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. An air gap between the stator and rotor allows it to turn. The motor shaft extends outside of the motor, where it satisfies the load. 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.
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.
The stator surrounds the rotor, and usually holds field magnets, which are either electromagnets (wire windings around a ferromagnetic iron core) 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.
The armature consists of wire windings on a ferromagnetic core. Electric current passing through the wire causes the magnetic field to exert a force (Lorentz 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.
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 commutator reverses the current direction in the rotor windings with each half turn (180°), so the torque applied to the rotor is always in the same direction.
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 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. André-Marie Ampère developed the first formulation of the electromagnetic interaction and presented Ampère's force law, describing 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.
The first commutator DC electric motor 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. German-Russian Moritz von Jacobi created the first real useful rotating electric motor in May 1834. A major turning point came in 1864, when Antonio Pacinotti first described the ring armature. The first commercially successful DC motors followed the developments by Zénobe Gramme who, in 1871, reinvented Pacinotti's design. In 1886, Frank Julian Sprague invented the first practical DC motor, a non-sparking device that maintained relatively constant speed under variable loads.
In 1824, French physicist François Arago formulated the existence of rotating magnetic fields, termed Arago's rotations. In the 1880s many inventors were trying to develop workable AC motors. The first alternating-current commutatorless induction motor was invented by Galileo Ferraris in 1885. Possible industrial development was envisioned by Nikola Tesla, who invented independently his induction motor in 1887 and obtained a patent in May 1888. George Westinghouse acquired rights from Ferraris and bought Tesla's patents. Steadfast in his promotion of three-phase development, Mikhail Dolivo-Dobrovolsky invented the three-phase induction motor in 1889. The General Electric Company began developing three-phase induction motors in 1891.
Electric motors operate on one of three physical principles: magnetism, electrostatics and piezoelectricity. Motors can be designed to operate on DC current, on AC current, or some types can work on either. AC motors can be either asynchronous or synchronous. Synchronous motors require the rotor to turn at the same speed as the stator's rotating field. Asynchronous rotors relax this constraint.
Most DC motors are small permanent magnet (PM) types. They contain a brushed internal mechanical commutation to reverse motor windings' current in synchronism with rotation. A commutated DC motor has a set of rotating windings wound on an armature mounted on a rotating shaft. The shaft also carries the commutator.
Some of the problems of the brushed DC motor are eliminated in the BLDC design. In this motor, the mechanical "rotating switch" or commutator is replaced by an external electronic switch synchronised to the rotor's position. BLDC motors are typically 85%+ efficient, reaching up to 96.5%, while brushed DC motors are typically 75–80% efficient. BLDC motors are commonly used where precise speed control is necessary, as in computer disk drives or video cassette recorders.
The switched reluctance motor (SRM) has no brushes or permanent magnets, and the rotor has no electric currents. Torque comes from a slight misalignment of poles on the rotor with poles on the stator. SRMs are used in some appliances and vehicles.
A commutated, electrically excited, series or parallel wound motor is referred to as a universal motor because it can be designed to operate on either AC or DC power. Universal motors are often used in sub-kilowatt applications. They are also commonly used in portable power tools, such as drills, sanders, circular and jig saws.
AC induction and synchronous motors are optimized for operation on single-phase or polyphase sinusoidal or quasi-sinusoidal waveform power such as supplied for fixed-speed applications by the AC power grid or for variable-speed application from variable-frequency drive (VFD) controllers.
An induction motor is an asynchronous AC motor where power is transferred to the rotor by electromagnetic induction, much like transformer action. Polyphase induction motors are widely used in industry. Induction motors may be divided into Squirrel Cage Induction Motors (SCIM) and Wound Rotor Induction Motors (WRIM).
A synchronous electric motor is an AC motor. It includes a rotor spinning with coils passing magnets at the same frequency as the AC and produces a magnetic field to drive it. It has zero slip under typical operating conditions. One type of synchronous motor is like an induction motor except that the rotor is excited by a DC field.
The coreless or ironless DC motor is a specialized permanent magnet DC motor. Optimized for rapid acceleration, the rotor is constructed without an iron core. Because the rotor is much lower in mass compared to a conventional rotor, it can accelerate much more rapidly, often achieving a mechanical time constant under one millisecond.
The printed armature or pancake motor has windings shaped as a disc running between arrays of high-flux magnets. This design is commonly known as the pancake motor because of its flat profile. Pancake motors are widely used in high-performance servo-controlled systems, robotic systems, industrial automation and medical devices.
A servomotor is a motor that is used within a position-control or speed-control feedback system. Servomotors are used in applications such as machine tools, pen plotters, and other process systems. Motors intended for use in a servomechanism must have predictable characteristics for speed, torque, and power.
Stepper motors are typically used to provide precise rotations. An internal rotor containing permanent magnets or a magnetically soft rotor with salient poles is controlled by a set of electronically switched external magnets. Stepper motors are often used in computer printers, optical scanners, and digital photocopiers to move the active element.
A linear motor is essentially any electric motor that has been "unrolled" so that, instead of producing torque (rotation), it produces a straight-line force along its length. Linear motors are most commonly induction motors or stepper motors. Linear motors are commonly found in roller-coasters and are also used in maglev trains.
Electric motor output power is given as P<sub>em</sub> = Tω = Fv, where ω is shaft angular speed, T is torque, F is force, and v is velocity. In Imperial units a motor's mechanical power output is given by P<sub>em</sub> = (ω<sub>rpm</sub>T)/5252 (horsepower), where ω<sub>rpm</sub> is shaft angular speed in rpm and T is torque in foot-pounds.
To calculate a motor's efficiency, the mechanical output power is divided by the electrical input power. Electric motors have efficiencies ranging from around 15%-20% for shaded pole motors, up to 98% for permanent magnet motors, with efficiency also dependent on load. Peak efficiency is usually at 75% of the rated load. Efficiency also depends on motor size; larger motors tend to be more efficient.
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