ELECTRIC MOTOR

any of a class of devices that convert electrical energy to mechanical energy, usually by employing electromagnetic phenomena such as the coupling between the electrical and mechanical systems. The electric motor is the complement of the electric generator, a device that transforms mechanical energy into electrical energy (see also electric generator). In general, generators may function as motors, and, conversely, motors may function as generators; they differ only in some construction details and their auxiliary equipment. The operation of an electric motor involves two general principles. The first, called Ampre's law, states that a conductor experiences a force if a component of an electric current in that conductor flows at right angles to a magnetic field. The second principle, Faraday's law of induction, holds that a potential difference exists between the ends of a conductor if that conductor is given a component of motion perpendicular to the magnetic field. Given these principles, every electric motor must have two basic components: a rotor and a stator. The rotor, which in most cases comprises the moving part, contains conductors to produce and shape magnetic fields that will interact with magnetic fields generated by the stator. In addition, the rotor has a contacting device (e.g., slip rings) to connect it electrically with the external circuit, as well as various other mechanisms, including a drive shaft with which it transmits mechanical power to another machine. The stator is made of similar magnetic materials and electrical conductors that serve to establish and shape magnetic fields. Electric motors are commonly classified into two broad types, depending on whether direct or alternating current is used. These general types may be subdivided into induction motors, synchronous motors, and commutator motors, based on the way in which the magnetic fields are controlled. An induction motor typically has two sets of insulated wire windings: one, usually on the stator, is connected to an external power source; the other, on the rotor, consists of continuous wire loops. The magnetic field established when currents flow in these conductors is concentrated in a narrow air gap between the stator and the rotor. The purpose of the windings connected to the external energy source is to produce a rotating field that interacts with the induced currents in the rotor conductors, thereby establishing forces on those conductors. The revolving field can only be created if there are at least two windings that carry currents that are not in phase (i.e., whose waves are not at maximum magnitude simultaneously). Alternating-current induction motors basically are constant-speed devices and are most frequently used as such. They are the most widely employed electric motors because of their simple construction, efficiency, and low cost. Moreover, they are extremely rugged and in the smaller sizes (under 25 horsepower) can generally withstand a sudden application of full voltage at standstill without damage. Like induction motors, synchronous devices operate on the principle of a rotating magnetic field. In most cases the stator produces this field. The rotor serves to generate a constant unidirectional field by energizing a direct-current winding from a direct-current source (usually of 125 or 250 volts). This field interacts with the rotating field. In small synchronous motors, it is possible to eliminate the need for a direct-current power source by constructing the stator from metals used for permanent magnets. In such a case, the stator is given a permanent residual magnetic field that locks in with the rotating field at synchronous speed. These so-called hysteresis motors have a lower power-to-weight ratio than motors energized by direct current, but they are useful in certain low-load applications where constant speeds are cruciale.g., gyro-spin motors employed in navigational equipment, tape recorders, and electric clocks. Direct-current commutator motors, commonly regarded as forerunners of induction and synchronous devices, utilize a stationary field instead of a rotating field. Such a motor consists of a field magnet (stationary magnet), an armature (rotor coil), and a commutator, which is a mechanism that reverses the direction of the current in the armature. The armature turns between the poles of the field magnet until its own poles are next to the opposite poles of the magnet. At this point the direction of the current flowing through the armature is reversed, enabling the armature to make another half-turn. As the armature revolves, it produces torque, the turning force of the motor. One variety of direct-current commutator device, the series motor, is able to produce very high torque under heavy load, making it useful for traction applications. The development of the electric motor can be traced to the work of the Danish scientist Hans Christian rsted in the early 19th century, when he discovered that electricity in motion generates a magnetic field. In seeking to demonstrate the converse of this finding, the English physicist and chemist Michael Faraday constructed a primitive model of the electric motor in 1821. By the early 1870s the Belgian-born electrical engineer Znobe-Thophile Gramme had developed the first commercially viable electric motor. In 1883 the Serbian-American engineer Nikola Tesla invented the first alternating-current induction motor, a device generally considered the prototype of the modern electric motor. The electric motor has become the dominant motive power for industrial, business, transportation, and household applications. Not only are electric motors able to meet diverse service requirements, as, for example, accelerating, braking, and holding a load, but they can be made in a wide variety of sizes with output ranging from a fraction of a watt to thousands of horsepower. any of a class of devices that convert electrical energy to mechanical energy, usually by employing electromagnetic phenomena such as the coupling between the electrical and mechanical systems. In special situations, the electric supply may come from a battery, as, for example, in an automobile. Most motors develop their mechanical torque by the interaction of conductors carrying current in a direction at right angles to a magnetic field. The various types of electric motor differ in the ways in which the conductors and the field are arranged and also in the control that can be exercised over mechanical output torque, speed, and position. Each of the major kinds is delineated below. Additional reading Overviews may be found in the following texts: Syed A. Nasar (ed.), Handbook of Electric Machines (1987); G.R. Slemon and A. Straughen, Electric Machines (1980); Syed A. Nasar and L.E. Unnewehr, Electromechanics and Electric Machines, 2nd ed. (1983); Vincent Del Toro, Electric Machines and Power Systems (1985); and George McPherson and Robert D. Laramore, An Introduction to Electrical Machines and Transformers, 2nd ed. (1990). Gordon R. Slemon