Motors And Generators

Type of physical science: Classical physics

Field of study: Electromagnetism

The myriad force-driven processes that employ electricity as a source of power generally use electric motors that convert electrical energy into mechanical energy. The electricity that drives those motors is usually created by devices known as generators that convert mechanical energy into electrical energy. The design for motors and generators is identical, even though the functions are different.

Overview

Electricity is used primarily as a source of motive power. While it may be perceived by the public as primarily a source of power for the millions of lights that turn night into day in urban centers, more than 90 percent of all electricity is used to operate motors that drive machines. From the small electric clock at the bedside to the huge fan that works in tandem with others to circulate air inside the tallest skyscraper, electric motors keep the world moving. They are the most efficient, cleanest, and least costly means of driving motive systems. With increasing public awareness of the need to protect the environment and conserve resources, electricity and related devices promise to be an important energy technology in the twenty-first century.

The design of the electric motor is based upon magnetic attraction and repulsion. A flow of current is induced in a magnet by applying an external electromagnetic force that causes an armature to turn inside the motor. As the armature turns, mechanical energy is created, which, when harnessed, provides the power for many applications.

A magnet has two poles, customarily referred to as north and south poles; each is attracted to the other. Like poles repel, and unlike poles attract. The degree to which an object is one or the other is referred to as its polarity. The force with which they are attracted to or repelled by each other is related to the distance between the poles. Magnets are often bent into the shape of a horseshoe so that both poles are within close enough proximity to each other that they are able to create a magnetic field of sufficient intensity to attract materials that are subject to magnetic force fields or to maintain an electromagnetic field between them for some intended purpose.

The principle of magnetism has been understood for centuries. In the early eighteenth century, it was learned that electricity is related to magnetism in ways that can be predicted and demonstrated. It was discovered that when a wire carrying an electric current is placed over a compass needle, the needle is deflected to the left in relation to the direction in which the current is flowing. When the current is reversed in the same wire and the viewing perspective remains the same, the needle is deflected to the right. In essence, it was found that a magnetic field is created at right angles to the wire carrying the current flow. It was also learned that the force field thus created spins in a direction around the conductor that is determined by the direction of current flow. If the flow is reversed, the direction of the force field, referred to as the flux, is also reversed.

These principles were eventually employed by physicists who designed and demonstrated the first electromagnet. This device, a core of soft metal surrounded by a coil of wire through which an electric current was transmitted, performed like a magnet but a with a force whose strength varied, depending on the amount of current applied to the coil from some external source. The greater the current, the greater the force. Conversely, a decrease in the amount of current resulted in a weaker magnetic force. If no current was applied, then there was no appreciable force present.

Ultimately, a device was designed that combined an electromagnet, an armature, and a commutator. An armature is a coil that rides on a shaft attached to a magnet in such a way that lets it rotate freely. A commutator is a device attached to the armature that reverses the direction of current flow. The armature acts in the same way as the compass needle that was deflected by the flow of current through the wire. When the armature coil is either attracted or repelled by the force field, it moves on its free axis. Theoretically, it should then be alternately repelled and attracted by the magnetic poles of the magnet to the point where it would be in equilibrium. The commutator reverses the flow of current at precisely the time when the armature is being most forcefully repelled or attracted to one of the magnetic poles, thereby continuing the process of repulsion or attraction without interruption. Just as it is being repelled by the south pole of the magnet and attracted to the north pole, it is repelled by the north pole whose force field has suddenly changed polarity. The armature keeps moving in the same direction, alternately attracted to and repelled by the south pole, then the north pole, and so on. This process has the effect of causing the armature shaft to rotate in one direction continually, providing the means whereby the electric motor has emerged as the primary source of motive power in the mechanical age.

There are many kinds of electric motors and generators. They include the basic direct current (DC) motor. There are also alternating current (AC) motors called alternators that are either synchronous motors or induction motors. Some motors are single phase, a reference to the fluctuation of voltage that occurs during operation, while others are polyphase. Direct current generators are identical to DC motors except that the current flows in the opposite direction.

Electric motor torque is created by magnetic reaction, which induces a voltage in the armature windings that flows opposite to that of the voltage from the outside power source. As the motor rotates, the voltage rises and is discharged through slip rings at the end of the windings. Once a load, or force, is applied against the movement of the motor as happens when the motor is engaged to drive a conveyor or lift an electric crane, more current is applied from the outside and the voltage increases inside the motor as it seeks equilibrium with that outside voltage. In this way, electric motors are able to perform tasks.

The most common motors are two-phase and three-phase AC motors. In these machines, two or three separate windings are incorporated into the design to maximize the amount of voltage available to the motor at any time during operation. In a three-phase design, as the voltage in one winding is approaching its peak, the voltage in another winding is in mid-cycle between peak positive and peak negative. The third winding is at or very near peak negative voltage. This design has proved to be both reliable and economical.

In a DC polyphase motor, the coils are wound around the armature, which rotates within the motor housing. In AC motors, however, the magnetic field rotates for reasons of efficiency and economy. Some polyphase motors are used as synchronous motors that work in unison with each other to perform specific tasks. Two principle types of polyphase motors are squirrel-cage motors and wound-rotor motors. There are also fractional horsepower motors, a reference to the wide range of load capacities that can be built into them. The simplest of all electric motors is the squirrel-cage design that consists of three fixed coils. The motor gets its name from its physical appearance, which resembles a cage used to exercise pet squirrels. The motor housing contains a core, in which a series of conducting elements are embedded that run the length of and are parallel to the core. Current flowing in the armature, which is stationary, creates a magnetic field that causes the rotor to turn. Squirrel-cage motors have low starting torque characteristics.

The wide variety of electric motors in use is illustrative of the need for custom motor designs in most applications. A relationship exists among the amount of force--described as horsepower--required from the motor, the number of amperes it draws, the location of the motor in relation to its power source, and the voltage required to drive it optimally. In order to provide the precise requirements for a specific application, this relationship must be calculated carefully.

In some applications where significant torque is required, motors are designed so that one phase starts the motor and gets it moving before another phase kicks in to drive it at optimum speed.

Electric induction motors operate at peak efficiency when handling light loads.

Accordingly, it is important to select motors of the proper size to perform given tasks. The selection process must also take into account the voltage level available at the power source.

Electric motors and generators come in a wide variety of sizes and shapes. Each plays a key role in a machine age powered by electricity.

Applications

Since the dawn of the electronic era, electric motors and generators have been the means whereby electrical energy is turned into mechanical energy and vice versa. Electric motors are found in industry, institutional settings, and the home. They provide motive power for a thousand uses. They drive air through heating and cooling systems, drive conveyor belts and pumps, appliances and tools, and vehicles used for transportation.

There are many different variations on the basic electric motor design. Synchronous electric motors are used in applications that require constant speed and constant loads. They are usually characterized as either high speed or low speed, the designation indicative of performance range. High-speed synchronous motors, for example, are used as centrifugal pumps, reciprocating compressors, fans, blowers, and DC generators, along with many other applications. Low-speed synchronous motors are used as centrifugal pumps, vacuum pumps, and reciprocating pumps. Synchronous motors are also used for regulating voltage at the end of long electrical transmission lines where voltage levels tend to be unstable.

Squirrel-cage motors are the most widely used because of their simple design and versatility. They are used to power conveyors, large fans, presses, some compressors, and machines that employ flywheels. The tasks performed by these machines (sometimes called class-A motors), starting and stopping frequently at low inertia loads, require that the motors accelerate quickly without disrupting electrical system current voltage. Where there is a limitation on the amount of starting current available, class-B motors that are designed to provide high torque at low voltage levels are used. If greater torque is required, class-C motors are often used.

Another type of motor is the wound-rotor motor. Identical to the squirrel-cage motor in most respects, it has an external resistor in the power circuit for additional control. Wound-rotor motors are used in applications where starting and stopping of the motor is necessarily regulated to avoid damage to the motor or related machinery. They are also used where it is necessary to reverse the motor or where the motor must occasionally be run at speeds below those required for synchronization with other motors. These include faceplate starters, multiswitch starters, magnetic starters, speed regulators, and drum controllers.

Direct current motors come in three variations: series-wound, shunt-wound, and compound-wound. Series motors are used where there is a requirement for high torque without significantly increasing current as with the operation of overhead cranes. Shunt motors drive machinery designed to run constantly at nonvarying speeds, while compound motors drive elevators, printing presses, paper cutters, conveyors, and hoisting machinery--those applications that experience sudden load fluctuations but in which continuous, unwavering speeds are not required.

Another group falls under the heading of fractional-horsepower motors. These motors are used extensively in household appliances such as vacuum cleaners, hair dryers, fans, and blowers. Others are used to power sewing machines, portable power tools, film projectors, and appliances and machines that require cooling fans. The main attraction of fractional-horsepower motors is their ability to operate on both AC and DC power.

Sometimes it is desirable to change from alternating current to direct current and vice versa. Sometimes, direct voltage levels need to be changed; at other times AC frequency or phase needs to be changed. Special machines have been designed to accomplish these tasks. Usually, they take the form of a motor that operates from an available power supply to drive a generator that then creates the proper current flow at the appropriate voltages.

It is probable that electric motors will once again find applications in the automotive industry as they did early in the twentieth century. With oil resources becoming increasingly scarce and more expensive, electricity as a source of power for vehicles of all kinds is becoming increasingly attractive. Major automobile manufacturers began mass-marketing electric cars and trucks in the early 1990's in anticipation of a significant shift from fossil fuels to electricity for automotive power. The challenge is to come up with a design that will provide levels of power and range comparable to those of gasoline-powered vehicles.

Electric motors can also be powered by solar receptors, another promising source of energy for the twenty-first century. As solar technology is refined, it will increasingly provide power for electric motors designed to operate free from the standard umbilical connections that hook users to the sophisticated utility infrastructures of the industrialized world. This technology will be a boon to the Third World, already benefiting from satellite-delivered telecommunications services, because it will provide a means for employing electric motors to upgrade standards of living for millions of people.

Context

There can be no doubt that the electronic age will continue indefinitely. Nearly every human enterprise undertaken since the beginning of the twentieth century has incorporated some form of electrical application. Generators create electricity that electric motors transform into thousands of productive processes efficiently, economically, and cleanly. The astonishing speed at which the electronic era first emerged is testimony to the wide range of applications for electrical power, and the demand for this technology will not subside in the foreseeable future. In fact, the promise of current research into superconductivity is a vast new world of high-speed trains suspended electromagnetically above conducting rails and pocket computers with hundreds of megabytes of storage capacity. Perhaps the greatest promise, however, is the tremendous leap in efficiency that will result from the use of materials that conduct electricity without resistance.

Resistance results in a significant loss of energy during electrical transmission and operation of electronic systems. This inefficiency requires that generators create significantly more power than reaches the user, and it requires that user systems be designed with cooling devices to expel heat generated by the inherent inefficiency of those systems.

As generators have been used to transform mechanical energy into electricity, that mechanical energy has customarily been generated by burning fossil fuels, harnessing water power, or inducing nuclear reaction. In the twenty-first century, electricity will emerge as the energy source of choice for myriad economic and environmental reasons. Electrical generators will continue to play the major role in providing that energy resource, and for the same reasons, electric motors will continue to drive global production and provide the means to maintain modern living standards.

One of the major factors that makes electricity attractive as a source of motive power is its versatility. In addition to the efficiency and economy it affords, its portability seems limitless.

Computers and other chip-based technologies that rely on microcircuitry are being designed to operate virtually anywhere, including the hostile conditions of outer space. These electronic systems must be able to operate under environmental circumstances that would preclude the use of other energy resources. As supplies of fossil fuels dwindle, attention has refocused on solar and other renewable energy resources, and the need to put more emphasis on the development of electrical systems becomes more urgent.

The future holds great promise for increased efficiency in the design and operation of those systems. Many of today's electric motors are used to drive exhaust fans that expel heat created in the process of transmitting electrical energy. As current flows through conducting materials, it encounters various levels of resistance. The molecular structures of this transmission media and physical impurities contained therein interfere with the movement of electrons from one end of the medium to the other, which results in the radiation of heat energy. This heat energy must then be removed to protect system components. Motor-driven fans are found in computers, refrigerators, air conditioners, and most other appliances and electronic systems.

Ongoing research is expected to result in the development of new materials that will allow electricity to be transmitted without resistance. These so-called superconducting materials are expected to contain no obstructions to the flow of electrons and therefore no heat will be expended. Accordingly, no cooling fans will be required to expel that heat. The efficiency of electronic systems will improve dramatically, as will portability. The load factors built into electric motors will be reduced significantly, as current voltages will be more easily controlled.

Circuitry will be reduced to subminiature size and the amount of energy required to operate it will be reduced exponentially. The electronic era is experiencing a major technological leap forward that will ensure its place as the preeminent source of energy in the twenty-first century.

Principal terms

ALTERNATING CURRENT (AC): electric current that changes direction at regular intervals

ALTERNATOR: an electric generator or dynamo that produces alternating current

ARMATURE: in an electric motor or generator, the part that revolves; a series of coils of wire wound around an iron core

COMMUTATOR: a revolving part in an electric motor or generator that either collects electric current from brushes or distributes it to them; also used to reverse the direction of alternating current flow at regular intervals

CURRENT: the flow of electric force in a conductor from a point of higher potential to one of lower potential

DIRECT CURRENT (DC): electric current that flows in one direction only; also called continuous current

ELECTROMAGNET: a core of some substance that can be magnetized when wrapped in a wire to which an electric current is applied

FLUX: the rate of flow of electric current over a surface

INDUCTION: the generation of an electric current in a conductor by the influence of another electromagnetic force

POLARITY: the condition of being positive or negative with respect to an electromagnetic field

Bibliography

Albert, Arthur Lemuel. ELECTRONICS AND ELECTRON DEVICES. New York: Macmillan, 1956. This volume contains a lucid and comprehensible discussion of basic electronic theory. Areas covered include amplifiers, rectifiers, oscillators, semiconductors, and photoelectric devices. Contains illustrations, diagrams, index.

Anderson, Edwin P. ELECTRIC MOTORS. Indianapolis, Ind.: Howard W. Sams, 1968. Contains an excellent introduction to the principles of electric motor and generator design and applications. Includes chapters on synchronous, squirrel-cage, wound-rotor, DC, and fractional-horsepower motors. Each chapter contains a trouble-shooter's guide to operational problems and solutions. Also includes chapters on motor testing and maintenance. Contains excellent diagrams, charts, schematics, and calculation formulas. Index.

Lurch, E. Norman. FUNDAMENTALS OF ELECTRONICS. New York: John Wiley & Sons, 1981. In addition to a good discussion of fundamental electronic principles, this volume contains an excellent in-depth discussion of the concept of amplification in electronic circuitry. Contains diagrams, illustrations, index.

Meyer, Herbert W. A HISTORY OF ELECTRICITY AND MAGNETISM. Cambridge, Mass.: MIT Press, 1971. This novel look at the history of electricity contains an entertaining chronology of the evolution of the technology that led to the invention of the electric motor, the generator, and alternating current. Particularly interesting are the brief biographical sketches of the personages that led the way to the electronic era. Index.

Rosenblatt, Jack, and M. Harold Friedman. DIRECT AND ALTERNATING CURRENT MACHINERY. New York: McGraw-Hill, 1963. Contains an overview of the principles of electromagnetic induction and its application to the design and operation of electric motors and generators. Contains chapters that describe in detail the design of single-phase and polyphase induction motors. Index.

Charges and Currents

Conductors and Resistors

Electrons and Atoms

Forces on Charges and Currents

Insulators and Dielectrics

The Measurement of Magnetic Fields