Electric Motor Systems
ELECTRIC MOTOR SYSTEMS
Electric motors are everywhere. These ubiquitous devices come in a wide variety of sizes and power outputs, ranging from a fraction of a watt to huge multikilowatt applications. Tiny ones operate computer disk drives, power windows/mirrors, and windshield wipers; moderate-sized ones run appliances such as fans, blenders, electric shavers, and vacuum cleaners; a large drive pumps, elevators, sawmills, and electric trains and vehicles.
There has been not only growth in the total number of electric motors (more standard appliances in use), but also a proliferation in their use for new, novel applications. Both trends will continue to increase demand for the electricity to run electric motors. In the United States, electric motors are responsible for consuming more than half of all electricity, and for the industrial sector alone, close to two-thirds. Since the cost of the electricity to power these motors is enormous (estimated at more than $90 billion a year), research is focused on finding ways to increase the energy efficiency of motors and motor systems.
Electric motors are devices that convert electrical energy into mechanical energy. Devices that do the opposite—convert mechanical energy into electrical energy—are called generators. An important example is power plants, where large gas, steam, and hydroelectric turbines drive generators to provide electricity. Since generators and motors work under the same principles, and because construction differences are minimal, often generators can function as motors and motors, as generators, with only minor changes.
The early development of electric motors and generators can be traced to the 1820 discovery by Hans Christian Oersted that electricity in motion generates a magnetic field. Oersted proved the long-suspected promise that there is indeed a relationship between electricity and magnetism. Shortly thereafter, Michael Faraday built a primitive electric motor, which showed that Oersted's effect could be used to produce continuous motion.
Electric motors consist of two main parts: the rotor, which is free to rotate, and the stator, which is stationary (Figure 1). Usually each produces a magnetic field, either through the use of permanent magnets, or by electric current flowing through the electromagnetic windings (electromagnets produce magnetism by an electric current rather than by permanent magnets). It is the attraction and repulsion between poles on the rotor and the stator that cause the electromagnet to rotate. Repulsion is at a maximum when the current is perpendicular to the magnetic field. When the plane of the rotator is parallel to the magnetic field (as in Figure 1) there is no force on the sides of the rotator. The left side of the rotator receives an upward push and the right side receives a downward push; thus rotation occurs in a clockwise direction. When the rotator is perpendicular to the magnetic field, forces are exerted on all its sides equally, canceling out. Therefore, to produce continuous motion, the current must be reversed when reaching a vertical direction. For alternating-current motors this occurs automatically, since the alternating magnetic attraction and repulsion change directions 120 times each second; for direct current-motors, this usually is accomplished with a device called a commutator.
Before useful electric motors could be developed, it was first necessary to develop practical electromagnets, which was primarily the result of work done by Englishman William Sturgeon as well as Joseph Henry and Thomas Davenport of the United States. In 1873 Zénobe-Théophile Gramme, a Belgian-born electrical engineer, demonstrated the first commercial electric motor. A decade later Nikola Tesla, a Serbian-American engineer, invented the first alternating-current induction motor, the prototype for the majority of modern electric motors that followed. Magnetic fields today are measured in units of teslas (symbol T), to acknowledge Tesla's contribution to the field.
The electrical power supplied to electrical motors can be from a direct-current (dc) source or an alternating-current (ac) source. Because dc motors are more expensive to produce and less reliable, and because standard household current is ac, ac motors are far more prevalent. However, the growing demand for portable appliances such as laptop computers, cordless power drills, and vacuums ensures a future for dc motors. For these portable applications, the alternating current charges the battery, and the direct current of the battery powers the dc motor.
Based on the way magnetic fields are generated and controlled in the rotor and the stator, there are several additional subclassifications of direct- and alternating-current motors.
Direct Current Motors
Direct-current motors are classified as separately excited motors, series motors, shunt motors, and compound motors. The field winding of a separately excited motor is in a circuit that is energized by a separate dc source; the field winding is not physically connected to the armature circuit (containing the armature winding).
For series, shunt, and compound motors, only one power supply is needed. In a series motor the field winding is connected in series with the power supply and the armature circuit. In a shunt motor the field winding is connected across, or in parallel with, the dc supply, which energies the armature winding. In a compound motor, also known as a cumulative compound motor, the field winding is physically in parallel with the armature winding circuit and is magnetically coupled to a coil or a winding in series with the armature winding.
Direct current motors are most appropriately used in applications where a dc power supply is available or where a simple method of speed control is desired. The fans used in automobile heating and air conditioning systems are driven by direct-current motors.
Alternating-current motors are classified as induction motors or synchronous motors. Faraday found that a stationary wire in a magnetic field produced no current. However, when the wire continues to move across magnetic lines of force, it produces a continual current. When the motion stops, so does the current. Thus Faraday proved that electric current is only produced from relative motion between the wire and magnetic field. It is called an induced current—an electromagnetic induction effect.
Induction motors usually entail insulated wiring windings for both the rotor and the stator, with the stator connected to an external electric power source. Between the narrow gap of the stator and the rotor, a revolving magnetic field is established. A current can be established only when the waves of the rotor and stator windings are not in phase—not at a maximum simultaneously.
The induction motor is the most common motor in industrial applications, and are also very prevalent for smaller applications because of their simple construction, reliability, efficiency, and low cost. Historically they have been found in applications that call for a constant speed drive, since the alternating-current power supply is of constant voltage and frequency; however, the continual development of more powerful and less inexpensive solid-state electronic devices has allowed for electronic inverters (which control voltage and frequency) to more accurately control the speed and torque of induction motors, thereby matching the control performance of a motor. Since induction motors are less expensive, more compact, more efficient, and more reliable (better voltage overload capabilities), it is likely that induction motors will continue to replace motors in most applications.
Synchronous motors operate like induction motors in that they rely on the principle of a rotating magnetic field, usually produced by the stator. Synchronous motors differ in that the rotor generates a constant unidirectional field from a direct-current winding powered by a direct-current source. This field interacts with the rotating field. To get around the need for direct current-power, the stator can be constructed from permanent magnets so that the permanent magnetic field and rotating field can be synchronized. Synchronous motors are most useful for low-load applications where constant speed control is crucial, such as in phonographs, tape recorders, and electric clocks.
ADVANCES IN PERFORMANCE, EFFICIENCY, AND RELIABILITY
Innovations in designs and materials led to continual advances in the performance, efficiency and reliability, of electric motors throughout the Twentieth century. The best results for motor designers occur when starting with the function at hand and working back toward the power source, optimizing each element along the way, foremost being the improvement of the end use of mechanical energy. If end use mechanical energy is curtailed, the demand for power generated from an electric motor declines, and consequently the demand for electricity to run the motor. For a factory or warehouse conveyer belt system, a streamlined design, better bearings, and lighter components can yield far greater energy savings than replacing a standard-efficiency motor with an energy-efficient model.
There are few shortcuts. If you want a powerful electric motor, it is going to have to be large and entail an extensive amount of copper windings. That is why it is much more cost-effective to rebuild many large (more than 100 horsepower) industrial motors than to replace them with new motors.
In theory, electric motors can be more than 95 percent efficient. Since electric motors can to convert almost all the electrical energy into mechanical energy, it partly explains the continued growth of electrical technology at the expense of competing technologies. It is widely believed that the electric motor will eventually replace even the internal-combustion engine (in which only about 25 percent of the heat energy is converted to mechanical energy) once the costs of better-performing battery and fuel cell technologies decline.
In practice most electric motors operate in the 75 to 90 percent range, primarily because of core magnetic losses (heat losses and electric current loss), copper resistive losses, and mechanical losses (friction in the bearings, and windings, and aerodynamic drag). At low speeds core magnetic losses are greatest, but as speeds get higher, core magnetic losses decline, and copper resistive losses become more dominant. The mechanical losses do not vary much, remaining fairly constant at both low and high speeds.
There were no standards for energy-efficient motors until the National Electrical Manufacturers Association (NEMA) developed design classifications for energy-efficient, three- phase induction motors in 1989. This standard was made the national minimum efficiency level by the Energy Policy Act of 1992, which went into effect in October 1997. Manufacturers responded to this higher efficiency level by reducing losses through the use of better materials, improved designs, and precision manufacturing.
Aside from the efficiency of the motor itself, energy efficiency is very dependent upon proper sizing. While the efficiency of a motor is fairly constant from full load down to half load, when a motor operates at less than 40 percent of its full load, efficiency drops considerably, since magnetic, friction, and windage losses remain fairly constant regardless of the load. Moreover, the power factor drops continuously as the load drops. The problem is most discernible in small motors.
The obvious answer is to properly size the motor for an application. However, properly sizing a motor is difficult when motors are required to run at a wide spectrum of loads. In many cases a decrease in the motor's speed would reduce the load while maintaining the efficiency.
A major hurdle to greater efficiency is the constant-speed nature of induction and synchronous motors. Nevertheless, considerable advances have been made in improving motor speed controls that essentially better optimize the motor speed to the task at hand, resulting in substantial energy savings, decreased wear of the mechanical components, and usually increased productivity from the user.
Usually the lowest first cost solution is to use multispeed motors with a variety of torque and speed characteristics to match the different types of loads encountered. The more costly and more energy-efficient choice is to use electronic adjustable speed drives that continuously change the speed of AC motors by controlling the voltage supplied to the motor through semiconductor switches. Energy efficiency is also achieved by converting the 60-hertz supply frequency to some lower frequency, thereby enabling induction motors to operate at slower speeds, and thus consuming less energy. Slower speeds may be desirable for many applications such as fans and conveyor belts. There are several different types of electronic adjustable-speed drives, yet no one technology has emerged as superior to all others. That is because the multitude of different motors, different sizes (horsepower) and speeds, and control requirements make it difficult for one control system technology to be superior for all applications.
The cost premium for a motor equipped with speed control can be substantial, sometimes costing twice that of a single-speed motor. But the energy savings from speed control can be substantial, especially for fan and pump systems. Electronic adjustable speed drives continue to become more attractive because the costs of microelectronics and power electronics technologies continue to fall as performance and energy efficiency improve.
Electric motors have proven to be very reliable and continue to become more reliable because of better materials and designs. However, because of the excellent reliability record, long life cycles, and the lower first cost of rebuilding motors instead of purchasing new, more energy-efficient models, it will be decades before energy-efficient motors significantly penetrate the market.
Even when the time comes to make a purchasing decision, an energy-efficient motor purchase is not a certainty. Sometimes an energy-efficient motor will be the economically efficient choice; at other times, not. The capital investment decision is based on the cost in relation to performance, efficiency and reliability. Moreover, the decision depends on the application and the amount of time the motor is in operation. It can be the major component of a product (drill or mixer), or a minor component (computer disk drive); it can be the major component cost of a product (fan), or it can be a minor component cost (stereo tape deck); it can run almost constantly (fan, pump, and machinery), or only a few minutes a day (vacuums and power tools). For example, contractors purchase circular saws almost solely based on performance and reliability. Time is money, and since the saw is operating only a few minutes a day and the contractor is often not responsible for the electricity costs to run the motor, energy efficiency is not a consideration; performance and reliability are what matter most. On the other hand, an industrial user, who runs huge electric motors twenty-four hours a day to work pumps, machinery, and ventilation equipment, is very concerned with energy efficiency as well as performance and reliability.
See also: Batteries; Capital Investment Decisions; Consumption; Economically Efficient Energy Choices; Electricity; Electric Power, Generation of; Faraday, Michael; Fuel Cells; Fuel Cell Vehicles; Magnetism and Magnets; Oersted, Hans Christian; Tesla, Nikola.
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Beaty, H., and Kirtley, J. (1998). Electric Motor Handbook. New York: McGraw-Hill.
Electric Power Research Institute. (1992). "Electric Motors: Markets, Trends and Applications." EPRI Report TR-100423. Palo Alto, CA: Author.
Nadel, S.; Shepard, M.; Greenburg, S.; Katz, G.; and Almeida, A. (1992). Energy-Efficient Motor Systems: A Handbook on Technologies, Programs and Policy Opportunities. Washington, DC: American Council for an Energy Efficient Economy.