energy stored in the battery. When that is used up either by internal inefficiency or by doing external work the machine will stop. Because the generator can never turn all the mechanical power from the motor back into electrical energy the battery will always go flat. To start it again the battery must be recharged from an external source of energy. This is an example of the first law of thermodynamics.