power source. The three-phase induction is now used for the vast majority of commercial applications. Mikhail Dolivo-Dobrovolsky claimed that Tesla's motor was not practical because of two-phase pulsations, which prompted him to persist in his three-phase work. The General Electric Company began developing three-phase induction motors in 1891. By 1896, General Electric and Westinghouse signed a cross-licensing agreement for the bar-winding-rotor design, later called the squirrel-cage rotor. Induction motor improvements flowing from these inventions and innovations were such that a 100-horsepower induction motor currently has the same mounting dimensions as a 7.5-horsepower motor in 1897. Twenty-first century In 2022, electric motor sales were estimated to be 800 million units, increasing by 10% annually. Electric motors consume ~50% of the world's electricity. Components The two mechanical parts of an electric motor are the rotor, which moves, and the stator, which does not. It also includes two electrical parts, a set of magnets and an armature, one of which is attached to the rotor and the other to the stator, together forming a magnetic circuit: Field magnets - The magnets create a magnetic field that passes through the armature. These can be electromagnets or permanent magnets. The field magnet is usually on the stator and the armature on the rotor, but in some types of motor these are reversed. Bearings The rotor is supported by bearings, which allow the rotor to turn on its axis. The bearings are in turn supported by the motor housing. Rotor The rotor is the moving part that delivers the mechanical power. The rotor typically holds conductors that carry currents, which the magnetic field of the stator exerts force on to turn the shaft. Alternatively, some rotors carry permanent magnets, and the stator holds the conductors. Permanent magnets offer high efficiency over a larger operating speed and power range. An air gap between the stator and rotor allows it to turn. The width of the gap has a significant effect on the motor's electrical characteristics. It is generally made as small as possible, as a large gap weakens performance. It is the main source of the low power factor at which motors operate. The magnetizing current increases and the power factor decreases with the air gap, so narrow gaps are better. Conversely, gaps that are too small may pose mechanical problems in addition to noise and losses. The motor shaft extends through the bearings to the outside