One classification of perpetual motion machines refers to the particular law of thermodynamics the machines purport to violate:[2]
A perpetual motion machine of the first kind produces work without the input of energy. It thus violates the first law of thermodynamics: the law of conservation of energy.
A perpetual motion machine of the second kind is a machine which spontaneously converts thermal energy into mechanical work. When the thermal energy is equivalent to the work done, this does not violate the law of conservation of energy. However it does violate the more subtle second law of thermodynamics (see also entropy). The signature of a perpetual motion machine of the second kind is that there is only one heat reservoir involved, which is being spontaneously cooled without involving a transfer of heat to a cooler reservoir. This conversion of heat into useful work, without any side effect, is impossible, according to the second law of thermodynamics.
A more obscure category is a perpetual motion machine of the third kind, usually (but not always)[3] defined as one that completely eliminates friction and other dissipative forces, to maintain motion forever (due to its mass inertia). Third in this case refers solely to the position in the above classification scheme, not the third law of thermodynamics. Although it is impossible to make such a machine, as dissipation can never be 100% eliminated in a mechanical system, it is nevertheless possible to get very close to this ideal (see examples in the Low Friction section). Such a machine would not serve as a source of energy but would have utility as a perpetual energy storage device.
The USPTO has granted a few patents for motors that are claimed to run without net energy input. Some of these are:
Howard R. Johnson, U.S. Patent 4,151,431
USPatent4151431-1.
Robert George Adams UK patent GB2282708 "Electrical motor-generator" 12 April 1995.
http://www.intalek.com/Index/Projects/Patents/GB2282708.pdf
Johnson, Howard R., U.S. Patent 4,151,431 "Permanent magnet motor", April 24, 1979[16]
Baker, Daniel, U.S. Patent 4,074,153 "Magnetic propulsion device", February 14, 1978
Hartman; Emil T., U.S. Patent 4,215,330 "Permanent magnet propulsion system", December 20, 1977 (this device is related to the Simple Magnetic Overunity Toy (SMOT)),
Flynn; Charles J., U.S. Patent 6,246,561 "Methods for controlling the path of magnetic flux from a permanent magnet and devices incorporating the same", July 31, 1998
Patrick, et al., U.S. Patent 6,362,718 "Motionless electromagnetic generator", March 26, 2002
Green, Willie A., U.S. Patent 6,526,925 "Piston Driven Rotary Engine", March 4, 2003 "Fluid driven device utilizing a leveraged system with minimal displacement"
Goldenblum, Halm, U.S. Patent 6,962,052 "Energy generation mechanism, device and system", November 8, 2005 "A chamber with a partition which lets gas molecules flow one way and not the other. The pressure which builds up on one side of the partition is used to drive a generator."
Flynn, Joe, U.S. Patent 6,246,561 "Methods for controlling the path of magnetic flux from a permanent magnet and devices incorporating the same", June 12, 2001
Gates; Glenn A., U.S. Patent 6,523,646 "Spring driven apparatus", February 23, 2003 "Energy is stored in the springs and power is generated by way of the various forces which cause the springs to wind and unwind."
McQueen; Jesse, U.S. Patent 7,095,126 "Internal energy generating power source", August 22, 2006 "An external power source such as a battery is used to initially supply power to start an alternator and generator. Once the system has started it is not necessary for the battery to supply power to the system. The battery can then be disconnected. The alternator and electric motor work in combination to generate electrical power." Examiners: Schuberg, Darren ; Mohandesi, Iraj A.
Haisch, et al. U.S. Patent 7,379,286 "Quantum vacuum energy extraction", May 27, 2008 "[...] converting energy from the electromagnetic quantum vacuum available at any point in the universe to usable energy in the form of heat, electricity, mechanical energy or other forms of power. [...] When atoms enter into suitable micro Casimir cavities a decrease in the orbital energies of electrons in atoms will thus occur. Such energy will be captured in the claimed devices. Upon emergence form such micro Casimir cavities the atoms will be re-energized by the ambient electromagnetic quantum vacuum. [...] process is also consistent with the conservation of energy in that all usable energy does come at the expense of the energy content of the electromagnetic quantum vacuum."
Low friction
In flywheel energy storage, "modern flywheels can have a zero-load rundown time measurable in years."[24]
Once spun up, objects in the vacuum of space—stars, black holes, planets, moons, spin-stabilized satellites, etc.—continue spinning almost indefinitely with no further energy input. Tide on Earth is dissipating the gravitational energy of the Moon/Earth system at an average rate of about 3.75 terawatts.[25][26]
In certain quantum-mechanical systems (such as superfluidity and superconductivity), dissipation-free "motion" is possible.
http://en.wikipedia.org/wiki/Perpetual_motion