Electric motor
An electric motor is a machine that converts electrical energy into mechanical energy. Most electric motors operate through the interaction between the motor's magnetic field and electric current in a wire winding to generate Laplace force in the form of torque applied on the motor's shaft. An electric generator is mechanically identical to an electric motor but operates inversely, converting mechanical energy into electrical energy.
Electric motors can be powered by direct current sources, such as from batteries or rectifiers, or by alternating current sources, such as a power grid, inverters or electrical generators. Electric motors may also be classified by considerations such as power source type, construction, application and type of motion output. They can be brushed or brushless, single-phase, two-phase, or three-phase, axial or radial flux, and may be air-cooled or liquid-cooled.
Standardized electric motors provide power for industrial use. The largest are used for marine propulsion, pipeline compression and pumped-storage applications, with output exceeding 100 megawatts. Other applications include industrial fans, blowers and pumps, machine tools, household appliances, power tools, vehicles, and disk drives. Small motors may be found in electric watches. In certain applications, such as in regenerative braking with traction motors, electric motors can be used in reverse as generators to recover energy that might otherwise be lost as heat and friction.
Electric motors produce linear or rotary force intended to propel some external mechanism. This makes them a type of actuator. They are generally designed for continuous rotation, or for linear movement over a significant distance compared to its size. Solenoids also convert electrical power to mechanical motion, but over only a limited distance.
Components
An electric motor has two mechanical parts: the rotor, which moves, and the stator, which does not. Electrically, the motor consists of two parts, the field magnets and the armature, one of which is attached to the rotor and the other to the stator. Together they form a magnetic circuit. 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 these may be reversed.Rotor
The rotor is the moving part that delivers the mechanical power. The rotor typically holds conductors that carry currents, on which the magnetic field of the stator exerts force to turn the shaft.Stator
The stator surrounds the rotor, and usually holds field magnets, which are either electromagnets or permanent magnets. These create a magnetic field that passes through the rotor armature, exerting force on the rotor windings. The stator core is made up of many thin metal sheets that are insulated from each other, called laminations. These laminations are made of electrical steel, which has a specified magnetic permeability, hysteresis, and saturation. Laminations reduce losses that would result from induced circulating eddy currents that would flow if a solid core were used. Mains powered AC motors typically immobilize the wires within the windings by impregnating them with varnish in a vacuum. This prevents the wires in the winding from vibrating against each other which would abrade the wire insulation and cause premature failures. Resin-packed motors, used in deep well submersible pumps, washing machines, and air conditioners, encapsulate the stator in plastic resin to prevent corrosion and/or reduce conducted noise.Gap
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. Conversely, gaps that are too small may create friction in addition to noise.Armature
The armature consists of wire windings on a ferromagnetic core. Electric current passing through the wire causes the magnetic field to exert a force on it, turning the rotor. Windings are coiled wires, wrapped around a laminated, soft, iron, ferromagnetic core so as to form magnetic poles when energized with current.Electric machines come in salient- and nonsalient-pole configurations. In a salient-pole motor the rotor and stator ferromagnetic cores have projections called poles that face each other. Wire is wound around each pole below the pole face, which become north or south poles when current flows through the wire. In a nonsalient-pole motor, the ferromagnetic core is a smooth cylinder, with the windings distributed evenly in slots around the circumference. Supplying alternating current in the windings creates poles in the core that rotate continuously. A shaded-pole motor has a winding around part of the pole that delays the phase of the magnetic field for that pole.
Commutator
A commutator is a rotary electrical switch that supplies current to the rotor. It periodically reverses the flow of current in the rotor windings as the shaft rotates. It consists of a cylinder composed of multiple metal contact segments on the armature. Two or more electrical contacts called brushes made of a soft conductive material like carbon press against the commutator. The brushes make sliding contact with successive commutator segments as the rotator turns, supplying current to the rotor. The windings on the rotor are connected to the commutator segments. The commutator reverses the current direction in the rotor windings with each half turn, so the torque applied to the rotor is always in the same direction. Without this reversal, the direction of torque on each rotor winding would reverse with each half turn, stopping the rotor. Commutated motors have been mostly replaced by brushless motors, permanent magnet motors, and induction motors.Shaft
The motor shaft extends outside of the motor, where it satisfies the load. Because the forces of the load are exerted beyond the outermost bearing, the load is said to be overhung.Bearings
The rotor is supported by bearings, which allow the rotor to turn on its axis by transferring the force of axial and radial loads from the shaft to the motor housing.History
Early motors
Before modern electromagnetic motors, experimental motors that worked by electrostatic force were investigated. The first electric motors were simple electrostatic devices described in experiments by Scottish monk Andrew Gordon and American experimenter Benjamin Franklin in the 1740s and 1750s. The theoretical principle behind them, Coulomb's law, was discovered but not published, by Henry Cavendish in 1771. This law was discovered independently by Charles-Augustin de Coulomb in 1785, who published it so that it is now known by his name. Due to the difficulty of generating the high voltages they required, electrostatic motors were never used for practical purposes.The invention of the electrochemical battery by Alessandro Volta in 1799 made the production of persistent electric currents possible. Hans Christian Ørsted discovered in 1820 that an electric current creates a magnetic field, which can exert a force on a magnet. It only took a few weeks for André-Marie Ampère to develop the first formulation of the electromagnetic interaction and present the Ampère's force law, that described the production of mechanical force by the interaction of an electric current and a magnetic field.
Michael Faraday gave the first demonstration of the effect with a rotary motion on 3 September 1821 in the basement of the Royal Institution. A free-hanging wire was dipped into a pool of mercury, on which a permanent magnet was placed. When a current was passed through the wire, the wire rotated around the magnet, showing that the current gave rise to a close circular magnetic field around the wire. Faraday published the results of his discovery in the Quarterly Journal of Science, and sent copies of his paper along with pocket-sized models of his device to colleagues around the world so they could also witness the phenomenon of electromagnetic rotations. This motor is often demonstrated in physics experiments, substituting brine for mercury. Barlow's wheel was an early refinement to this Faraday demonstration, although these and similar homopolar motors remained unsuited to practical application until late in the century.
File:An_electric_motor_presented_to_Kelvin_by_James_Joule_in_1842,_Hunterian_Museum,_Glasgow.jpg|thumb|An electric motor presented to Kelvin by James Joule in 1842, Hunterian Museum, Glasgow
In 1827, Hungarian physicist Ányos Jedlik started experimenting with electromagnetic coils. After Jedlik solved the technical problems of continuous rotation with the invention of the commutator, he called his early devices "electromagnetic self-rotors". Although they were used only for teaching, in 1828 Jedlik demonstrated the first device to contain the three main components of practical DC motors: the stator, rotor and commutator. The device employed no permanent magnets, as the magnetic fields of both the stationary and revolving components were produced solely by the currents flowing through their windings.
DC motors
The first commutator capable of turning machinery was invented by English scientist William Sturgeon in 1832. Following Sturgeon's work, a commutator-type direct-current electric motor was built by American inventors Thomas Davenport and Emily Davenport, which he patented in 1837. The motors ran at up to 600 revolutions per minute, and powered machine tools and a printing press. Due to the high cost of primary battery power, the motors were commercially unsuccessful and bankrupted the Davenports. Several inventors followed Sturgeon in the development of DC motors, but all encountered the same battery cost issues. As no electricity distribution system was available at the time, no practical commercial market emerged for these motors.After many other more or less successful attempts with relatively weak rotating and reciprocating apparatus German-Russian Moritz von Jacobi created the first real useful rotating electric motor in May 1834. It developed remarkable mechanical output power. His motor set a world record, which Jacobi improved four years later in September 1838. His second motor was powerful enough to drive an electric boat with 14 people across a wide river. It was also in 1839–1840 that other developers managed to build motors with similar and then higher performance.
In 1827–1828, Jedlik built a device using similar principles to those used in his electromagnetic self-rotors that was capable of useful work. He built a model electric vehicle that same year.
A major turning point came in 1864, when Antonio Pacinotti first described the ring armature. This featured symmetrically grouped coils closed upon themselves and connected to the bars of a commutator, the brushes of which delivered practically non-fluctuating current. The first commercially successful DC motors followed the developments by Zénobe Gramme who, in 1871, reinvented Pacinotti's design and adopted some solutions by Werner Siemens.
A benefit to DC machines came from the discovery of the reversibility of the electric machine, which was announced by Siemens in 1867 and observed by Pacinotti in 1869. Gramme accidentally demonstrated it on the occasion of the 1873 Vienna World's Fair, when he connected two such DC devices up to 2 km from each other, using one of them as a generator and the other as motor.
The drum rotor was introduced by Friedrich von Hefner-Alteneck of Siemens & Halske to replace Pacinotti's ring armature in 1872, thus improving the machine efficiency. The laminated rotor was introduced by Siemens & Halske the following year, achieving reduced iron losses and increased induced voltages. In 1880, Jonas Wenström provided the rotor with slots for housing the winding, further increasing the efficiency.
In 1886, Frank Julian Sprague invented the first practical DC motor, a non-sparking device that maintained relatively constant speed under variable loads. Other Sprague electric inventions about this time greatly improved grid electric distribution, allowed power from electric motors to be returned to the electric grid, provided for electric distribution to trolleys via overhead wires and the trolley pole, and provided control systems for electric operations. This allowed Sprague to use electric motors to invent the first electric trolley system in 1887–88 in Richmond, Virginia, the electric elevator and control system in 1892, and the electric subway with independently powered centrally controlled cars. The latter were first installed in 1892 in Chicago by the South Side Elevated Railroad, where it became popularly known as the "L". Sprague's motor and related inventions led to an explosion of interest and use in electric motors for industry. The development of electric motors of acceptable efficiency was delayed for several decades by failure to recognize the extreme importance of an air gap between the rotor and stator. Efficient designs have a comparatively small air gap. The St. Louis motor, long used in classrooms to illustrate motor principles, is inefficient for the same reason, as well as appearing nothing like a modern motor.
Electric motors revolutionized industry. Industrial processes were no longer limited by power transmission using line shafts, belts, compressed air or hydraulic pressure. Instead, every machine could be equipped with its own power source, providing easy control at the point of use, and improving power transmission efficiency. Electric motors applied in agriculture eliminated human and animal muscle power from tasks such as handling grain and pumping water. Household uses of electric motors reduced heavy labor in the home and made higher standards of convenience, comfort and safety possible. Today, electric motors consume more than half of the electric energy produced in the US.