An electric motor is a machine that converts electrical energy into mechanical motion. Most motors use electromagnetic interactions to produce rotation, while linear motors produce movement along a straight path. Their operating principles are closely related to those of an electric generator, which converts mechanical input into electrical output. Motors are central components of industrial drives, household equipment, and electric transportation. (ocw.mit.edu)
Operating principles
In a typical electromagnetic motor, electric current and a magnetic field interact to produce force. For a current-carrying conductor, this interaction can be described through the Lorentz force. Conductors arranged around an axis produce torque, turning the motor shaft. Other designs develop torque through the tendency of a magnetic structure to move toward a configuration of lower magnetic reluctance. Continuous rotation requires an appropriate relationship between the rotor position and the magnetic fields. (ocw.mit.edu)
Mechanical power is expressed as
[ P=T\omega, ]
where (P) is power, (T) is torque, and (\omega) is angular velocity. Thus, torque and power are distinct: a motor can exert holding torque while its shaft is stationary, but its mechanical output power is then zero. Rotation also produces an induced voltage, commonly called back electromotive force, which influences the current drawn from the supply. (ocw.mit.edu)
Construction
Most rotating motors have a stationary stator and a moving rotor, separated by a narrow air gap. The rotor is attached to a shaft supported by bearings. Depending on the design, the stator and rotor contain current-carrying windings, permanent magnets, or shaped magnetic steel. The rotor commonly lies inside the stator, although the opposite arrangement is also possible. (ocw.mit.edu)
Windings are coils of insulated electrical conductor. Magnetic cores guide flux and are often assembled from thin steel laminations to limit eddy-current losses. Winding insulation and cooling determine allowable temperature and current density, and therefore constrain continuous output. Motor design balances electromagnetic performance with mechanical strength, heat removal, and manufacturing requirements. (ocw.mit.edu)
Principal motor types
Direct-current motors. A conventional brushed DC motor uses direct current and a segmented commutator with brushes to switch current through the rotating armature windings. This switching maintains the torque-producing relationship between the armature and the field. The field may come from permanent magnets or separately energized windings. Brush contact introduces wear and maintenance requirements. (ocw.mit.edu)
Induction motors. An induction motor normally operates with alternating current. Stator currents establish a rotating magnetic field that induces rotor currents through electromagnetic induction. In ordinary motoring operation, the rotor turns more slowly than the field; this difference, called slip, is necessary for induction and torque production. A squirrel-cage rotor contains conducting bars joined by end rings and requires no electrical brush connection. Its robust construction makes this design important in industrial drives. (ocw.mit.edu)
Synchronous motors. A synchronous motor rotates in synchronism with the stator field during steady operation. Its rotor field may be supplied by energized windings or permanent magnets; synchronous reluctance motors instead exploit differences in magnetic reluctance. For a machine supplied at electrical frequency (f), the synchronous speed is
[ n_s=\frac{120f}{p}, ]
where (p) is the number of magnetic poles and (n_s) is measured in revolutions per minute. A four-pole machine supplied at 60 hertz therefore has a synchronous speed of 1,800 rpm. (ocw.mit.edu)
Brushless motors. A brushless DC motor replaces mechanical commutation with electronic switching. Despite its name, it commonly has a permanent-magnet rotor and operates as a synchronous machine; a DC source supplies electronics that generate the required winding currents. The term describes the drive arrangement rather than direct current flowing unchanged through every winding. (ocw.mit.edu)
Speed control and efficiency
Motor speed depends on machine type, supply conditions, and load. A variable-frequency drive controls an AC motor by adjusting the frequency and voltage supplied to it. More advanced drive methods regulate torque and magnetic flux for demanding motion-control applications. Motor drives therefore combine the machine with power electronics and control functions rather than treating the motor as an isolated component. (natural-resources.canada.ca)
Efficiency is the ratio of useful mechanical output power to electrical input power. Losses include winding resistance losses, magnetic-core losses, bearing friction, windage, and additional stray losses. Efficiency varies with operating conditions. System efficiency also depends on the driven equipment: adjusting motor speed to match demand can reduce energy consumption in suitable pump and fan applications, even when motor efficiency itself changes little. (energy.gov)
Development and applications
In 1821, Michael Faraday demonstrated continuous electromagnetic rotation using a current-carrying wire and a magnet, establishing an early electric motor. Later developments produced machines suitable for practical power delivery. An 1888 Westinghouse two-phase induction motor designed by Nikola Tesla illustrates the emergence of commercial AC motors without commutators or contact brushes. (rigb.org)
Industrial motors drive pumps, fans, compressors, conveyors, mixers, and processing machinery. In electric vehicles, motors convert energy supplied by a battery into propulsion. With suitable electronics, the traction machine can operate as a generator during regenerative braking, recovering some of the vehicle’s motion energy as electrical energy. Linear motors provide direct translational movement for applications such as weaving-machine shuttles and equipment handling. (betterbuildingssolutioncenter.energy.gov)